Download Global Biogeochemical Cycles and the Physical Climate System

Global Biogeochemical Cycles and the
Physical Climate System
by
Fred T. Mackenzie
Atmosphere
Ecosphere
Hydrosphere
Lithosphere
University Corporation for Atmospheric Research
National Center for Atmospheric Research UCAR Office of Programs
Understanding Global Change: Earth Science and Human Impacts
Global Biogeochemical Cycles and
the Physical Climate System
by
Fred T. Mackenzie
School of Ocean and Earth Science and Technology
University of Hawaii
National Oceanic and Atmospheric Administration
Understanding Global Change: Earth Science and Human Impacts
Understanding Global Change: Earth Science and Human Impacts
Global Biogeochemical Cycles and the Physical Climate System
by Fred T. Mackenzie
An instructional module produced by the Global Change Instruction Program of the University
Corporation for Atmospheric Research with support from the National Science Foundation.
GCIP Staff
Advisory Committee
Tom M.L. Wigley, Scientific Director
National Center for Atmospheric Research
Arthur Few
Rice University
Lucy Warner, Program Manager
University Corporation for Atmospheric Research
John Firor
National Center for Atmospheric Research
Carol Rasmussen, Editor
University Corporation for Atmospheric Research
William Moomaw
Tufts University
Linda Carbone, Secretary
University Corporation for Atmospheric Research
Ellen Mosley-Thompson
The Ohio State University
Jack Rhoton
East Tennessee State University
John Snow
University of Oklahoma
©1999 by the University Corporation for Atmospheric Research. All rights reserved.
Any opinions, findings, conclusions, or recommentations expressed in this publication are those of the
authors and donot necessarily reflect the views of the National Science Foundation.
For more information on the Global Change Instruction Program, contact the UCAR Communications
office, P.O. Box 3000, Boulder, CO 80307-3000. Phone: 303-497-8600; fax: 303-497-8610;
[email protected] or [email protected]
http://home.ucar.edu/ucargen/education/gcmod/contents.html
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Global Biogeochemical Cycles and the Physical Climate System
A note on this series
This series has been designed by college professors to fill an urgent need for interdisciplinary materials
on global change. These materials are aimed at undergraduate students not majoring in science. The
modular materials can be integrated into a number of existing courses—in earth science, biology,
physics, astronomy, chemistry, meteorology, and the social sciences. They are written to capture the
interest of the student who has little grounding in math and technical aspects of science but whose intellectual curiosity is piqued by concern for the environment. For a complete list of materials contact
UCAR Communications (see previous page).
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Global Change
Instruction Program
Contents
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .vii
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
Chapter 1: Bigeochemical Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
Chapter 2: Biogeochemical Cycles and Climate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Chapter 3: The Modern Coupled C-N-P-S-O System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
Chapter 4: Carbon Cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
Chapter 5: The Important Nutrient Nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29
Chapter 6: Phosphorus and Sulfur . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36
Chapter 7: The Water Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44
Study Questions and Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45
Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .60
Supplementary Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .68
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .69
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Global Change
Instruction Program
Preface
discipline than in another. Also, each discipline
has a unique vocabulary. (Most of the interdisciplinary vocabulary in this module is defined
within the text or in the extensive glossary.)
Furthermore, the language of chemical equations
is used to describe processes operating within the
ecosphere. Therefore, it may take some additional
work and perhaps reference to basic texts in
chemistry, ecology, meteorology, etc., to digest
the material of this module.
The text begins by introducing some important biogeochemical processes. This material is
not a laundry list of processes but a selection of
such processes as photosynthesis, weathering,
and deposition of sediments in the ocean as
examples of the nature and variety of biogeochemical processes. The next subject is the historical (geological) nature of environmental change
on the earth. Emphasis is on the biogeochemical
cycles of atmospheric carbon dioxide and oxygen
through the past 600 million years of the history
of the earth. The major processes controlling
these cycles and their tie to climate are discussed.
We will see that for much of this time, the planet
has had a more equable climate than at present.
Finally, the text deals with parts of the modern biogeochemical cycles of five of the most
important elements essential for life: carbon,
nitrogen, phosphorus, sulfur, and oxygen. These
elements, along with hydrogen and a suite of
nutrient trace elements, interact through the
processes of photosynthesis and respiration
and/or decay. Processes and feedbacks within
the cycles are described in the context of the
potential for a global warming brought about by
human activities that have changed the composition of the atmosphere. Keep in mind that the
approach can be used to interpret the interaction
between biogeochemical cycles and climatic
change of any nature—warming or cooling—and
at various space and time scales.
Global environmental change is a subject of
considerable public and scientific interest today.
Any discussion of change must involve the substances that are transported in cycles about the
earth’s surface—through its air, water, soil, rocks,
ice, and living and dead organic matter. Thinking
about these global biogeochemical cycles and
their role in environmental change requires us to
cross the usual boundaries between biology, ecology, oceanography, meteorology, chemistry, and
geology. Because of the impact of human activities on the cycles, and consequently the climate,
the subject also involves the effects and consequences of natural and human-induced change
for ecosystems, humans, and human infrastructures. This leads the discussion into the fields of
sociology, economics, and political science. Such
a broad and interdisciplinary topic is difficult to
capture completely in a module of this size. I
have made no attempt to do so but have concentrated on the biogeochemical cycles of five of the
major elements important to life—carbon, nitrogen, phosphorus, sulfur, and oxygen—and their
role in climatic change.
Biogeochemistry is the discipline that links
various aspects of biology, geology, and chemistry to investigate the surface environment of
the earth. This environment, the ecosphere (see
Figure 1), encompasses the biosphere (living and
dead organic matter) and parts of the other large
subdivisions (reservoirs) of the earth’s surface of
atmosphere (air), hydrosphere (water), shallow
crust (soils, sediments, and crustal rocks), and
cryosphere (ice). In this module, I focus on the
role biogeochemistry plays in regulating and
interacting with the climate system.
This module covers a great deal of material,
much of which is interdisciplinary. This presents
a problem for the writer of the material, the
teacher, and the student. Both teacher and student generally will have more knowledge in one
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Understanding Global Change: Earth Science and Human Impacts
An extensive glossary, study questions and
answers, and a supplementary reading section
conclude the module. The glossary is interdisciplinary and should help in understanding the
diverse material of the module. The study questions are designed to enable students to review
the text, to integrate the material, and to expand
their knowledge of the topics covered. Many of
the questions require calculations using standard
arithmetic. Mathematics is the foundation of science, and it is necessary for students to get their
feet wet. The readings are broad in scope and of
a general nature.
I would like to thank John Firor, Dave
Schimel, and especially Tom Wigley for their
comments on the initial draft of this module.
Some of the material in this module comes from
research supported by the National Science
Foundation and the National Oceanographic and
Atmospheric Administration. The final version of
this module was written while I was a Fellow at
the Wissenschaftskolleg zu Berlin. I thank Prof.
Dr. Wolf Lepenies, rector of the institute, for providing space, facilities, and peace of mind to
accomplish the task. Many thanks to Michael
Shibao for drafting and in so doing substantially
improving the original illustrations for this module. Finally, I am extremely indebted to Carol
Rasmussen of the University Corporation for
Atmospheric Research for her critical and laborious editing. Without her, this module would not
have been completed.
Fred T. Mackenzie
School of Ocean and Earth
Science and Technology
University of Hawaii
June 1996
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Global Change
Instruction Program
Introduction
The global ecosphere is the thin film around
the earth where living things (the biosphere 1 )
interact with the atmosphere (air), hydrosphere
(water), cryosphere (ice), and lithosphere (soils and
shallowly buried rocks) in a complex system
involving biological, geological, and chemical
processes and cycles (Figure 1). This biogeochemical system of spheres and processes is powered
mainly by energy from the sun.
The ecosphere is made up of individual
ecosystems, such as tropical forests, grasslands,
tundra, coral reefs, and estuaries. Matter and
energy flow between and within these ecosystems in interconnected biogeochemical cycles.
Gaseous chemical compounds are produced and
consumed in the ecosystems and exchanged
between them and the air. In the atmosphere,
they may react to form other compounds before
returning to the earth’s surface. Some of these
chemical species are greenhouse gases, like carbon
dioxide (CO2) and methane, which act in the
atmosphere to warm the planet. Others, like
dimethylsulfide gas, react with other atmospheric
chemicals to form minute airborne particles
(aerosols) that directly or indirectly help to cool the
climate.
The most common way of studying the global movements of these chemicals is by mathematical modeling of biogeochemical cycles at the
earth’s surface. Modeling also allows scientists to
estimate the effects of human activities on natural
biogeochemical cycles. A model is simply a set of
Figure 1. The ecosphere, our life support system, showing its relationship to the other important spheres of the surface system of the earth
(after Christensen, 1991).
Atmosphere
Ecosphere
Lithosphere
Hydrosphere
1 Terms in italics are defined in the glossary at the end of the text.
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Understanding Global Change: Earth Science and Human Impacts
equations that describe some of the processes
found in the real world. Biogeochemical cycling
models generally include processes that move
materials and their rates of transfer among a limited number of well-studied spheres of the earth.
Biogeochemical cycles, however, have certain
properties that are inherently difficult to describe
and model. These include: (1) irreversibility, that
is, the system does not return to its exact previous state if it goes through a disturbance; (2) transitional phenomena, that is, the system tends to
switch from one state to another and another and
yet others, and perhaps back again, rather than
simply moving from “before” to “after”; (3) evolution, in which the system progressively changes
in a particular direction; and (4) processes that
either enhance the original perturbation to the
system (positive feedback) or relieve the perturbation (negative feedback).
In Chapter 1 of this module, we shall first consider some examples of biogeochemical processes.
In Chapters 2 and 3, we shall discuss how the biogeochemical cycles interact with climate, both in
previous eras and at present. In Chapters 4–6, we
shall discuss the present-day global biogeochemical cycles of several elements that are important
biologically and that interact with the climate system. The cycles are looked at in the context of
global warming from an enhanced greenhouse
effect.
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Global Change
Instruction Program
Biogeochemical Processes
Innumerable biological, geological, and chemical processes cycle elements throughout the ecosphere. The few discussed in this section should
give the reader an idea of their variety and complexity. As an example, consider a group of organisms called the prokaryotes: the bacteria and bluegreen algae. The processes that these organisms are
involved with (summarized in Table 1) include:
• methane production and oxidation
• sulfur reduction and oxidation
• nitrogen fixation, nitrification, and
denitrification.
This list is given only as an example; some of
these processes will not be discussed in the text.
These prokaryotic processes may take place
in a variety of ways, such as (1) autotrophy, in
which the organisms convert inorganic carbon in
the environment to organic matter; (2) heterotrophy, in which the products from the breakdown
of organic compounds are used to make new
organic materials; and (3) mixotrophy, in which
both inorganic and organic compounds are used
to make organic matter.
• the capture of carbon dioxide from the
atmosphere and its conversion to organic
matter (fixation of CO2)
• the release of CO2 back to the atmosphere
(through respiration and decay)
• fermentation of sugar
Table 1. Biogeochemical reactions involving prokaryotes
Element
Process
Summary of partial
chemical reactions
Examples of organisms
involved in process
Carbon
CO2 fixation
CO2 + H2 ⇒ (CH2O)n +
A2 (A = O, S)
Photoautotrophs:
cyanobacteria, purple and green
sulfur bacteria
Chemoautotrophs:
sulfur and iron oxidizing bacteria
Methanogenesis
COO- + H2 ⇒ CH4
Methanogenic bacteria
Methanotrophy
CH4 + O2 ⇒ CO2
Methanotrophic bacteria
Fermentation
(CH2O)n + O2 ⇒ CO2
Anaerobic heterotrophic bacteria
Respiration
(CH2O)n + O2 ⇒ CO2
Aerobic heterotrophic bacteria
Sulfur reduction
SO4 + H2 ⇒ H2S
Sulfur-reducing bacteria
Sulfur oxidation
H2S ⇒ S0
Purple and green sulfur phototrophs
S0 + O2 ⇒ SO4
Sulfur oxidizing bacteria
N2 fixation
N2 + H2 ⇒ NH4
Phototrophic bacteria, nitrogen-fixing
heterotrophic bacteria
Nitrification
NH4 + O2 ⇒ NO2, NO3
Nitrifying bacteria
Sulfur
Nitrogen
Denitrifying bacteria
Denitrification
NO2, NO3 ⇒ N2O, N2
__________________________________________________________________
After Stolz et al., 1989
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Understanding Global Change: Earth Science and Human Impacts
Principles of chemical reactions
Atoms and elements
Every object in the universe is composed of matter. Because matter can be converted to energy, it is essentially a form of energy. Matter is composed of atoms, which are the smallest particles of an element that can
exist either alone or in combination. An atom is also the smallest particle that can enter into a chemical reaction. Most atoms never change; they only combine with other atoms to make different substances. Radioactive
atoms, however, do change and eventually decay into stable, nonradioactive atoms.
Elements consist of atoms of the same kind and, when pure, cannot be decomposed by a chemical change.
There are 106 known elements; 103 are listed in the periodic table (Figure 2). The elements most used commercially by people, in order of use, are carbon (C), in the form of coal, oil, and gas; sodium (Na), in table salt
and other products; iron (Fe), used in the steel industry; and nitrogen (N), sulfur (S), potassium (P), and calcium (Ca), all used in fertilizers or as soil conditioners for our food supply.
Compounds
When two or more atoms are bonded together in a definite proportion, a compound is formed. Examples of
compounds discussed in this text are water (H2O), carbon dioxide (CO2), salt (NaCl), and sugar (e.g., glucose,
C6H12O6 ). (All of the compounds named in the text are listed in Table 2.) The numbers in these chemical formulas are the number of atoms of each substance in the compound. If only one atom of a substance is in the
compound, no number is given. The universe is composed of millions of these compounds, all created from the
elements given in the periodic table. The smallest particle of a compound that can exist and exhibit the properties of that compound is called a molecule.
A compound is a pure substance that can be decomposed by a chemical change. The atoms in the chemical
compound may rearrange themselves, or they may separate from the compound to form different compounds.
These changes and interactions among compounds are called chemical reactions.
Chemical equations
A chemical equation expresses a chemical reaction involving compounds or elements. The chemicals that
react together, called reactants, generally are shown on the left-hand side of the equation and the products on
the right-hand side. Consider the decay of plant material (represented by the chemical compound CH2O, a carbohydrate), which requires the oxygen gas (the chemical compound O2) in the earth’s atmosphere. The simplest chemical equation representing this process is
CH2O + O2 ⇒ CO2 + H2O (1)
The arrow pointing right indicates that this process is irreversible; the plant material will be completely oxidized to CO2 and H2O in the presence of atmospheric oxygen. Other processes are highly reversible, and these
are usually represented by a double arrow. For example, the equilibrium between calcium carbonate and its
dissolved calcium and carbonate ions (atoms or molecules that have lost or gained electrons, with the number
lost or gained shown as a positive or negative superscript) is represented as
CaCO3 ⇔ Ca2+ + CO32-
(2)
In chemical processes, matter cannot be created or destroyed. Thus, when a chemical equation is written,
the total number of atoms of any particular element on the left-hand side of a chemical equation must be
made to equal the total number of atoms of that element on the right-hand side of the equation. This is the
process of balancing a chemical equation. Balancing the equation expresses the fact that molecules usually
react in such a way as to bear simple, integral, numerical relationships to one another.
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Global Biogeochemical Cycles and the Physical Climate System
If these relationships are known, it is possible to calculate the masses of reactants and products by using
known atomic and molecular weights. In chemical terms, the amount of a substance is expressed in moles. One
mole of a substance is the amount that contains as many elementary entities as there are atoms in 12 grams of
carbon. This number is termed Avogadro’s constant, and its value is equal to 6.022 x 1023. In the chemical
equation given above for the equilibrium of CaCO3 and its dissolved chemical species, one mole of CaCO3 will
dissolve in water to make one mole of Ca2+ and one mole of CO32-. In terms of mass, 100 grams of CaCO3 will
react to give 40 grams of Ca2+ and 60 grams of CO32-. If only 10 grams of CaCO3 were to dissolve, then the
same proportions of Ca2+ and CO32- would be present at the equilibrium: 4 and 6 grams, respectively.
Figure 2. Periodic table of the elements. Each box includes an element’s atomic number,
chemical symbol, and atomic weight.
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Understanding Global Change: Earth Science and Human Impacts
Table 2.
Chemical formulas and names used in this module
Al2Si2O5(OH)4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .kaolinite
Ca2+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .calcium ion
CaCO3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .calcium carbonate
CaSiO3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .calcium silicate
Ca5(PO4)3(OH,F) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .carbonate fluoroapatite
CH2O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .carbohydrate
(CH2O)106(NH3)16H3PO4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .organic matter in marine phytoplankton
CH4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .methane
CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .carbon dioxide
CO32- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .carbonate ion
CS2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .carbon disulfide
C6H12O6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .sugar (glucose)
DIC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .dissolved inorganic carbon
DMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .dimethyl sulfide, (CH3)2S
HCO3- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .bicarbonate ion
HNO3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .nitric acid
H2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .molecular hydrogen
H2O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .water
H2S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .hydrogen sulfide
H2SO4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .sulfuric acid
H3PO4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .phosphoric acid
H4SiO40 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .monomeric silicic acid
KAlSi3O8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .orthoclase feldspar
MSA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .methane-sulfonic acid
NaAlSi3O8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .albite
NaCl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .sodium chloride, common table salt
NH3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ammonia
NH4+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ammonium ion
NH4NO3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ammonium nitrate
(NH4)2SO4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ammonium sulfate
NMHC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .nonmethane hydrocarbon
NO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .nitric oxide
NO3- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .nitrate ion
NOx . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .oxides of nitrogen
N2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .diatomic nitrogen
N2O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .nitrous oxide
OCS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .carbonyl sulfide
OH* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .hydroxyl radical
OH- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .hydroxyl ion
O2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .diatomic oxygen (pure oxygen molecules)
O3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ozone
PAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .peroxylacetyl nitrate
PH3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .phosphine or swamp gas
PO43+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .phosphate ion
SO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .sulfur dioxide
SO42- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .sulfate ion
SOx . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .oxides of sulfur
SiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .silica
6
Global Biogeochemical Cycles and the Physical Climate System
Incidentally, the carbon dioxide and donor
molecule used for photosynthesis are not the
only requirements for plant growth. Plants also
need nitrogen, phosphorus, sulfur, potassium,
and a dozen or so trace elements, like zinc and
iron. As we shall see below, human activities are
changing the atmospheric concentrations of these
nutrients as well as that of carbon, with various
possible effects on plants.
The photosynthetic reactions that produce
organic matter on land differ from those in the
ocean because the proportions of carbon, nitrogen, sulfur, and phosphorus in land vegetation
differ from those in marine plankton. The ratio of
C:N:S:P in marine plankton is 106:16:1.7:1.
Known as the Redfield ratio, this proportion is
fairly constant for the surface-dwelling, microscopic plants (phytoplankton) of the world’s
oceans. The C:N:S:P ratio for land plants is more
variable but averages 882:9:0.6:1. The amount of
carbon is so much greater in land vegetation
because it is stored as cellulose in the structural
tissues of trees and grasses.
From this summary, it can be seen that
photosynthesis (among other things) links the
biogeochemical processes and cycles of the individual organic elements of carbon, nitrogen,
phosphorus, and sulfur. These elements, plus
hydrogen and oxygen, are the major constituents
of organic matter. Those six elements and about a
dozen or so minor elements are necessary for the
maintenance of organic structures and the physiological functions of living organisms.
Photosynthesis
We begin with perhaps the most important
biogeochemical process of all, photosynthesis. It is a
photoautotrophic process, that is, an autotrophic
reaction in the presence of light. Nutrients such as
phosphate (PO43-) and nitrate (NO3-) are also necessary for this reaction to occur.
In the early stages of our planet’s formation,
the atmosphere was very different from that of
today. There was no free molecular oxygen (O2),
which most of today’s life forms require. In fact,
oxygen was a very powerful poison for the simple organisms that lived in this early, oxygendeficient (anaerobic) world. Both the organisms
and the earth had to evolve to a stage where the
organisms produced oxygen and emitted it to
their environment before more advanced life
forms could evolve. Photosynthesis, the process
of constructing complex organic molecules from
simple inorganic ones in the presence of light,
was a critical step in the evolution of life and
allowed the mass of living organisms to grow to
the level of today. In our world, the mass of living
organisms on earth is equivalent to about 600 billion tons of carbon. More than 99% of this carbon
is in land plants; the remainder is stored in
marine plants and in animals.
Photosynthesis is basically a chemical reaction
or process in which carbon-, hydrogen-, and oxygen-bearing chemical compounds (carbohydrates)
are synthesized from atmospheric CO2 and H2O or
another chemical compound that can act as a
hydrogen donor. The generalized reaction is
Respiration and Decay of
Organic Matter
energy + nCO2 + 2nH2A ⇒ (CH2O)n +
(3)
nH2O + 2nA
In the life cycle, photosynthesis in plants is
balanced by the complementary processes of respiration and decay in plants and animals. In
plants, respiration is the breakdown of the complex organic molecules that were formed during
photosynthesis. The chemical reactions for respiration and decay are the reverse of those shown
above for the production of organic material. The
generalized reaction is
C6H12O6 + 6H2O + 6O2 ⇒ 6CO2 +
12H2O + energy
(5)
where H2A is a hydrogen donor molecule,
(CH2O) is a carbohydrate, and n stands for any
number. In higher plants, the donor molecule is
water, and n = 6. Thus for these plants the specific
reaction is
energy + 6CO2 + 12H2O ⇒ C6H12O6 +
6H2O +6O2
(4)
For photosynthetic sulfur bacteria the donor molecule is hydrogen sulfide (H2S), and for nonsulfur
purple bacteria it is organic compounds.
DRAFT
7
Understanding Global Change: Earth Science and Human Impacts
Compare this with reaction 4. The amount of
energy released is about 686 kilocalories (kcal) for
each mole of C6H12O6 (glucose, the most common
form of sugar in living things) that is broken down.
In animals, the respiratory oxidation of foods
—that is, the loss of electrons from the carbon in
carbohydrates, occurring during digestion—
provides energy for a variety of uses, including
maintenance of body temperature, muscular
movement, and synthesis of complex organic
compounds.
During the oxidation of organic matter, CO2,
nitrogen- and phosphorus-bearing nutrients, and
bioessential trace elements (e.g., iron) are returned
to the environment to be used again in the production of more organic matter. When O2 is available, it is the oxidizing agent (oxidant); however,
in oxygen-depleted (anoxic) waters, sediments,
and soils, other oxidants are used. These include
nitrate, sulfate, and iron and manganese oxides.
The chemical equations for respiration and
decay, either in an oxygenated or in an anoxic
environment, are more complex than the generalized reaction for photosynthesis given above. For
example, the chemical composition of average
marine phytoplankton—a relatively simple form
of life—is (CH2O)106(NH3)16H3PO4: 106 molecules of carbohydrate, 16 of ammonia, and 1 of
phosphoric acid. When dead phytoplankton react
with O2 in an oxygenated environment, the products are carbon dioxide, nitric acid, phosphoric
acid, and water:
Weathering of Rocks
Another very important set of biogeochemical processes is that involved with the breakdown of rocks exposed to rain, wind, and ice.
Weathering prepares rock for erosion and transportation. Its products are dissolved chemical
species and solids derived from changes in the
primary minerals of the rock being weathered.
The solid products are predominantly clay minerals; there are also dissolved products, predominantly calcium, carbon, and silicon. Ultimately,
the products of weathering are either carried by
water, blown as dust, or carried by glaciers to the
ocean. Of the approximately 20 billion tons of
solids and dissolved materials reaching the ocean
annually from the land, more than 80% is delivered by rivers. However, high-temperature chemical reactions in the presence of seawater along
the great submarine midocean ridges are significant sources of dissolved calcium, silica, and iron
for the oceans.
An example of a chemical weathering reaction is the weathering of the mineral albite (the
inorganic chemical compound NaAlSi3O8), found
in igneous rocks like basalt, to the clay mineral
kaolinite [Al2Si2O5(OH)4]. The reaction takes
place principally in the presence of soil water
and groundwater that contain significant amounts
of dissolved CO2. Although the ultimate source
of the CO2 is the atmosphere, much of it does not
come directly from the air but is produced in
soils by the respiration of plants and the decay of
dead plants and animals. Because of these
processes, the concentration of CO2 in soils may
be one or more orders of magnitude greater than
that of the atmosphere. The elevated CO2 levels
give rise to acidic soil solutions, and these corrosive, low-pH soil solutions are responsible for the
weathering of rock minerals like albite:
(CH2O)106(NH3)16H3PO4 + 138O2 ⇒
106CO2 + 16HNO3 + H3PO4 + 122H2O
(6)
For an example of respiration and decay in an
anoxic environment, let us consider the reduction
of sulfur in sulfate (SO42-) in the pore waters of
anoxic sediments. Bacteria use the oxygen originally bound in the sulfate to oxidize organic matter. Again using phytoplankton as the organic
matter, the equation for this chemical reaction is
2NaAlSi3O8 + 2CO2 + 11H2O ⇒
Al2Si2O5(OH)4 + 2Na+ + 2HCO3- + 4H4SiO40 (8)
(CH2O)106(NH3)16H3PO4 + 53SO42- ⇒
106CO2 + 16NH3 + H3PO4 + 53S2- + 106H2O (7)
The products of this reaction, besides the
kaolinite, are sodium ion, bicarbonate ion, and
monomeric silicic acid.
In regions where human activities such as
coal burning release considerable amounts of sulfur and nitrogen oxide gases to the atmosphere,
This time, in addition to carbon dioxide, phosphoric acid, and water as in reaction 6, the products include ammonia (NH3) and sulfide (S2-).
8
Global Biogeochemical Cycles and the Physical Climate System
inorganic carbon brought to the oceans annually
by rivers. The other half of the riverborne carbon
is released to the ocean and atmosphere when
skeletal carbonate minerals are formed.
Dissolved silica is also removed from the
oceans in the skeletons of marine organisms.
Certain of these organisms—planktonic diatoms
(algae), radiolarians (protozoans), dinoflaggelates
(protozoans), and benthic sponges—use dissolved silica to form their shells of opaline silica.
After these organisms die, most of the opal dissolves, because the oceans throughout their
extent are undersaturated with respect to this
chemical compound. Only about 40% of the total
annual production of skeletal silica sinks below
the parts of the ocean that daylight reaches (the
euphotic zone). Most of this siliceous material dissolves en route to the seafloor; only 5% of that
produced in the euphotic zone accumulates in
marine sediments. This amount is about equivalent to the annual input of dissolved silica to the
oceans by rivers.
such as the midwestern and eastern United States
and southern China, the pH of rainwater and
consequently soil water may be lower (more
acid) than natural values. This happens because
the gases oxidize and react with water in the
atmosphere and then rain out as sulfuric and
nitric acids, respectively. This phenomenon is the
environmental problem of acid deposition (often
called acid rain), which in extreme forms can be
responsible for increased fish mortalities in lakes
and decreased agricultural production.
Deposition in the Oceans
When the solid and dissolved products of
weathering reach the ocean, the solids settle out
because of their weight and are deposited on the
seafloor as gravel, sand, silt, and mud. How long
the dissolved products remain in the ocean
depends on how long it takes them to enter into a
chemical or biochemical reaction. As an example
of the periods involved, dissolved sodium in the
ocean has a long residence time, about 55 million
years. At the other end of the time scale, the residence time of dissolved silica is only 20,000 years.
Sodium and magnesium
In contrast to carbon and silica, which are
removed from the ocean primarily by biological
processes, riverborne dissolved sodium and magnesium are removed to a significant extent by
inorganic chemical reactions. Both of these elements are involved in hydrothermal reactions
between seawater circulating through midocean
ridges and the basalt rock making up the ridges.
In the hydrothermal reaction process, sodium
and magnesium are removed from the seawater.
Sodium is also removed from the ocean by the
precipitation of halite (common table salt, sodium
chloride) from seawater. This process is very
important as a removal mechanism for sodium
and chlorine, but only occurs when the right set
of climatic and tectonic conditions are achieved.
Only seawater in relatively isolated arms of the
sea can be sufficiently evaporated to reach halite
saturation. Thus, because such environments are
scarce today, it is likely that sodium and chlorine
brought to the oceans by rivers are currently
accumulating in seawater.
Some magnesium is also removed from seawater by chemical processes in the pore waters of
Calcium and silica
Many of the processes by which dissolved
constituents are removed from the ocean involve
marine organisms. In today’s oceans, dissolved
calcium and bicarbonate are precipitated as carbonate minerals in the skeletons of several kinds
of marine organisms: planktonic foraminifera (protozoans), pteropods (mollusks), and Coccolithophoridae (algae), and bottom-dwelling (benthic)
corals, echinoids, mollusks, and coralline algae. Of
the total production of skeletal carbonate in the
oceans, equivalent to about 1 billion tons of carbon
per year, 80% is redissolved in the ocean as skeletal debris sinks to the seafloor. This efficient recycling is due to the fact that although the surface
ocean is oversaturated with respect to calcium carbonate, the deeper sea is undersaturated with
respect to this mineral. The remaining 20% of the
ocean’s carbonate production accumulates in shallow-water and deep-sea sediments.
The amount of carbon in these sediments
only accounts for about one-half of the dissolved
9
Understanding Global Change: Earth Science and Human Impacts
sediments. These processes taking place during
the burial of sediments are collectively referred to
as diagenesis. The relative importance of diagenetic and hydrothermal reactions for the removal of
magnesium from seawater is a topic of current
scientific research and debate.
We can conclude from the above discussion
that the circulation of material through the ecosphere is complex and involves myriad chemical,
biological, and geological processes. The system
is truly biogeochemical in nature. On all time and
space scales, if the composition of the ecosphere
is regulated, the regulation is controlled by a
complex of interwoven inorganic and organic
processes. The maintenance of the equable environment, including climate, that is required for
life to exist on earth is a product of this interacting and interwoven web of biogeochemical
processes and cycles.
10
Global Change
Instruction Program
Biogeochemical Cycles and Climate
In this chapter, we will look at some representative global biogeochemical cycles and their
role in climate. The elements whose cycles are
discussed are intimately connected through the
organic processes of photosynthesis and respiration and/or decay. These elements are carbon
(C), nitrogen (N), sulfur (S), oxygen (O), and—for
completeness, because it is an important biological nutrient—phosphorus (P). We will begin with
a discussion of gases whose production or consumption on the earth’s surface is accomplished
by biological reactions (biogenic gases) and the
possible effects of these gases on the earth’s climate. To set the stage, the greenhouse effect is
discussed briefly here.
Greenhouse gases, which are all naturally
biogenic in origin, allow incoming shortwave solar
radiation to pass through the atmosphere to the
earth’s surface, but when part of that radiation
(about 45%) is reradiated back toward space as
heat (infrared radiation), the gases absorb it and
thus retain it in the atmosphere. This is the greenhouse effect. We can thank the natural greenhouse effect for the earth’s equable climate.
Without it, the planet would be about 33°C cooler
than its mean global temperature of 15°C, that is,
–18°C. (See the Global Change Instruction
Module The Sun-Earth System, by John Streete.)
There can be, however, too much of a good
thing. In recent years, the concentrations of these
gases in the atmosphere have been rising because
of fossil fuel combustion, biomass burning, rice
paddy cultivation, and other human activities.
This buildup may absorb increased amounts of
outgoing infrared radiation, leading to an
enhanced greenhouse effect and global warming.
It is interesting and informative to put this
present-day worry in the context of public and
scientific concern about climate during the 1950s
and 1960s. Between about 1940 and 1970, global
mean temperatures remained nearly constant, or
even declined slightly. There was considerable
discussion and concern in the scientific literature
and in public forums about global cooling and
perhaps another ice age. Much of the discussion
below in the context of global warming is applicable to a scenario of global cooling as well. The
difference is that in a global cooling, many of the
feedbacks mentioned would act in the opposite
direction and would probably have different
magnitudes of change.
The Biogenic Gases and Climate
It is very likely that during the next century
the earth’s climate will change due to natural
causes. Changes in the amount of solar radiation
received by the planet, in the circulation of the
atmosphere and the oceans, and in volcanism can
affect climate on this time scale. On the longer
time scale, if left to its own recourse, the planet
will most likely enter another ice age about
10,000–30,000 years from now.
On the other hand, human-induced climate
change during the next century is also very likely. The flywheel of population growth and fossil
fuel burning is turning rapidly and will be difficult to slow in this time. The global population is
growing at a rate of 1.5% per year, a doubling
time of 45 years. This rate of growth implies a
population of about 10 billion by 2050. All these
people will require energy to sustain themselves
and to develop their industries, farms, and cities.
Most scenarios of future global energy use project a continuous heavy reliance on fossil fuel
into the 21st century. Continued fossil fuel burning will result in continued emissions of CO2,
methane (CH4), and nitrous oxide (N2O) to the
atmosphere. These greenhouse gases will be
accompanied by emissions of trace metals, nonmethane hydrocarbons (NMHCs), oxides of sulfur (SOx), and the most reactive oxides of nitrogen (NO and NO2, collectively known as NOx).
11
Understanding Global Change: Earth Science and Human Impacts
The latter three groups of chemical compounds
react with other chemical components of the climate system, particularly the hydroxyl radical
(OH*). Also, SOx and NOx are the principal constituents in acid deposition, and NOx and
NMHCs are involved in the formation of ozone
(O3), another greenhouse gas, in the troposphere.
The unchecked accumulation of these gases in
the atmosphere could lead to an uncomfortably
warm planet.
The burning of fossil fuels and the burning of
forests and other biomass are the principal
human-induced, or anthropogenic, emissions of
most biogenic gases to the earth’s atmosphere.
Also, fossil fuel burning and changes in land use
(such as deforestation) are responsible for many
of the global environmental problems the people
of the world face today. Fossil fuel burning alone
accounts for perhaps 80% of sulfur dioxide (SO2)
emissions from the land surface to the atmosphere, 50% of carbon monoxide, 50% of NOx,
20% of methane, 20% of NMHCs, 5% of ammonia, and 4% of nitrous oxide. It is also responsible
for 70–90% of anthropogenic CO2 emissions to
the atmosphere. This amount is equivalent to
about 10% of the natural CO2 emissions from respiration and decay.
As mentioned previously, C, N, P, and S,
besides O and hydrogen (H), are the principal
elements that make up living matter. The biogeochemical cycles of these elements are intimately
coupled through biological productivity and respiration and/or decay. Ecosystems take energy from
their surrounding environment. The net result is
production of organic matter, more disorder on
the planet (increased entropy), and waste. The
waste may act as a pollutant. The biogenic gases
of carbon, nitrogen, and sulfur are a consequence
of this entropy production. Their fluxes maintain
the earth’s atmosphere in a state of disequilibrium.
The natural sources of these biogenic gases
are processes at the earth’s surface or chemical
reactions in the atmosphere. The processes by
which biogenic gases and other components
cycle through the coupled C-N-P-S-O system,
although in some environments operating close
to equilibrium, are principally controlled by the
rates at which the processes operate.
During the last half century, scientists have
tended to specialize. Consequently, most global
environmental systems have been little studied
or studied only in a piecemeal fashion. Only
recently has attention been paid to the coupled
earth-surface system of atmosphere, hydrosphere, biosphere, cryosphere, and shallow
lithosphere. Basic information on global reservoir
sizes and fluxes (e.g., biological productivity) is
lacking or is only partly known. An example of
this lack of data is the estimates of tropical forest
biomass, which vary by a factor of 2 or 3 for
Amazonia alone.
It is very unlikely that the anthropogenic
fluxes of gases to the atmosphere will substantially decline as we enter the 21st century.
Population growth and our global reliance on
fossil fuels as an energy source make such a scenario highly improbable. Thus, continued global
environmental change is a virtual inevitability. It
is likely that, by the middle of the next century,
the atmospheric concentration of CO2 will be
double what it was before the Industrial
Revolution (to date, it has increased about 30%),
and concentrations of other greenhouse gases
will also increase. Such a change in the composition of the atmosphere portends a strong probability of climate change.
Historical Framework
It is worthwhile considering at this stage
how the earth’s biogeochemical cycles and climate system functioned prior to human interference. It is impossible to consider the functioning
of all the biogeochemical cycles of concern
because of space limitations. Only the global biogeochemical cycles of carbon and oxygen are
used as examples in this section. We will end
with a brief summary of environmental conditions just prior to major human interference in
the biogeochemical cycles and climate system.
Carbon
Carbon composes approximately 50% of all
living tissues. In the form of carbon dioxide, it is
necessary for plants to grow. Carbon dioxide also
helps to sustain an equable climate on earth. The
concentration of carbon dioxide in the
12
Global Biogeochemical Cycles and the Physical Climate System
(a) Photosynthesis-respiration
CO2
CO2
Decay
Uptake by
rocks in
weathering
Organisms use
carbon from the
ocean/atmosphere
to construct
organic matter and
shells of calcium
and carbonate,
CaCO3
(c) Stored oil, gas, coal,
and kerogen
(b) Subduction
Figure 3. The biogeochemical cycle of carbon prior to human interference, showing (a) the short-term cycle, i.e., photosynthesis and
respiration; (b) the long-term cycle, involving accumulation of organic C and CaCO3 in marine sediments, their subduction, their alteration,
and the return of CO2 to the atmosphere via volcanism; and (c) the medium-term cycle, involving storage of C in organic materials in sedimentary rocks. Ultimately this carbon is returned to the earth’s surface and undergoes weathering; in the process, O2 is taken out of the
atmosphere and CO2 is returned.
atmosphere has varied during the geologic past,
but has remained within limits that permit life to
exist on earth. Carbon dioxide is cycled throughout the spheres of earth on different time scales.
We can refer to these scales as short-, medium-,
and long-term. Figures 3 and 4 illustrate the
processes involved in these time scales.
surface waters of the oceans. The primary producers, the photosynthetic phytoplankton and
benthic plants in the oceans and plants on the
terrestrial surface, transform inorganic carbon as
carbon dioxide into organic carbon within their
tissues. Light and nutrients, like phosphate and
nitrate, are necessary for this reaction to occur.
Some of the energy from the light is used in the
growth of plants, and some remains stored in the
tissues of plants as carbohydrates.
Plants remove about 100 billion tons of carbon as carbon dioxide from the global atmosphere each year, which is about 14% of the
atmosphere’s total carbon. Most, but not all, of
the carbon dioxide taken from the atmosphere
during photosynthesis is returned to the atmosphere during respiration and decay. The annual
The short-term carbon cycle
Photosynthesis is part of the short-term carbon cycle (on the order of years). We can look at
the short-term cycling of carbon as carbon dioxide by beginning with the producers of organic
carbon, the plants. Carbon in the form of atmospheric carbon dioxide is removed from the air
by plants. This removal occurs both on land—for
example, in forests and grasslands—and in
water—for example, in lakes, rivers, and the
13
Understanding Global Change: Earth Science and Human Impacts
ATMOSPHERE
Carbon in CO2 gas
Living plants
extract CO2
CO2 from
deforestation
CO2
from
cement
manufacturing
CO2 to
weathering
of limestone
and silicate
CO2
from
burning
of coal,
oil, and
gas
Buried
organic
matter
CO2 from
decaying
organic matter
and
respiration
BIOSPHERE
Organic matter in
plants and animals
Oceans
absorb CO2
Aquatic plants
put CO2 into water
CO2 from
alteration of
organic
matter and
CaCO3
HYDROSPHERE
CO2 dissolved in ocean
Precipitation
of CaCO3
Coal, oil, gas
LITHOSPHERE
Carbon in buried plants, animals, and sediments
Kerogen
Figure 4. The major reservoirs and fluxes in the biogeochemical cycle of carbon. The shapes surrounding the spheres are called boxes.
Arrows represent the processes and their directions that transfer carbon from one box to another. The carbon cycle can be conceived of as
a series of interlocking circuits in the reservoirs of atmosphere, biosphere, hydrosphere, and shallow lithosphere (crust). In our time, the
cycle would be in balance if it were not for human interference by burning of fossil fuels, cement manufacturing, and land-use activities
(e.g., deforestation) (after Skinner and Porter, 1987).
removal rate of atmospheric carbon dioxide in
photosynthesis is slightly larger on land than in
the ocean.
After photosynthesis, carbon may next be
transferred to a consumer organism if the plant is
eaten for food. The carbon stored in the tissue of
the plant enters an animal’s body and is used as
energy or stored for growth. Land animals, such
as cows and deer, are the primary consumer
organisms. Aquatic plants are eaten by zooplankton (small sea animals) and larger animals. When
an animal breathes, some of this carbon that was
14
Global Biogeochemical Cycles and the Physical Climate System
taken up from plants is released from the animal’s body as carbon dioxide gas.
Besides the carbon stored above ground in
living and dead vegetation, there is carbon below
ground in the root systems of terrestrial plants.
When the plants die, some of this carbon may be
released as carbon dioxide or methane gas to the
air trapped in the soil, or it may accumulate in
the soil itself as dead organic material. This dead
organic matter may be ingested by consumer
organisms, such as insects and worms living in
the soil.
Some of the organic carbon generated in land
environments is weathered and eroded, and the
organic debris is transported by streams to the
ocean. In the ocean, some of this debris, along
with the organic detritus of dead marine plants
and animals, settles to the ocean floor and accumulates in the sediments. However, some of the
debris is respired in the ocean to carbon dioxide.
This carbon dioxide may leave the ocean and be
transported over the continents, where it is used
again in the production of land plants.
The long-term carbon cycle
The long-term carbon cycle (on the
order of tens to a hundred million
years) requires that we consider the
earth’s history over the last 600 million
years or so—the period covered by the
fossil record. Figure 5 defines the
terms and intervals of geologic time.
The long-term cycling of carbon
(Figures 3 and 4) involves interconnections between the cycling of the
minerals calcium carbonate (CaCO3)
and calcium silicate (CaSiO3). This
series of processes dates back to the
beginning of plate tectonics. This
cycling includes not only the land and
ocean reservoirs but also that of limestone rocks. Limestone rocks are mainly composed of calcium carbonate and
are the fossilized skeletal remains of
marine organisms or, less commonly,
inorganic chemical precipitates of calcium carbonate. Limestones are great
storage containers for carbon. Most of
the carbon near the earth’s surface is
found in these rocks or in fossil organic matter in
sedimentary rocks. Weathering and erosion of the
earth’s surface result in the leaching of dissolved
calcium, carbon, and silica (SiO2) from limestones
and rocks containing calcium silicate.
The dissolved substances produced by
weathering are transported to the ocean by
rivers. As discussed in on p. 9, they are then used
to form the inorganic skeletons of benthic organisms and plankton, which are composed of calcium carbonate and silica. During formation of the
calcium carbonate skeletons, the carbon dioxide
derived from the weathering of limestone is
returned to the atmosphere.
When marine animals and plants die, their
remains settle toward the seafloor, taking the carbon stored in their bodies with them. En route,
their organic matter is decomposed by bacteria,
just as on land. Some shells may dissolve. Thus,
animal and plant organic and skeletal matter is
turned back into dissolved carbon dioxide, nutrients, calcium, and silica in the ocean. This carbon
dioxide is stored in the deeper waters of the
Figure 5. The geologic time scale—the calendar of the earth. Geologic time is
divided into the intervals of eon, era, period, and epoch. The boundaries of these
intervals are based on absolute age dating using the radioactive decay of certain
elements (e.g., uranium, potassium, rubidium, and carbon) in rocks; the distribution of fossilized plants and animals found in the rocks; and certain worldwide
geologic events recorded in the rocks (after Skinner and Porter, 1987).
Eon
Phanerozoic
Era
Period
Epoch
Cenozoic
Quaternary
Holocene
Pleistocene
Tertiary
Pliocene
Miocene
Oligocene
Eocene
Paleocene
Mesozoic
Cretaceous
Jurassic
Triassic
Paleozoic
Permian
Carboniferous
Devonian
Silurian
Ordovician
Cambrian
Precambrian:
• Proterozoic
• Archean
Millions of years ago
Today
0.01 (10,000 years ago)
1.6
5.3
23.7
36.6
57.8
65.0
144
208
245
286
360
408
438
505
545
2500
~3800
Hadean
4600
15
Understanding Global Change: Earth Science and Human Impacts
oceans for hundreds to a thousand or so years
before being returned to the atmosphere when
the deep water moves upward (upwelling), usually because of divergent movements of surface
water.
Some of the animal and plant plankton sinks
to the bottom, where the carbon in the organic
matter and shells escapes degradation and
becomes part of the sediment. As the seafloor
spreads through plate tectonics, the sediments
containing the remains of marine plants and animals are carried along to subduction zones, where
they are transported down into the earth’s mantle.
At the severe pressures and high temperatures in
the subduction zones, organic matter is decomposed and calcium carbonate reacts with the silica found in the subducted rocks to form rocks
containing calcium silicate.
During this metamorphism, carbon dioxide is
released and makes its way into the atmosphere
in volcanic eruptions and via hot-spring discharges. Once in the atmosphere, it can then
combine with rainwater. The rainwater falls on
the land surface and seeps down into the soils,
where it picks up more carbon dioxide from
decaying vegetation. This water, enriched in carbon dioxide, weathers and dissolves the compounds of calcium and silica found in rocks of
the continents. The cycle begins again.
This series of processes has been active for at
least 600 million years, since the advent of the
first organisms that made shells (and were therefore the first to leave fossils). The processes were
important even earlier in earth’s history, when
calcium carbonate was deposited in the ocean by
inorganic processes.
Figure 6. Model calculation of atmospheric carbon dioxide during the last 600 million years. The horizontal axis shows time in millions of years
before the present (top) and geological time period (bottom). The left vertical axis shows the number of times today’s C level that existed in the
atmosphere of the time; the right vertical axis shows the amount of CO2 in the atmosphere. For example, 500 million years ago there was
about 14 times as much CO2 in the atmosphere as there is today, with a total amount of about 37 x 1018 grams (after Berner, 1991).
Time
Millions of years before present (BP)
500
400
300
200
100
0
50
18
C
O
S
D
C
P
Tr
J
K
T
16
14
12
10
-
Cambrian
Ordovician
Silurian
Devonian
Carboniferous
Permian
Triassic
Jurassic
Cretaceous
Tertiary
8
40
30
20
6
4
2
0
10
Present day
C
O
S
D
C
Paleozoic
P
Tr
J
K
Mesozoic
16
T
0
Cenozoic
Atmospheric CO2 (1018 grams)
CO2 in the atmosphere relative to present day
600
Global Biogeochemical Cycles and the Physical Climate System
weathering rates and withdrawal of CO2 from
the atmosphere.
The important point is that atmospheric CO2
has varied by a factor of perhaps more than ten
during the last 600 million years of the earth’s
history. This variation certainly has had climatic
implications, because CO2 is an important greenhouse gas. In fact, for much of the last 600 million years, the planet had a different atmospheric
composition and a more equable climate than
that of today.
Atmospheric CO2 levels of the past
One outcome of changes in the rates of
processes in the long-term biogeochemical
cycling of carbon is that atmospheric carbon
dioxide has varied in a quasicyclic fashion during the last 600 million years of earth’s history.
Robert A. Berner of Yale University and colleagues have developed models of the carbon
cycle to calculate these variations. Figure 6 shows
the results of one such calculation.
Periods of high atmospheric carbon dioxide
levels are the result mainly of intense plate tectonic activity, with increased metamorphism of
limestone and release of carbon dioxide to the
atmosphere from volcanoes. These high carbon
dioxide periods are often referred to as hothouses
or greenhouses. They are also periods of relatively high sea level; for example, in the Cretaceous
and Cambrian periods, much of what would
become the present continent of North America
was covered by water. This flooding of the continental landscapes was due principally to the
large size and volume of the midocean ridges,
caused by intense plate tectonic activity. The
increase in ridge volume led to the displacement
of ocean water onto the continents of the time.
Periods with lower atmospheric carbon dioxide levels, such as the Carboniferous through
Triassic and much of the Cenozoic, are the outcome of less intense plate tectonic activity and
increased removal of carbon dioxide from the
atmosphere by weathering. These intervals,
which include the present era, are extended cool
periods (ice houses) in the climatic history of the
earth. They are also times of relatively low sea
level due to a decrease in the volume of the
midocean ridges.
Biological and other factors also play a role
in regulating atmospheric carbon dioxide levels
over the long term. For example, the lowering of
atmospheric CO2 from the high levels of the midPaleozoic era, about 400 million years ago, was
not simply the result of the waning intensity of
plate tectonic processes. It followed the evolution
of land plants, which withdrew CO2 from the
atmosphere by photosynthesis. Similarly, the
lowering after the Cretaceous period, about 100
million years ago, followed the appearance of
flowering plants, which resulted in an increase in
The medium-term carbon cycle
Medium-term cycling of carbon dioxide (millions to tens of millions of years) involves organic
matter in sediments; coal, oil, and gas; and
atmospheric oxygen (Figures 3 and 4). It commences, as does the short-term cycling, with the
removal of carbon dioxide from the atmosphere
by its incorporation into plants and the accumulation of the dead plant and animal carbon in
sedimentary organic matter (the dead and fossilized remains of plants and animals). When
dispersed throughout a sedimentary rock, this
organic matter is termed kerogen. Shales are very
fine-grained sedimentary rocks that are often rich
in kerogen. Coal, oil, and gas deposits are also
the altered remains of the soft tissues of plants
and animals which have accumulated in a geographically restricted area.
Coal is derived mainly from terrestrial plant
material, which is often deposited in swampy
environments. The plant material is altered when
the swamp sediments are buried. If buried deep
enough, the dead plant material may be substantially changed because of the increased temperatures and pressures at depth. Different types of
coal are formed by varying conditions of temperature and pressure. Anthracite—a hard, dense,
black coal—is formed by alteration of plant material at a relatively high temperature and pressure.
Bituminous, brown coal is formed under less
intense conditions. Peat, used as a fuel in some
parts of the world, is little-altered plant material
that has not been buried deeply. It is high in carbon. Peat is an important component of tundra
areas in the Northern Hemisphere.
Oil and gas represent highly altered organic
matter, principally the altered remains of marine
17
Understanding Global Change: Earth Science and Human Impacts
phytoplankton that were deposited on the
seafloor. During burial, these organic materials
are broken down at elevated temperature and
pressure, forming oil and gas. The oil and gas
may migrate hundreds of miles in the subsurface
before coming to rest in large accumulations in
the voids of rocks. Often, oil and gas are formed
in shales; but as temperature and pressure
increase with depth, they move to more coarsegrained rocks like sandstones and limestones. It
is in these latter rocks, dating from the Cretaceous and Cenozoic periods, that the great oil
and gas reserves of the world are found, like
those of the Persian Gulf.
These deposits of coal, oil, and gas come
from organic carbon that has escaped respiration
and decay. Thus, they represent carbon dioxide
that has been removed from the atmosphere. The
same is true of the kerogen dispersed as finegrained materials in the sedimentary rocks.
Because these materials were buried, the oxygen
that would have been used in their decay has
remained in the atmosphere. Eventually, however, the carbon in these deposits and in kerogen is
recycled into the atmosphere, returning carbon
dioxide to that reservoir and removing oxygen.
This happens when these fossil fuel deposits and
kerogen are uplifted by plate tectonic forces after
millions of years of burial and exposed to the
atmosphere. When this occurs, the oxygen that
previously accumulated in the atmosphere reacts
with the coal, oil, gas, and kerogen. The reaction
involves the decay of these organic materials in
the presence of oxygen (equation 5). This results
in the removal of oxygen from the atmosphere
and the return of carbon dioxide to the atmosphere. The ongoing dynamic cycle is complete.
Fossil fuel is a nonrenewable energy source,
because coal, oil, and gas deposits take millions
of years and specific environmental conditions to
form. The mining of these deposits brings these
materials back to the surface much more rapidly
than natural processes do. The stored energy
from the long-dead organisms is released in the
form of heat when the coal, oil, and gas are
burned. This fossil fuel energy keeps us warm,
powers our cars, and moves the machinery of
industry. It is also a main cause of environmental
pollution, because a byproduct of fossil fuel
burning is the release of gases and particulate
materials into the environment. Climatic change
is an important potential global environmental
effect of the release of carbon dioxide and other
gases to the atmosphere by combustion.
In summary, carbon is found in all four major
surface spheres of the earth. In the ecosphere, it is
essential to every life form, occurring in all
organic matter. In the atmosphere, it is found as
the gases carbon dioxide, methane, and several
other compounds. It occurs as carbon dioxide
dissolved in lakes, rivers, and oceans in the
hydrosphere. In the earth’s crust—part of the
lithosphere—it is found as calcium carbonate
originally deposited on the seafloor, as kerogen
dispersed in rocks, and as deposits of coal, oil,
and gas. It is because carbon is stored in the large
sedimentary reservoirs of limestone and fossil
organic carbon, and not in the atmosphere, that
life on earth is possible. If all this carbon were
stored in the atmosphere, there would be about
30 times the existing amount of CO2. The result
would be a substantial heating of the atmosphere
because of the increased absorption of the longwave infrared radiation emitted from the surface
of the earth. The greenhouse effect would be very
strong, and our planet would probably have a
temperature like that of Venus: 460°C!
Oxygen
Oxygen composes 20.9% of the gases of the
atmosphere. The cycling of oxygen (Figure 7) is
strongly coupled to that of carbon (Figure 4).
Oxygen is produced by plants during photosynthesis, when carbon dioxide is consumed. It is
removed by respiration and decay, when carbon
dioxide is produced. This is a short-term, nearly
balanced cycle on land, because the amount of
oxygen produced yearly by land plants is about
equivalent to the amount they use in the processes
of respiration and decay. On average, it takes
about a decade for oxygen and carbon dioxide to
cycle through living plants. However, a little terrestrial organic matter that has not undergone
respiration or decay, such as leaves and trunks of
trees and smaller-sized organic debris, leaks from
the land to the ocean via rivers. In the ocean,
some of this organic detritus escapes destruction
and is deposited in the sediments.
18
Global Biogeochemical Cycles and the Physical Climate System
In the oceans, phytoplankton produce slightly more oxygen than is consumed during the respiration and decay of marine life. As a result,
oxygen is released to the atmosphere. The organic carbon not decayed by this oxygen, along with
some of the terrestrial organic detritus mentioned
in the preceding paragraph, is deposited on the
seafloor and accumulates in the sediments of the
ocean. If this accumulation were not counteracted
by other processes, and no other factors were to
intervene, all the carbon dioxide in the atmosphere would disappear in less than 10,000 years.
The oxygen content of the atmosphere would
double in less than several million years.
Fortunately, the overproduction of oxygen in the
oceans is balanced by the weathering of fossil
organic carbon and other materials in rocks on
land. During this process, carbon dioxide is
returned to the atmosphere.
If the burial of organic carbon in sediments
were enhanced, oxygen might accumulate in the
atmosphere. In fact, it seems to be the case that
times of high organic carbon burial in the past
gave rise to high atmospheric oxygen levels.
Figure 8 is a model calculation of atmospheric
oxygen concentration variations during the last
600 million years. Just before the Carboniferous,
vascular plants evolved and spread over the continents. Their remains were a new source of organic matter resistant to degradation by atmospheric
oxygen. During the Carboniferous and Permian,
large quantities of vascular plant organic matter
were buried in the vast coastal lowlands and
swamps of the time. This material became the
coal deposits mined from rocks of Carboniferous
and Permian age today. This large accumulation
of organic matter gave rise to the high atmospheric oxygen levels of the late Paleozoic. Coal
deposits are also important in Cretaceous- and
early Cenozoic–age rocks, another time of high
atmospheric oxygen concentrations.
Environmental Conditions Before
Human Interference
Figure 7. The biogeochemical cycle of oxygen. This cycle is strongly coupled to that of carbon (Figure 4). The boxes represent the
major reservoirs of oxygen, and the arrows the fluxes of oxygen
from one box to another. The heavier the arrow, the larger the flux.
The broken lines show the flow of carbon in sedimentation on the
ocean floor, burial in sediments, and uplift by plate tectonic
processes. When uplifted, organic C is oxidized by oxygen in the
atmosphere, and CO2 is released (after Andreae, 1987).
From the above discussion, we can conclude
that the carbon and oxygen biogeochemical
cycles have changed throughout geologic time.
These changes have led to changes in atmospheric composition and climate. Just prior to significant human interference in the ecosphere, environmental conditions were also changing naturally because of a variety of factors. These factors
include external forcings, such as changes in the
orbit of the planet earth and in the flux of solar
radiation to the planetary surface, and internal
forcings, involving the behavior of the earth’s
ocean-atmosphere-land-biota-cryosphere system.
For about 1.6 million years, the earth has
experienced a series of cold and icy times known
as ice ages or glacial stages. These periods of
extensive glaciation of the continents alternated
with shorter and warmer periods, interglacial
stages like today. During the interglaciations, the
great glaciers that covered the continents of
Europe and North America to depths of two kilometers or more melted. The glaciers retreated to
geographical positions similar to those of the present continental ice sheets of Greenland and
Antarctica. As the glaciers waxed and waned
Respiration
Atmospheric
CO2
Photosynthesis
Atmospheric oxygen
Land biota
Oxygen dissolved
in the ocean
Ph
oto
syn
the
sis
tion
ira
sp
Re
Marine biota
Continental rocks
Marine sediments
(organic carbon)
19
Understanding Global Change: Earth Science and Human Impacts
through the last 2 million years, global temperatures went up and down, as did sea level. The
composition of the atmosphere and other environmental conditions changed.
The earth, before extensive human interference in its biogeochemical cycles, was recovering
from the climax of the last great glaciation 18,000
years ago. The recovery has not been smooth.
There have been times in the past 18,000 years
when the planet cooled quickly. Also, there have
been periods when the earth was warmer than
today. However, during the past several centuries, the global environment has changed substantially and rapidly. Atmospheric trace gas concentrations, matter in runoff, temperature, and
other indicators have increased in magnitude. A
major reason for all the changes is the impact of
human activities on the environment.
Figure 8. Model calculation of atmospheric oxygen during the past 600 million years. The dashed line across the figure shows today’s
atmospheric concentration of O2. The left vertical axis shows the atmospheric O2 level; the right vertical axis shows O2 as a percentage of
the total atmospheric gases. The horizontal axis shows time in millions of years before the present and geological time period (after Berner
and Canfield, 1989).
Time
Millions of years before present (BP)
20
500
C
O
S
D
C
P
Tr
J
K
T
-
400
300
200
100
0
Cambrian
Ordovician
Silurian
Devonian
Carboniferous
Permian
Triassic
Jurassic
Cretaceous
Tertiary
35
30
25
Present day
20
10
15
10
5
0
C
O
S
D
C
P
Paleozoic
Tr
J
K
Mesozoic
20
T
0
Cenozoic
O2 as percentage of atmospheric gases
Oxygen (1020 grams)
600
30
Global Change
Instruction Program
The Modern Coupled C-N-P-S-O System
The modern coupled C-N-P-S-O system is
multidimensional and complex, with numerous
processes, reservoirs, and fluxes. All these attributes are difficult to portray in detail. To see the
relationships between biogeochemical cycles and
climate, we will examine those biogeochemical
compounds that play a role in climatic change. In
particular, we will focus on the problem of an
enhanced greenhouse effect and global warming
brought about by human activities. Keep in mind
that the cycles also respond to a global cooling
and can feed back into any such cooling. The
processes discussed below would respond to
cooling in an opposite sense, to some extent, and
probably with different magnitudes of change.
Figures 9–20 show the biogeochemical cycles
of interest. The figures show that the burning of
forests and other biomass and fossil fuel
Figure 9. Part of the modern global biogeochemical cycle of methane, emphasizing exchanges of methane between the earth’s surface and
atmosphere and the fate of the gas in the atmosphere. The atmospheric reservoir is shown as a circle, including the amount of carbon in
teragrams (1012 grams), equivalent to million metric tons (Mt). Flux ranges are also given in Mt C/yr; arrows indicate the directions of the
fluxes. The proportion of C in the atmosphere is given in parts per million by volume (ppmv), the residence time in years, and the accumulation rate per year in both Mt and parts per billion by volume (ppbv). The chemical reaction at the top of the figure shows the fate of methane
that escapes to the stratosphere (after Mackenzie, 1995; Houghton et al., 1996).
To stratosphere
46
CH4 + OH* → H2O + CH3
Climate sensitivity
0.3°C/CH4 doubling
17% greenhouse
n
epletio
H*d
O
0
y
inl
– 45
Ma 300
CO
CH4 hyd
rate
0 – 76
Land
ean
Oc 16
4–
nds
ral wetla
Natu
76 – 150
Freshwater
1 – 19
Production from livestock
56 – 150
Soil uptake
11 – 34
CH4
3750 Mt C
1.72 ppmv
Residence time = 10 y
Accumulation:
28 Mt C/y
8 ppbv
ing
Fos
sil
fue
34 l bu
– 7 rn
Biom
6
as s
bu
16
r
– 3 nin
1
g
Lan
dfil
16 – ls
53
Rice
pa
19 – ddies
127
METHANE
(fluxes = Mt C/y)
Ocean
21
Understanding Global Change: Earth Science and Human Impacts
combustion are the major human sources of most
biogenic gases in the earth’s atmosphere. Many
of the natural exchanges of gases between the
earth’s surface and the atmosphere in these
cycles are driven by biological processes, which
involve bacteria. In the atmosphere, many of the
gases are oxidized, usually by hydroxyl radical
(OH*), a trace gas that is the atmosphere’s main
cleansing mechanism. The oxidized materials
then return to the earth’s surface, through biological production of plants on land and in the
ocean and by wet and dry deposition.
Many of the natural processes shown in these
figures have feedbacks that affect the accumulation of trace gases in the atmosphere and hence
affect the global climate. Because these feedbacks
are linked to biological processes, they are termed
biotic feedbacks. We shall explore the nature of
the feedbacks in the C-N-P-S-O system below.
Figure 10. Part of the modern global biogeochemical cycle of carbon monoxide, emphasizing exchange of the gas between the earth’s
surface and its atmosphere and the fate of the gas in the atmosphere. See Figure 9 for an explanation of the units and abbreviations
used. Notice, as with methane, the role of OH* as an agent of oxidation of the reduced carbon gases (after Mackenzie, 1995; Houghton
et al., 1996).
CARBON MONOXIDE
(fluxes = Mt C/y)
CO
228 Mt C
0.11 ppmv
Residence time = 0.2 y
Accumulation:
2 Mt C/y
1 ppbv
ing
Soil
48
Biom
ass
bu
312 rnin
g
tion
ple
e
* d 12
OH 12
CO2
epletion
OH* d
360
NMHC
ean
Oc 2
7
Fos
sil
fue
l
33 bur
6
n
n
OH*
dep
l
32 etio
4
Soil uptake
192
CH4
To stratosphere
48
CO + OH* → CO2 + H
Land
Ocean
22
Global Change
Instruction Program
Carbon Cycles
The CH4-CO-CO2 Connection
anthropogenic emission sources are located. The
rate of increase in the 1980s was approximately 1
ppbv per year, principally because of the same culprits, fossil fuel and biomass burning (Figure 10).
The rates of increase of all three atmospheric
carbon gases slowed during the late 1980s and
early 1990s because of a variety of natural and
human-induced causes.
Figures 9–11 illustrate the biogeochemical
cycles of methane (CH4), carbon monoxide (CO),
and carbon dioxide between the earth’s surface
and atmosphere. We have a reasonably good
understanding of the major processes associated
with the exchange of these gases between the
surface and the atmosphere and of the gases’
atmospheric reservoir sizes. However, the fluxes
associated with the various processes are less
well quantified.
All three of these gases have been accumulating in the atmosphere because of human activities. Since the late 18th century, global atmospheric CO2 concentrations have risen from
about 280 to about 360 parts per million by volume (ppmv), a change of approximately 30%.
This increase is principally the result of fossil fuel
burning and land-use activities such as deforestation, which add CO2 to the atmosphere when the
burning carbon combines with atmospheric molecular oxygen. Because of this increase in CO2
from human activities, the atmospheric reservoir
of oxygen gas has been reduced by less than 1%.
Thus, in the burning of fossils fuels, there is no
concern about depletion of atmospheric CO2.
The global concentration of CH4 during this
period has more than doubled from about 800 to
1,720 parts per billion by volume (ppbv). As with
CO2, this increase is due to human activities, and
again as with CO2 those activities include burning of biomass and fossil fuels. For methane,
though, rice paddies, farm and ranch animals
(which produce methane as they chew their cud),
and rotting landfills are also important. Landfills
emit CH4 as methanogenic bacteria decompose the
wastes in an anaerobic environment.
Carbon monoxide has also been increasing in
concentration in the atmosphere, particularly in
the Northern Hemisphere where important
Major processes in the CH4 and CO cycles
CH4 and CO are reduced gases, that is, they
may react with other chemical compounds by the
loss of electrons from the carbon in the compounds. The carbon in CH4 has a valence of –4;
the carbon in CO has a valence of +2. The biogeochemical cycles of these two gases and CO2 are
connected because OH* oxidizes CH4 and CO to
CO2, which has a valence of +4. This CO2 can
then be used by plants and other organic matter.
The decay of this organic matter, in turn, leads to
the release of CH4 and CO from the earth and
oceans.
The major natural processes involving
exchange of CH4 between the earth’s surface and
the atmosphere (shown on the right-hand side of
Figure 9) are
• evasion (release) to the atmosphere from the
ocean and natural wetland and freshwater
ecosystems,
• leakage from underground natural gas
deposits into the atmosphere (in Figure 9,
this is included in the fossil fuel burning
flux), and
• uptake of CH4 in soils due to the activity of
methanotrophic bacteria.
The major sink of CH4 is the atmosphere,
where it is oxidized principally by reaction with
OH* (top right-hand side of Figure 9).
Approximately 300–450 million tons of carbon
23
Understanding Global Change: Earth Science and Human Impacts
annually are oxidized this way. During the past
hundred years, the change in the concentration of
methane in the atmosphere could be responsible
for about 20% of the warming due to an
enhanced greenhouse effect. If CH4 concentration
were to double in the atmosphere, one would
expect, based on the ability of the gas to warm
the atmosphere, a rise of approximately 0.3°C in
global mean temperature.
Soils are both a natural source of CO to the
atmosphere and a natural sink of the gas from
the atmosphere (Figure 10). The source is the bacterial decomposition of organic matter in soils,
and bacteria and algae in the ocean. The sink is
also the result of bacterial processes.
The atmosphere is also both a source and
sink of CO. It is the most important sink of CO,
as with CH4, through the oxidation of CO to CO2
via OH*. This process transfers approximately
1,200 million tons of carbon to the atmospheric
CO2 reservoir annually (Figure 10). The atmosphere is a source of CO because nonmethane
hydrocarbons (NMHCs) are oxidized to that
compound by OH*.
methane escaping to the atmosphere from this
source is poorly known today (Figure 9).
Because CH4 and CO react easily with OH*
in the atmosphere, increases in their atmospheric
concentrations (and in those of the reduced N
and S gases) would affect the concentration of
OH*, thus having an impact on the atmosphere’s
ability to cleanse itself. It is possible that an
increase in the flux of CO to the atmosphere,
because of the gas’ short residence time there,
could deplete OH* and consequently cause less
CH4 to be removed from the atmosphere. If so,
CH4 would accumulate faster in the atmosphere,
leading to an increased rate of global warming.
However, the overall effect is likely to be small.
Furthermore, increased atmospheric CH4 would
probably lead to increased production of water
vapor in the stratosphere (Figure 9), where
methane and OH* react to form water vapor and
CH3. Water vapor is a greenhouse gas, and its
increased production in the stratosphere is a positive, but minor, feedback in a scenario of a
warming earth.
In summary, although the various effects are
difficult to quantify, it is likely that an initial
warming of the climate would lead to a net
increase in fluxes of CH4 and CO to the atmosphere and accumulation there. This situation
constitutes a positive feedback on the concentration of these gases in the atmosphere and hence
on global warming. Any global cooling would
probably result in an opposite effect.
CH4 and CO feedbacks to global climate change
Respiration and the bacterial decomposition
of organic matter emit CH4 and CO from soils to
the atmosphere. Because the rates of these
processes increase with increasing temperature,
the emissions might increase in a world that was
warming. This would be a positive feedback on
any initial warming of the earth. However,
because soils are also sinks for atmospheric CH4
and CO, and emissions of these gases are sensitive to the amount of moisture in the soil, the situation is more complex.
In general, it is likely that warmer temperatures in the high northern latitudes will lead to
an increase in CH4 fluxes from CH4 trapped in
permafrost, decomposable organic matter frozen
in permafrost, and decomposing CH4 gas
hydrates—substances in which CH4 is locked in a
structure of water ice. These hydrates are stable
only at low temperatures or under pressures
exceeding ten atmospheres. They are stored in
sediments under shallow seas, particularly in the
Arctic region, and also in permafrost. The flux of
The CO2 Cycle
The most important trace gas contributing to
the potential of an enhanced greenhouse effect is
CO2. From 1765 to 1995, CO2 accounted for about
60% of the human-induced warming due to the
accumulation of greenhouse gases in the atmosphere. A doubling of the concentration of CO2 in
the atmosphere could eventually lead to an
increase in global mean temperature of about
1.5–4.5°C.
Figure 11 shows the major processes, both
natural and human, affecting the exchange of
CO2 between the atmosphere and earth’s surface.
Estimates of the fluxes associated with the
processes are also shown. Let us begin our
24
Global Biogeochemical Cycles and the Physical Climate System
exploration of Figure 11 by considering the ocean
fluxes. The total exchange of CO2 between the
ocean and atmosphere amounts to about 90 billion tons of carbon per year. Much of this carbon
exchange involves
photoautotrophic marine plankton. A slightly
larger amount of CO2 is returned to the atmosphere by respiration and decay in the ocean. The
difference between the two amounts implies that
the ocean, prior to human interference in the
global biogeochemical cycle of carbon, was a net
heterotrophic system and supplied CO2 to the
atmosphere. The CO2 released from the ocean was
subsequently used in organic production on land.
In contrast, the terrestrial biota were a net
autotrophic system before humans interfered with
the carbon cycle. In other words, the land biota
produced more organic carbon than was consumed. (Compare the rates for net primary
• evasion of CO2 from the warm surface
waters of the tropics and from upwelling
regions, and
• invasion (uptake) of CO2 into higher-latitude,
colder, surface waters (see equation 9, p. 27).
Almost half of this exchange—close to 45 billion tons of carbon annually—is involved in net
primary production due to CO2 fixation by
Figure 11. Part of the modern global biogeochemical cycle of carbon dioxide, emphasizing its exchanges between the earth’s surface and its
atmosphere. See Figure 9 for an explanation of the units and abbreviations used. If there were a doubling of the CO2 concentration in the
atmosphere, one would expect a rise in temperature of 2–3°C at the earth’s surface and in the lower atmosphere. CO2 accounts for about
60% of the enhanced greenhouse effect. Note that fossil fuel burning and land-use practices released to the atmosphere about 7,700 Mt of
carbon in 1995. The atmosphere was a major sink for this anthropogenic carbon, but the ocean and the terrestrial biosphere took up about
50% of it. Compare this figure with Figure 4 (after Mackenzie, 1995; Houghton et al., 1996).
CARBON DIOXIDE
(fluxes = Mt C/y)
To terrestrial realm
2412
CO
To ocean
2000
Excess
4412
Climate sensitivity
2.5°C/CO2 doubling
60% greenhouse
OH* d
eple
121 tion
2
Ne
tp
n
t io
uc
rod
yp
00
ay
50
ec
-d
ion
2
ion
25
itat
45
cip
pre
168
rim
a
4 r
ira
t
3
anic
Volc
60
Land
CO
Ca
Lan
Late diagenesis-me
tamor
p h is
60
m
Re
sp
il CH2O
Oxidation of foss
36
d
16 use
Fos
0
0
sil f
u el
610 burn
0
ing
Resp
irat
io
614 n-dec
40
ay
Net pr
imar
yp
630 roduc
00
tio
n
Weatherin
g
216
CO2
752000 Mt C
360 ppmv
Residence time = 6.5 y
Accumulation:
3100 Mt C/y
1.5 ppmv
Ocean
25
Understanding Global Change: Earth Science and Human Impacts
production and respiration-decay in Figure 11.)
This excess organic carbon, approximately 400
million tons annually, eventually made its way
into river waters in particulate and dissolved
forms. It was then transported to the oceans,
where some portion of it decayed. The CO2 generated by this decay led to the pristine heterotrophic state of the ocean. In addition, the precipitation of calcium carbonate in the ocean also
led to the evasion of CO2 to the atmosphere
(Figure 11).
During the past several centuries, as fossil
fuel burning and land-use changes added CO2 to
the atmosphere, the ocean went from a net source
to a net sink of CO2 from the atmosphere. The
concentration of CO2 in the atmosphere rose, and
the gradient of CO2 concentration between the
atmosphere and ocean changed. This change
favored the net uptake of CO2 in the ocean by
solution of the gas in surface seawater (see
Equation 9, p. 27). This oceanic sink of anthropogenic CO2 was on the order of 2,000 million
tons of carbon per year during the 1980s,
although it varies annually.
Continuing with the exploration of Figure 11,
we see that volcanic emissions of CO2 from land
and under the sea amount to about 60 million
tons of carbon annually. This flux varies each
year with the intensity of volcanic activity. The
processes of diagenesis and metamorphism create a flux of about the same size as that of volcanism. These processes occur as buried sedimentary materials change; calcium carbonate and
silica are converted to calcium silicate and CO2.
The CO2 is consequently released to the atmosphere via volcanism, hot springs, and seepage
from deep within sedimentary basins.
Weathering of minerals on land removes CO2
from the atmosphere, and weathering of sedimentary organic matter (kerogen and fossil fuel)
at the land surface by oxygen adds CO2 to the
atmosphere (Figure 11). As mentioned in Chapter
2, the balance between the weathering fluxes and
those of volcanism, diagenesis, and metamorphism has changed throughout geologic time.
These changes are in part responsible for the
long-term variation in atmospheric CO2 during
geologic time (Figure 6).
Keep in mind, as mentioned in the previous
section, that CO2 is coupled to the reduced carbon gases through the oxidation of CH4 and CO
by OH* in the atmosphere. Therefore, any initial
warming of the planet and consequent enhanced
emissions of CH4 and CO from the earth’s surface because of human activities could potentially lead to an increase in the accumulation of CO2
in the atmosphere, a positive feedback.
The imbalance in the cycle and feedbacks
The notorious problem with atmospheric
CO2 today is the difficulty in balancing the
known sinks with the source from fossil fuel
combustion and land-use activities such as biomass burning. The problem is illustrated in
Figure 11.
We know that carbon is accumulating in the
atmosphere at the rate of 3,300 million tons annually. We also know that the amount of carbon put
into the atmosphere from land-use activities is
1,600 million tons of carbon per year (with an
uncertainty of 1,000 million tons, plus or minus),
and the flux from fossil fuel burning plus cement
manufacturing is 6,100 million tons per year—a
total of around 7,700 million tons. Subtracting the
known atmospheric accumulation of carbon from
the known source, we see that the fate of roughly
3,000 to 4,000 million tons of human-produced
carbon per year is not known. The lower number
is within the rather large error margins of recent
estimates of how much anthropogenic CO2 the
ocean takes up annually, but the upper amount is
out of the range of these estimates.
These numbers suggest that there may be
another important sink (sometimes called the
“missing sink”) of anthropogenic CO2 besides the
atmosphere and ocean. This sink has been
hypothesized to be the Northern Hemisphere’s
midlatitude forests and soils. If that is the case, a
rather strange situation has developed with
respect to the modern carbon cycle. While tropical rain forest ecosystems are sources of carbon to
the atmosphere because of deforestation at rates
of about 1% of the world’s forested area per year,
higher-latitude terrestrial ecosystems may be
sinks of carbon.
26
Global Biogeochemical Cycles and the Physical Climate System
Such a net sink implies that some processes
of carbon storage on land have changed. Some or
all of the following changes may have occurred.
respiration. The rates of both processes in vegetation and microbial life increase with increasing
temperature. However, respiration rates are more
sensitive to temperature change. In a warming
world, increased respiration could temporarily
increase the flux of CO2 to the atmosphere by as
much as 1–3 billion tons of carbon per year. This
increased flux is a potentially strong positive
feedback on CO2 accumulation in the atmosphere
and hence on global warming.
A change in the ocean temperature would
affect the amount of dissolved inorganic carbon
in seawater. The equation relevant to this effect
is
• Increased atmospheric levels of CO2 act as a
fertilizer and stimulate productivity in
plants. This leads to storage of carbon in biomass or in soil organic carbon.
• Plant productivity is stimulated by increased
NO3- and NH4+ from fertilizers used in farming and from deposition of anthropogenic N
from the atmosphere. Carbon once more is
stored in biomass or in soil.
• Vegetation regrows in previously disturbed
ecosystems, or grows in undisturbed ones.
-
CO2(g) + H2O + CO32- = 2HCO3
The first process on this list, carbon dioxide
fertilization, is a potentially strong negative feedback on the accumulation of anthropogenic CO2
in the atmosphere and hence on global warming.
We know from experiments on plants and small
ecosystems that almost all agricultural crops and
a few perennial plants, when subjected to
increased CO2 levels, will increase their rates of
photosynthesis and growth. If this enhancement
should also occur in large ecosystems, like
forests, CO2 would be withdrawn from the
atmosphere and stored in plant organic matter.
Humans have put excess phosphorus, nitrate,
and ammonia nutrients into the environment by
applying fertilizers to the land surface, burning
fossil fuels and biomass, and discharging sewage
containing nitrogen and phosphorus. These
nutrients can stimulate increased plant growth in
the soil and aquatic environments. This eutrophication of both land and marine environments is a
negative feedback on accumulation of carbon in
the atmosphere from anthropogenic sources and
hence is a negative feedback on warming of the
earth. Total land and ocean eutrophication may
amount to a billion tons of carbon per year.
(9)
The (g) is gas, and the equal sign implies the
reaction is an equilibrium representation of the
process.
For a warmer ocean surface, this reaction
moves from right to left—the bicarbonate ions
break down into carbon dioxide, water, and carbonate. Thus the concentration of CO2 in the
water will increase, and CO2 will evade from the
seawater to the atmosphere. For a temperature
increase of 1°C, the change in CO2 concentration
is on the order of 10 x 10-6 atmospheres (10 µatm).
This is a positive feedback that could amplify
a future atmospheric CO2 increase by about
5%.
At a constant temperature, as you add CO2
to the ocean, equation 9 goes from left to right,
consuming carbonate ion and producing bicarbonate ion. This process too has the effect of
reducing the ocean’s ability to take up CO2.
Feedbacks involving ocean circulation are
strongly linked to the biogeochemical cycling of
carbon and nutrients in the ocean. If the ocean
began to warm, the waters at the surface would
warm more than those at depth. This change
would decrease the amount of nutrients rising
from the deep ocean into the euphotic zone. With
fewer nutrients, there could be a consequent
decrease in biological productivity, followed by
less organic carbon escaping the euphotic zone
and settling into the deep sea in a set of processes
known as the biological pump. It is also conceivable that changes in wind patterns and wind
intensities along the western coastal margins of
Other processes and feedbacks involving CO2
What about other processes and feedbacks in
the biogeochemical cycle of CO2 that might
change with a global warming? Perhaps one of
the strongest potential positive biotic feedbacks
in the terrestrial environment is the effect of
increasing temperature on photosynthesis and
27
Understanding Global Change: Earth Science and Human Impacts
continents could lead to changes in the upwelling
of CO2 and nutrients to the surface ocean.
There are several other potential feedbacks
involving CO2, the ocean, and climatic change,
but it is difficult to determine their importance:
• An increase in the rate of decomposition of
dissolved organic carbon in the ocean could
occur. There are about 1,000 billion tons of
dissolved organic carbon in the ocean.
• The biological processes associated with
these possibilities could change the concentration of CO2 in surface waters and hence
the amount of anthropogenic CO2 the ocean
can take up.
• Increased ultraviolet radiation because of
stratospheric ozone depletion may affect the
capacity of certain marine ecosystems, e.g.,
around the Antarctic, to remove carbon from
the atmosphere.
• Finally, with a rise in atmospheric CO2 to
about 1,000 ppmv, the pH of the ocean could
fall sufficiently to make it difficult for the
organisms that build skeletons of calcite
enriched by magnesium (e.g., coralline algae
and sea urchins) and aragonite (e.g., corals)
to produce shells.
• With warming, the composition and distribution of algae, jellyfish, and other marine
species could change.
• The rates of delivery to the ocean of iron,
molybdenum, and other trace metals essential to marine life may change.
28
Global Change
Instruction Program
The Important Nutrient Nitrogen
Nitrogen forms part of the molecules that
make up living things, such as amino acids (the
building blocks of proteins) and DNA. The nitrogen in proteins bonds together various amino
acids to form the protein structure. The amount
of nitrogen in the atmosphere is very large compared to that in the oceans or rocks. Of the elements C, N, P, S, and O, only nitrogen is found in
more abundance in the atmosphere than in rocks.
The complete biogeochemical cycle of nitro-
gen is very complex. Figures 12–17 show only
portions of it. There are six major forms of atmospheric nitrogen: the gaseous forms of diatomic
nitrogen (N2), ammonia (NH3), nitrous oxide
(N2O), and NOx (NO and NO2), and the aerosols
of ammonium (NH4+) and nitrate (NO3-). In this
chapter, we will focus on the cycles of the first
four of these forms, and also discuss nonmethane
hydrocarbons, the cycles of which are closely
related to those of NOx.
Figure 12. Part of the modern global biogeochemical cycle of nitrogen, emphasizing interactions among the land, atmosphere, and ocean.
Fluxes between the ocean, land, and groundwater are shown as arrows, with quantities given in Mt N/yr. Fluxes within reservoirs are shown
as circling arrows. “Ind. fix” is industrially fixed N (for the manufacture of fertilizers), “Bio. fix” is biologically fixed N, DN is dissolved N, PON is
particulate organic nitrogen, and “pollutant” is the excess nitrogen that has resulted from human activities (modified from Mackenzie, 1995).
at
ion
NITROGEN
(fluxes = Mt N/y)
2 fi
x
Atmospheric
CO2
N
N2O
N2O
Denitrification
1.4 – 2.6
Evasion
Land
42 Rice cultivation
20 Combustion
78 Ind. fix.
126 Bio. fix.
Enhanced organic
production-burial
216 Mt C/y
Aerosol
14
560
Ocean
Human waste
20
Agriculture
9
River
35 DN
27 PON
>21 “pollutant”
8000
Groundwater
Organic N
28
Accumulation
29
Understanding Global Change: Earth Science and Human Impacts
and rice paddy cultivation add fixed nitrogen to
the earth’s surface. Because of these human activities, the amount of nitrogen on land is increasing
(Figure 12). About 30 million tons of nitrogen are
leached from agricutural fertilizers and human
waste each year and added to groundwater systems and runoff. Some of this nitrogen makes its
way to rivers and then to lakes and the coastal
oceans. On a global scale, rivers may already
carry more nitrogen from human activities than
was transported in the natural state (Figure 12).
This increased nitrogen flux to lakes, rivers, and
coastal marine environments is one cause of
increased regional and global eutrophication of
these systems. Note, however, that rivers supply
only a small percentage of nitrogen to the coastal
zone (Figure 13). Most of the nitrogen there, other
than that recycled in the zone, upwelled from the
deep ocean to the surface.
Scientists have calculated how much this
human-caused increase in nitrogen is likely to be
N2
The overwhelming majority of nitrogen in
the atmosphere is in the form of N2. The other
forms exist only in small quantities. Biological
fixation and denitrification are the major processes leading to exchange of nitrogen between the
earth’s surface and the atmosphere (Figure 12).
Biological fixation is the process whereby N2 is
withdrawn from the atmosphere and converted
to N compounds that plants can use (e.g., NH3
and subsequently NO3-). Denitrification is the
process by which nitrogen as N2 or as N2O is
returned to the atmosphere. Both processes are
mediated by a variety of bacteria living in soils
and water.
The exchange fluxes between the earth’s surface and the atmosphere are small compared to
the internal recycling of nitrogen within the land
and ocean realms (Figures 12 and 13). Combustion
practices, the production of commercial fertilizers,
Figure 13. River input of N to the ocean compared to the fluxes involved with the internal recycling of N in the ocean due to biological productivity and decay. Besides the ocean fluxes shown, about 90% of the nitrogen involved in biological production is simply recycled in the
shallow surface waters of the coastal and open oceans. Some nitrogen escapes from the surface ocean in organic matter that settles to the
deep ocean, where the organic matter is decayed and the nitrogen released. It then returns to the surface via upwelling in the coastal zone
and vertical mixing in the open ocean. Some nitrogen, about 30 Mt per year, is buried in marine sediments in organic matter (see Figure
12). Some N is transported to the open ocean from the coastal environment (after Mackenzie, 1995; Houghton et al., 1996).
ORGANIC NITROGEN
(fluxes = Mt N/y)
Rivers
Dissolved N 35
Particulate N 27
Coastal
zone
Export
200
Surface
open ocean
Organic
Vertical
Upw
matter
e
206lling mixing sedimentation
670
900
Deep ocean
30
Global Biogeochemical Cycles and the Physical Climate System
changing ocean productivity and the flux of
organic carbon from the ocean’s euphotic zone.
The calculations show that in the 1980s there may
have been an increased organic carbon flux from
the atmosphere to the oceanic environment of
about 200 million tons of carbon per year (Figure
12), which is buried in marine sediments. This
flux takes from the atmosphere about 3% of the
increase occurring today as a result of fossil fuel
burning. While relatively small, this is a possible
negative biotic feedback on atmospheric CO2 and
hence global climate change.
N2O
N2O is a natural product of biological denitrification in soils and in the ocean (Figure 14).
The N2O produced by denitrification is only
about 15% of all N returned to the atmosphere;
the rest is in the form of N2.
N2O is an important greenhouse gas, accounting for about 9% of the enhanced greenhouse
effect since the 18th century. It has a present
atmospheric concentration of 312 ppmv and a residence time of about 130 years. This concentration
is about 8% greater than in preindustrial time and
Figure 14. Part of the modern global biogeochemical cycle of nitrous oxide. Symbols and units are as in Figure 9. This gas is responsible for
5% of the enhanced greenhouse effect. A doubling of its atmospheric concentration could lead to about a 0.4°C increase in global temperature. Notice the reaction of this long-lifetime gas in the stratosphere, leading to the destruction of stratospheric ozone. The fluxes in this
cycle are not well known (modified from Mackenzie, 1995).
NITROUS OXIDE
(fluxes = Mt N/y)
~ 85%
+ hν
<15%
+ O3
NOx
To stratosphere
5–9
N2
Climate sensitivity
0.4°C/N2O doubling
5% greenhouse
Fertiliz
er
0.01 –
2.2
Biom
ass
0.02 burnin
g
– 0.
3
Com
bu
0.1 stion
–0
.3
ean
Oc – 3
1.5
So
il
~7 s
N 2O
1510 Mt N
312 ppbv
Residence time = 130 y
Accumulation:
3.6 Mt C/y
0.8 ppbv
Land
Ocean
31
Understanding Global Change: Earth Science and Human Impacts
sediments, and ocean water. Also, N2O fluxes
from nitrogen-bearing fertilizers applied to the
land surface and sewage discharges into aquatic
systems will be affected by warming. Because the
reactions involving N2O are bacterially mediated,
it is likely that an increase in temperature will
lead to enhanced evasion rates of N2O from the
earth’s surface. This is a positive biotic feedback
on accumulation of N2O in the atmosphere and,
hence, on global warming. It could also lead to a
small enhanced destruction of stratospheric
ozone (Figure 14).
is increasing at a rate of 0.2–0.3% per year because
of human activities, including the combustion of
fossil fuels, burning of biomass, and emissions
from urea and ammonium nitrate applied to croplands. These emissions amount to 0.13 to 2.8 million tons of nitrogen annually (Figure 14).
N2O is chemically inert in the troposphere. In
the stratosphere, it can be converted photochemically to nitric oxide (NO), which acts as a catalyst
in the destruction of stratospheric ozone (see
sidebar). The series of reactions by which this is
accomplished has been one of the regulators of
stratospheric ozone concentration through geologic time.
The flux of N2O from the earth to the atmosphere has been increasing because of the rapidly
increasing use of industrially fixed nitrogen (up
to the late 1980s), increases in fossil fuel burning
and biomass burning, and increases in organic
carbon in coastal waters. This last process is an
important link between the carbon and nitrogen
cycles. The rate of denitrification and consequently of N2O emissions from coastal waters
may have increased because rivers are bringing
more organic carbon to these systems or because
these systems are undergoing eutrophication as
they receive increased inputs of nutrients from
fertilizer, sewage, and the atmosphere.
With warming, the most important biotic
feedbacks involving N2O are changes in the
denitrification (and nitrification) rates in soils,
NH3
The biogeochemical cycle of ammonia (NH3)
is shown in Figure 15. Ammonia is released to the
atmosphere by organic decomposition and
volatilization. There, it reacts with water droplets to
form ammonium ion (NH4+) and hydroxyl ion
(OH-). NH4+ appears to be removed from the
atmosphere mainly by being deposited back on
the earth in the aerosols of ammonium sulfate
[(NH4)2SO4] and ammonium nitrate (NH4NO3).
Incidentally, (NH4)2SO4 links the nitrogen and sulfur biogeochemical cycles, since its deposition on
the earth is also one of the ways oxidized sulfur is
removed from the atmosphere; the other is by
deposition of sulfuric acid (H2SO4).
Two interactions in the NH3 cycle are important in considerations of global warming. The
first is its interaction with OH* to produce NOx.
In a warmer world, the decomposition that
releases NH3 would probably be enhanced,
which would slightly increase the stress on the
OH* concentration of the atmosphere and
enhance production of NOx (Figure 16). The
effects of increased NOx concentrations are discussed in the following section. The second important interaction is NH3’s reaction with NO3 and
SO4 to produce aerosols containing ammonium
(Figure 15). Aerosols are known to cool the planet,
although the amount of the effect is unclear. An
increase in atmospheric NH3 could lead to a small
negative feedback on potential warming.
The ammonia cycle also gives us information
on nitrogen fertilization of the terrestrial biosphere. About four-fifths of the N released to the
atmosphere each year in NH3 comes from human
N2O reactions leading to the destruction
of stratospheric ozone
NO formation in the middle stratosphere (20–30 km):
N2O + ultraviolet light ⇒ NO + N
N2O + O ⇒ 2NO
(10)
(11)
Ozone destruction:
NO + O3 ⇒ NO2 + O2
O3 + ultraviolet light ⇒ O2 + O
NO2 + O ⇒ NO + O2
_____________________________________
2O3 + ultraviolet light ⇒ 3O2 (net reaction) (12)
32
Global Biogeochemical Cycles and the Physical Climate System
activities—50 out of 62 million tons. Only about
12 million tons of ammonia nitrogen per year
comes from natural bacterial decomposition in
soils. About 25% of the human-produced flux is
transported away from the continents to the
oceanic atmosphere. The rest, about 37 million
tons of nitrogen per year, falls back on the land
surface and may be available for terrestrial
organic productivity.
Now it’s time for a back-of-the-envelope calculation. If this 37 million tons of nitrogen were
to fertilize land plant production with a ratio of C
to N of 100 to 1, the plants would require more
than 3 billion tons of carbon per year. The phosphorus accumulating on land each year from
agricultural fertilizers and sewage amounts to
about 8.5 million tons (see Figure 18, p. 36)—just
about the amount of phosphorus needed to
sustain this magnitude of land plant production.
This calculation gives some idea of the potential
of fertilization of the land as a sink for the excess
CO2 that we are emitting to the atmosphere by
fossil fuel burning and land-use practices.
NOx and NMHCs
This brings us to the cycles of NOx and the
NMHCs (Figures 16 and 17). We will begin with
NOx. It has several natural sources: on the earth,
bacterial decomposition of organic matter in
soils; in the atmosphere, lightning, mixing from
the stratosphere, and oxidation of ammonia. NOx
also has anthropogenic sources: fossil fuel and
biomass burning. The main sink of NOx is
deposition on earth of chemical products that were
produced in the atmosphere by photochemical
Figure 15. Part of the modern global biogeochemical cycle of ammonia, including that of the ammonium ion (NH4+). See Figure 9 for an
explanation of the units and symbols used. Most of the ammonia emissions from the land surface are due to human activities. Ammonia is
removed from the atmosphere mainly in rain and as small, solid aerosol particles after reaction with water and with nitrate and sulfate.
Through reaction with OH*, a small amount of NH3 is converted to nitrogen oxides, e.g., NO and NO2 (modified from Mackenzie, 1995).
AMMONIA
(fluxes = Mt N/y)
epletion
OH* d
6
NO 3
Wet-dry
ion
Comb
ustio
n
7
+ H2O
89
deposit
89
Land
+
NH4
2 Mt N
Variable conc.
Residence time =
0.01 y
tion
mposi
deco
a ni c
ion
Org olatilizat
v
33
Organic
deco
mp
osi
volat
ti o
iz a t i
n
o
n
55
NH3
2 Mt N
Variable conc.
Residence time =
0.01 y
NOx
Ocean
Land
33
SO 4
Ocean
Understanding Global Change: Earth Science and Human Impacts
reactions with NOx, such as HNO3 and organic
nitrates.
NMHCs (also called volatile organic compounds, or VOCs) are natural byproducts of
plant productivity in terrestrial and marine environments. Thus, their fluxes to the atmosphere
change greatly with the seasons. They also have
anthropogenic sources—once again, fossil fuel
and biomass burning. Their main sink is in the
atmosphere, through oxidation with OH*.
effect of increasing temperature is not at all
straightforward. The concentration of ozone in
the troposphere depends in a complex way on
the atmospheric concentrations of several other
biogenic trace gases, including CH4, CO, and the
NMHCs.
In general, when there is little NOx in the troposphere (5–30 pptv), increases in the concentrations of CH4, CO, and NMHCs lead to a decrease
in the concentration of O3. At high NOx concentrations (generally greater than about 90 pptv),
increases in these three gases lead to an increase
in ozone. The combination of high NOx and
NMHCs in the troposphere disrupts the natural
cycle of production and destruction of ozone,
and ozone accumulates. In urban areas, this contributes to air pollution.
Effects of NOx on ozone
Increasing temperature alone would probably increase the flux of NOx from soils to the
atmosphere, potentially depleting OH* and forming more methane and ozone in the troposphere
(Figure 16). For tropospheric ozone, however, the
nin
g
NITROGEN OXIDES
(fluxes = Mt N/y)
5
ht
Tropospheric
O3
on
OH
*d
ep
le t
6
i
ere
Lig
NH3
F ro m
stra
tos
1 ph
Figure 16. Part of the modern biogeochemical cycle of the nitrogen oxides. About one-half of the emissions of these gases to the atmosphere comes from the combustion of fossil fuels. In the atmosphere, the gases are converted to several other chemical species, mainly
HNO3 , and removed from the atmosphere both in rain and in dust. Notice the tie to tropospheric O3 (see text), a greenhouse gas and a
component of smog (modified from Mackenzie, 1995).
Climate sensitivity
Greenhouse gas
Biomass b
urnin
g
3
ustio
n
21
Comb
Photochemical
56
HNO3
PAN
Organic nitrates
Photolysis
Ther
mal decomposition
Wet-dry
ion
deposit
56
Soil
s
20
NOx
Land
Land
34
Ocean
Global Biogeochemical Cycles and the Physical Climate System
On the other hand, increases in CH4, CO, and
NMHCs will lead to lower levels of OH*.
One critical positive feedback is that increases in CO concentrations in the atmosphere could
lead to a reduction in OH*, because NOx has too
short a lifetime to counteract that effect on a global
scale. Decreased concentrations of OH* could lead
to an increase in the lifetime of CH4, a positive,
but small, feedback on the accumulation of CH4 in
the atmosphere and hence global warming.
Effects of NOx on OH*
The concentration of OH*, which is mainly
responsible for cleansing the atmosphere,
depends on the concentrations of trace gases, tropospheric ozone, and water vapor. Elevated concentrations of O3, NOx, and H2O will increase
OH* levels. (Generally, changes in NOx concentrations affect OH* in the same way they affect
ozone, described above, except to a lesser degree.)
Figure 17. Part of the modern global biogeochemical cycle of the nonmethane hydrocarbons. Land vegetation and phytoplankton naturally
produce these compounds. Their human sources include industrial practices, transportation, and fossil fuel combustion. These compounds
react in the atmosphere with OH* and are important in controlling that compound’s concentration in the troposphere. They are also responsible for disrupting the natural production and destruction of the ozone cycle in the troposphere. In conjunction with NOx , they can lead to
increased concentrations of O3 in the troposphere (modified from Mackenzie, 1995; Guenther et al., 1995).
OH
*
NONMETHANE
HYDROCARBONS
(fluxes = Mt C/y)
tion
p le
e
d 00
13
+ NOx
Climate sensitivity
Greenhouse gas
NMHCs
n
Terr
est
r ia
l
terp
ene vege
,
t
11 isop a
45 r
tio
Tropospheric
O3
e
en
Bioma
ss bu
rnin
combu
g
stion
, sol
150 vents
an
Oce
pene
, pro
ene 5
eth
Land
Ocean
35
Global Change
Instruction Program
Phosphorus and Sulfur
nutrient, if for no other reason than that the
atmosphere contains essentially an unlimited supply of nitrogen for fixation in aquatic systems.
The major difference between the phosphorus
cycle and the carbon, nitrogen, and sulfur cycles
is that no biological process generates an important gas flux of phosphorus from the earth’s surface to the atmosphere (Figure 18). A phosphorus
gas known as phosphine or swamp gas (PH3gas)
causes a small flux, but the amount is insignificant compared to other fluxes of phosphorus. As
with nitrogen, fluxes of phosphorus between the
The Important Nutrient Phosphorus
Phosphorus is a key nutrient, fueling organic
productivity on land and in water. A portion of its
cycle is shown in Figure 18. The P cycle is considered here both because it is closely coupled with
those of carbon and nitrogen and for completeness.
Controversy still exists, particularly between
marine biologists and geochemists, as to whether
phosphorus or nitrogen is the limiting nutrient in
the marine environment. Globally and on a long
time scale, phosphorus is probably the limiting
Figure 18. Part of the modern global biogeochemical cycle of phosphorus, emphasizing the exchange of P among the land, atmosphere, and
ocean. Units and symbols are as in Figure 9. Notice, as with nitrogen (Figure 12), that the internal recycling fluxes in the ocean and land reservoirs by organic production and decay are much larger than the exchanges. Also as with nitrogen, the land is gaining P because of its mining,
its use in the manufacture of fertilizers and detergents, and sewage inputs. “Wet-dry fallout” is the precipitation of phosphorus on the ocean as
particles and in rain. DP is dissolved phosphorus, and “pollutant” is excess phosphorus from human activities (modified from Mackenzie, 1995).
PHOSPHORUS
(fluxes = Mt P/y)
Atmospheric
CO2
Sea salt
0.03
Dust
3.1
Land
8.5 Acc.
186
Atmosphere
0.03 Mt P
Residence time =
2.4 days
Dust
3.72
Human
activities
0.47
Dust
1.09
Wet-dry
fallout
0.28
Sea salt
0.31
Enhanced organic
production-burial
140 Mt C/y
Ocean
1085
Rivers
2.9 DP (1.5 “pollutant”)
Organic P
1.6
Fisheries
0.31
Mining
12.1
DRAFT
Accumulation
36
Global Biogeochemical Cycles and the Physical Climate System
land, ocean, and atmosphere are small compared
to the amounts that cycle within the land and
ocean systems.
The second biological feedback has to do with
the land reservoir of phosphorus. Notice in Figure
18 that phosphorus is accumulating on land
because of mining, fertilizer use, and sewage discharges. Because most chemical weathering and
biological decomposition rates increase with
increasing temperature, a climate warming would
mean that phosphorus in this reservoir may be
more easily leached into aquatic systems. This
additional phosphorus could increase eutrophication in these systems and lead to increased accumulation of organic matter. This flux is a negative
feedback on CO2 accumulation in the atmosphere
and hence on global warming. However, the effect
is quite small. If all the phosphorus now stored on
land were leached and transported by rivers to
coastal systems, and all the other nutrients in
coastal marine systems could be used for plant
growth (which may not be the case), the increase
in organic carbon storage would be 800 million
tons. This is equivalent to only 13% of one year’s
flux of carbon to the atmosphere because of the
burning of fossil fuels.
Sinks and sources
Unlike nitrogen, phosphorus accumulates in
both organic and inorganic sediments. Because of
the direct tie between N and P in organic matter,
the only organic sink shown in Figure 16 is the
important phosphorus flux to sediments as P
incorporated in dead organic matter (bottom
right-hand corner of the figure). The inorganic
sinks, which are not shown in the figure, involve
the precipitation of the mineral carbonate fluoroapatite [Ca5(PO4)3(OH,F)], scavenging by iron
oxy-hydroxides, and incorporation in oxidized
iron coatings on the surfaces of calcium carbonate.
Phosphorus mining is the largest source of P
to the land surface. Much of the mined phosphorite rock is used to make commercial fertilizers.
The major impact of humankind on the phosphorus cycle results from the application of both commercial and organic fertilizers to croplands and
the disposal of sewage. Phosphorus from these
sources is introduced to streams via direct sewage
discharge or by leaching. The increased amount of
phosphorus in rivers, lakes, and coastal marine
waters results in increased rates of eutrophication
of these systems in cases where phosphorus is the
limiting nutrient.
Reduced and Oxidized Sulfur Gases
The reduced and oxidized sulfur cycles
(Figures 19 and 20) are closely tied, because the
reduced sulfur gases that dominate the earth’s
biological sulfur emissions are oxidized in the
atmosphere. These reduced gases are dimethylsulfide or DMS [(CH3)2S] and carbonyl sulfide
(OCS), which are emitted by the ocean surface,
and hydrogen sulfide (H2S), emitted by decaying
terrestrial vegetation. Oxidation converts these
gases to sulfur dioxide (SO2) gas and sulfate
aerosol, that is, microscopic sulfur-containing
particles. Sulfur enters the atmosphere as the
earth emits the reduced sulfur gases and leaves
the atmosphere as sulfate aerosol floats or is
washed to the earth.
The chemistry of DMS and OCS has two
major connections with climate, by way of cloud
condensation nuclei and stratospheric sulfate.
Feedbacks
Two major biological feedbacks in the phosphorus cycle may be of importance in global climate change. One involves the enhanced eutrophication of aquatic systems as a result of human
activities, as just mentioned. The global dissolved
phosphorus flux, carried by rivers to the coasts,
has doubled since preindustrial times because of
these activities (Figure 18). The resulting eutrophication of the coastal regions has led to a potential
accumulation of organic carbon in these systems of
about 140 million tons of carbon per year (Figure
18, top right). This enhanced flux is a negative
feedback on the accumulation of anthropogenic
CO2 in the atmosphere today and, as with nitrogen, could become more important in the future.
Cloud condensation nuclei
DMS is produced by bacteria in phytoplankton. Its concentration in the oceans is very low, but
it is found nearly everywhere at the sea surface,
37
Understanding Global Change: Earth Science and Human Impacts
where it may escape into the troposphere. Once
airborne, it is oxidized by OH* to sulfate within a
few days. Along with other chemical species in the
atmosphere, it condenses into small (micron-sized)
aerosol particles. These atmospheric sulfate
aerosols act as cloud condensation nuclei (CCN)—
the centers on which water droplets may form,
facilitating cloud formation. In the remote marine
atmosphere, DMS emissions are likely the main
source of the aerosols that act as CCN.
One hypothesis argues that a warming of
earth’s climate could lead to enhanced phytoplankton growth and thus greater emission of
DMS from the sea surface. The increased DMS
flux could result in increased production of sulfate
aerosols and CCN in the remote marine atmosphere, creating more and denser clouds. Besides
leading to more rain, clouds also reflect incoming
solar radiation and have a cooling effect on the
lower atmosphere and surface of the earth. Thus
an increase in cloud cover would give rise to a
cooling of the troposphere, a negative biotic feedback on global warming. To counteract a warming
owing to a doubling of atmospheric CO2 would
require a 25% increase in the number of cloud condensation nuclei.
The validity of this hypothesis is not yet confirmed by real-world evidence. For example, in ice
cores collected by drilling at Vostok Station in
Antarctica, a record of methane-sulfonic acid
(MSA) has been obtained from the ice. MSA is a
product of DMS oxidation. If this theory were correct, one would expect to find more MSA in ice
that dates from warmer periods, since increased
Figure 19. Part of the global biogeochemical cycle of reduced sulfur. See Figure 9 for an explanation of the units and symbols used. Reduced
sulfur gases are naturally produced both on land and in the ocean by biological processes. They are eventually removed from the atmosphere by reaction with OH* and conversion to SO2 and thence to sulfate aerosol (see Figure 20). Sulfate aerosol is produced by the oxidation of OCS in the stratosphere to form the sulfate veil during times of low volcanic activity. Sulfate aerosol is responsible for the scattering
and reflection of solar energy and thus cools the planet (modified from Mackenzie, 1995).
Climate sensitivity
DMS→SO4→
CCN→cooling
To stratosphere
0.06 OCS
Sulfate veil
Reduced S
0.5 Mt S
0.1 ppbv
Residence time =
2.8 days
REDUCED SULFUR
(fluxes = Mt S/y)
SO4
OH* depletion
67
Biolog
ical
25 DM decay
S, H
2S
etion
excr
ayS
Dec MS, OC
D
42
Land
Ocean
38
SO2
Global Biogeochemical Cycles and the Physical Climate System
levels of the compound would wash out of the
atmosphere. However, the record shows just the
opposite: higher levels of MSA in the ice from the
last ice age, which culminated 18,000 years ago,
than in this and past interglacial stages.
posphere and can enter the stratosphere. Once in
the stratosphere, OCS is destroyed by reacting
with ultraviolet light and atomic oxygen. It is converted to SO2 and then on to sulfate aerosol. This
reduced sulfur gas supplies about half of the
mass of sulfate aerosol found in the lower stratosphere (the so-called Junge layer).
If global warming causes any change in the
flux of OCS to the stratosphere, it will have an
effect on climate through change in the Junge
layer. An increased stratospheric sulfate burden
would give rise to cooling of the atmosphere; a
decreased burden would lead to warming. The
likely feedback effect on an initial warming is difficult to predict.
Stratospheric sulfate
OCS is produced mainly by the photolysis of
organic sulfur in the surface waters of the ocean
and by photochemical oxidation of the biogenic
gas carbon disulfide (CS2) in the atmosphere. It is
the most abundant reduced sulfur species in the
remote marine atmosphere. Because it is chemically inert, it has a long residence time in the tro-
Figure 20. Part of the global biogeochemical cycle of oxidized sulfur. See Figure 9 for an explanation of the units and symbols used. Notice
that the flux of oxidized sulfur from land as SO2 is dominated by the combustion of fossil fuels and biomass burning. This sulfur reacts with
OH* to produce sulfate aerosols of ammonium and hydrogen. About 40% of the sulfur falling back on the earth’s surface is derived from
these sources. Volcanism can add sulfate aerosols to the stratosphere and produce a temporary cooling of the planet, as after the eruption
of Mt. Pinatubo in 1991. Sulfate aerosols derived from combustion of fossil fuels and biomass burning exert a strong cooling effect, but only
in the regions where they are produced (modified from Mackenzie, 1995).
Sulfate veil
OXIDIZED SULFUR
(fluxes = Mt S/y)
Reduced S
OH
*d
n
tio
67
ep
le
Volcanism
To stratosphere
SO4
SO4 aerosol
2 Mt S
0.35 ppbv
Residence time =
4.6 days
Rain and dry
deposition
Earth surface
39
160
Volcanis
m
10
g
burn
in
7
Biomas
s
Land
49
C om
OH* depletion
115
Climate sensitivity
Cooling
sol
aero
Sea 45
bus
t
80 ion
SO2
0.5 Mt S
0.1 ppbv
Residence time =
1.2 days
To troposphere
Ocean
Understanding Global Change: Earth Science and Human Impacts
Oxidized sulfur
problem of acid precipitation. Of the total global
deposition of oxidized sulfur today, approximately
40% is derived from the human activities of combustion of fossil fuels, biomass burning, and
smelting of sulfide ores. The deposits from these
activities vary enormously spatially.
Recently a new hypothesis has emerged linking our sulfur emissions with climate. In the
Northern Hemisphere, emissions of SO2 have led
to an increased mass of sulfate aerosol. Because of
the aerosol’s short lifetime in the atmosphere, it
remains concentrated over the eastern half of the
continent and the western Atlantic Ocean. This
regional enhancement could be cooling the atmosphere enough to explain the discrepancy between
the observed temperature record of the past 100
years and the higher temperatures predicted by
climate models that include only the effects of
increased greenhouse gases and not increased
aerosols. On the other hand, when climate modelers attempt to introduce the cooling effect of
sulfate aerosols into their models, the modeled
temperatures are generally lower than those
observed in the recent past. This difficulty implies
that sulfate aerosols are an important component
of the temperature record, but that the amount by
which they cool the planet is not yet known.
To complete the picture of the global cycle of
sulfur gas species, the earth surface–atmosphere
oxidized sulfur cycle is shown in Figure 20.
Natural sources of oxidized sulfur in the atmosphere include oxidation of reduced sulfur gases
as mentioned above, volcanism, and aerosols from
the sea. The oxidized sulfur is removed from the
atmosphere by deposition of aerosols.
One of the most dramatic demonstrations of
the connection between volcanism, sulfate
aerosols, and climate was the eruption of Mt.
Pinatubo in the Philippines in 1991. The eruption
plume rose high into the stratosphere, where sulfate aerosol was generated and distributed over
much of the globe. The aerosol scattered and
reflected solar energy back to space. This event led
to a cooling of the planet of about 0.5°C during
1991–93. At its maximum in 1993, the cooling
reached –3–4 watts per square meter. This is considerably more than the enhanced greenhouse
forcing of +2.5 watts per square meter.
Human activities have strongly interfered
with the global biogeochemical cycle of oxidized
sulfur. This disturbance in the cycle has led to the
acidification of land and freshwater systems—the
40
Global Change
Instruction Program
The Water Cycle
The water cycle is a balanced system, with
water stored in many places at any one time
(Table 3). The cycle involves the transfer of water
in its various forms of liquid, vapor, ice, and
snow through the land, air, and water environments. Both matter and energy are involved in
the transfer. The transfer begins when heat from
the sun warms the ocean and land surfaces and
causes water to evaporate. The water vapor enters
the atmosphere and generally moves with the
The water cycle (Figure 21) is important in the
context of biogeochemical cycling in the C, N, P, S,
and O system, as well as being important in its
own right. Water circulating through the ecosphere
is part of a continuous hydrologic cycle that makes
life on earth possible. The water cycle is the driver
cycle for transport of many elements at the earth’s
surface. In the atmosphere, water vapor is the most
important greenhouse gas, and its behavior during
a global warming is of concern.
Figure 21. The global biogeochemical cycle of water (the hydrologic cycle), showing the major processes of water movement.
Numbers in parentheses show the water budget in thousands of cubic kilometers per year (after Maurits la Rivière, 1990.)
Atmospheric water vapor transport
(40)
So
lar
rad
iat
Precipitation of
Snow, ice, rain
(111)
(71)
ion
Cloud
Evaporation
r
cie
Su
a
Gl
rfa
ce
ru
Transpiration
no
Evaporation
(425)
ff
Surface water
Lake
River
Precipitation
(385)
Soils
Return flow (40)
Percolation
Land
Ocean
Groundwater flow
41
Understanding Global Change: Earth Science and Human Impacts
circulation of the air. On a global
Table 3. Distribution of water in the ecosphere
scale, warm air rises in the atmosphere and cooler air descends. The
Percent of total
Reservoir
Volume (106 km3)
________________________________________________________________
water vapor rises with the warm
Ocean
1370
97.25
air. The farther from the warm planCryosphere
etary surface the air travels, the
(ice caps and glaciers)
29
2.05
cooler it becomes. Cooling causes
Groundwater
9.5
0.68
water vapor to condense on small
Lakes
0.125
0.01
particles (cloud condensation
Soils
0.065
0.005
nuclei) in the atmosphere and to
Atmosphere
0.013
0.001
precipitate as rain, snow, or ice and
Rivers
0.0017
0.0001
fall back to the earth’s surface.
When the precipitation reaches the
Biosphere
0.0006
0.00004
___________________________________________________________
land surface, it is evaporated directTotal
1408.7
100
ly back into the atmosphere, runs
off or is absorbed into the ground,
After Berner and Berner, 1996.
or is frozen in snow or ice. Also,
plants require water and absorb it,
retaining some of the water in their
tissue. The rest is returned to the atmosphere
the planet will probably lead to more water
through transpiration. Precipitation on land is
vapor in the atmosphere. The increased water
equal to evapotranspiration plus runoff to the
vapor has the potential to absorb more infrared
ocean. That is, over the land, there is more preradiation reradiated from the planetary surface
cipitation than evapotranspiration.
and thus lead to further warming. This is another
For the ocean, the situation is reversed. Much
example of a positive feedback.
of the water evaporated from the ocean returns
Water droplets form in the troposphere by
there directly; however, a small amount (about
condensation of water on cloud condensation
8% of that evaporated) is carried by atmospheric
nuclei. These particles may absorb or reflect enerwinds over the continents, where it precipitates.
gy. The amount of water vapor in part deterOnce on the ground, the water finds its way to
mines the types and distribution of clouds that
streams, lakes, or rivers in runoff or by percolaform in the atmosphere. In terms of predicting
tion into and through groundwater. In due time,
the effects of increasing concentrations of greenthe water will return to the ocean, mainly in
house gases in the atmosphere on temperature
stream and river flows and less importantly in
and other climatic variables, general circulation
groundwater. This return flow balances the net
models (GCMs) and other types of models have
loss from the ocean surface by evaporation.
been used. The GCMs are very complex computSnow and ice may remain on the land for a
er representations of the atmosphere or atmolong time before the water in these forms of presphere-ocean system that are used in the modelcipitation evaporates to the atmosphere or
ing of global climate change. In these models, the
returns via rivers or as direct glacial meltwater to
effects of clouds on the radiant energy budget of
the oceans. The snow and ice that feed glaciers
the planet are a major source of uncertainty in
may remain locked up in the cryosphere for
attempting to predict future climate change.
thousands of years, but finally the ice will melt,
Clouds regulate the radiative heating of the
and the water will travel to another part of the
planet. They reflect a significant part of the
hydrologic cycle.
incoming solar radiation. Clouds also absorb
Water vapor in the atmosphere is the princilongwave, infrared radiation emitted by the earth.
pal greenhouse gas. Because the amount of water
At the cold tops of clouds, energy is emitted to
vapor in the atmosphere is dependent on the
space. In 1984 the Earth Radiation Budget
temperature of the planet, any initial warming of
Experiment (ERBE) was launched. This
42
Global Biogeochemical Cycles and the Physical Climate System
GCMs only perform calculations at widely
separated points over the globe and relatively
infrequently. Cloud formation involves very
dynamic processes at short time and spatial
scales.
Clouds may act as a positive or negative feedback in a future earth warmed by the enhanced
greenhouse effect. This ambiguity accounts in
part for the range of estimates of the average
temperature increase predicted by the GCMs for
a doubling of the atmospheric carbon dioxide
concentration.
A final comment on the water cycle is that it
is being significantly affected by water usage and
contamination of water stocks. Currently, humans
use an amount of water equivalent to about 25%
of total terrestrial evapotranspiration and 55%, or
6,800 cubic kilometers per year, of the runoff of
water from the continents that is accessible. Only
about 20% of the world’s drainage basins have
pristine water quality. There is little doubt that
the world’s water resources will be significantly
strained in the 21st century.
experiment involves a system of three satellites
that provide data on incoming and outgoing radiation. One result of this experiment so far is the
demonstration that clouds have a net cooling
effect on the earth. On a global scale, clouds
reduce the amount of radiative heating of the
planet by –13 watts per square meter. This is a
large number when compared to the +2.5 watts
per square meter attributed to the increase in
atmospheric greenhouse gases during the last
century. It is also large compared to the radiative
heating that could arise from a doubling of
atmospheric carbon dioxide concentrations in the
next century—about +4 watts per square meter.
Thus, a small change in the types and distribution
of clouds may have a large effect on the radiation
budget compared to the effect of changing greenhouse gas composition owing to human activities.
In a world already warmed by greenhouse
gases released from human activities, it is difficult
to predict what will happen to the types and distribution of clouds. Too little is known about the
complex processes of cloud formation and, in particular, the response of clouds to a warming earth.
Also, these complex processes are difficult to simulate in the general circulation models. One difficulty lies with the problem of reproducing cloud
physics in the models. A second difficulty is that
43
Global Change
Instruction Program
Conclusion
An analogy is commonly made between the
earth’s surface system and a giant chemical engineering factory. In the natural system, material
circulation is driven by energy from the sun and,
to a much lesser extent, from radioactive decay of
elements in the earth’s interior and motions of its
tides. This is a mechanical and inorganic view of
the earth. In another and more realistic sense,
the earth has a natural metabolism; materials
have circulated about its surface for millions of
years in a complex, interconnected web of biogeochemical cycles. An array of physical, chemical, and biological processes weather and erode
rocks and transfer materials in and out of the
atmosphere, from the atmosphere to the biota
and back again, to the oceans via rivers, and to
the continents by uplift. Each element has a natural biogeochemical cycle. It is these cycles and
their relationship to the physical climate system
that have led to the development of a relatively
stable and resilient surface system during
geologic time. Life has evolved in this system
and plays a strong role in the development and
maintenance of the system through processes,
fluxes, and feedbacks.
Human activities have contributed materials
to the biogeochemical cycles. Some of these materials enter element cycles already naturally in
operation; they are the same chemical species that
have circulated for millions of years. Other materials are synthetic compounds and are foreign to
the natural environment. These anthropogenic
fluxes are leading to a number and variety of
environmental issues, including the possibility of
global climate change. There is no doubt that
human activities have interfered in biogeochemical cycles and have modified the composition of
the atmosphere. Understanding the consequences
of this interference requires better quantitative
descriptions of these cycles, their interconnections, and, in particular, their coupled response to
perturbations, such as a change in climate.
44
Global Change
Instruction Program
Study Questions and Answers
Questions
Chemistry
1. The most simple chemical expression for the production of organic matter in plants is
CO2 + H2O = CH2O + O2
The chemical compound CH2O is organic matter; CO2 is carbon dioxide gas; O2 is diatomic
oxygen; and H2O is water. The atomic weights of the elements C, H, and O are, respectively, 12, 1,
and 16.
A. What are the gram molecular weights of the compounds of CH2O, CO2, O2, and H2O?
B. What is the weight of one mole of each of these compounds?
C. If 10 moles of plant matter have been produced, how many moles of CO2 did it take to produce
the plant matter? How many grams of CO2?
D. If the C:N ratio of the plant material in moles is 106:16 (that of marine phytoplankton), how many
moles of nitrogen were needed to produce the plant matter?
E. If the source of the nitrogen were nitrate (NO3-) in the euphotic zone of the ocean, how many
grams of nitrate were consumed in the process?
2. Write a balanced chemical reaction for the weathering of the mineral orthoclase feldspar (KAlSi3O8)
by water containing dissolved CO2. The products that are formed in this weathering reaction are:
kaolinite [(Al2Si2O10(OH)4], bicarbonate (HCO3-), monomeric silicic acid (H4SiO4o), and dissolved
potassium ion (K+). (Hint: to begin, balance the reaction on aluminum.)
3. The concentration of dissolved potassium (K+) in the ocean is 390 mg/kg. The atomic weight of
potassium is 39. The average density of seawater is 1.027 g/cm3.
A. What is the concentration of K+ in seawater in moles/kg?
B. What is its concentration in parts per million by volume of seawater?
4. The average molecular weight of the gases in the atmosphere is 29 and the mass of the atmosphere
is 52 x 1020 grams.
A. What is the total number of moles of gases in the atmosphere?
B. Carbon dioxide gas makes up 0.036 % of the atmosphere in moles. How many moles of CO2
are there in the atmosphere? How many grams?
45
Understanding Global Change: Earth Science and Human Impacts
5. Water vapor in the atmosphere averages about 0.2% of the atmosphere in weight.
A. What is the total mass of water vapor in the atmosphere?
B. How many moles of H2O vapor are there in the atmosphere?
The Oceans, Atmosphere, Sediments, and Rocks
1. The average depth of the ocean is 3.8 kilometers and the average upwelling rate of deep water into the
surface open ocean and into coastal environments is 4 m/yr. The area of the ocean is 3.6 x 1018 cm2.
A. About how long would it take to upwell the entire volume of the ocean?
B. The average nitrogen content of deep water is about 40 x 10-6 moles per liter (L). What is the
annual rate of addition of nitrogen to the surface water due to upwelling?
C. At a molar ratio of C:N of 106:16, what is the productivity in the global surface ocean in grams (g)
C/m2/yr sustained by the upwelling of nitrogen?
2. One source of the deep water of the world’s oceans is in the Norwegian Sea. Here water is sufficiently dense to sink to the bottom. This water mainly forms from the cooling by evaporation of water
carried northward by the Gulf Stream. The Gulf Stream carries heat to the high latitudes of the
North Atlantic Ocean which helps to moderate the climate of Europe. There is considerable interest
in the rate of formation of North Atlantic deep water (NADW) because of the role it plays in climate.
Possible changes in the rate of its formation have been cited as the cause of rapid climate change in
the past. Any global warming could modify the rate of deep water formation.
A. How would you expect the residence time of the deep water to change from the Atlantic Ocean to
the Pacific Ocean?
B. How does the deep water return to the surface?
C. If the continental glaciers were to begin melting because of a global warming, what would you
expect might happen to the rate of deep water formation and the climate of Europe?
3. The winds of the earth tend to blow along latitudinal lines around the planet. The major wind belts are
the Polar Easterlies, Westerlies, Trade Winds, and Equatorial Easterlies in both the Northern and
Southern Hemispheres. The winds drive the surface currents of the ocean. Cool air that has descended
at midlatitudes moves toward the equator as the Northeast and Southeast Trade Winds in the Northern
and Southern Hemispheres, respectively. These winds exert a force on the sea surface, and currents are
generated that flow toward the equator. Both the winds of the atmosphere and the surface currents of
the ocean converge near the equator. The warm equatorial air rises and moves north and south toward
the poles. The converging ocean currents generate westward-flowing equatorial currents.
A. Based on the above, would you expect a pollutant gas like carbon monoxide with major anthropogenic sources in the Northern Hemisphere and a short atmospheric residence time to be evenly
distributed in the troposphere?
B. Is there an equatorial barrier to the dispersal of floating tar balls produced when petroleum
tankers spill oil in the shipping lanes of the North Atlantic?
C. Does such a barrier exist for the deep waters of the ocean?
46
Global Biogeochemical Cycles and the Physical Climate System
4. The very finest particles of airborne dust carried by winds off the Sahara Desert travel in the troposphere for long distances westward across the Atlantic Ocean. These particles are deposited on the
ocean surface and settle out at a rate of 500 cm/yr. How long would it take such particles to reach
the bottom of the Atlantic Ocean at 4 km?
5. The global atmosphere in 1996 contained about 360 ppmv of CO2. This concentration is equivalent to
a partial pressure of CO2 (PCO2) of 10-3.45 atmosphere (atm).
A. What is the concentration of dissolved CO2 in the surface ocean in equilibrium with atmospheric
CO2 at a temperature of 25°C?
B. In the year 1700, when atmospheric CO2 was at a concentration level of 280 ppmv (PCO2 = 10-3.55
atm), what was the concentration of CO2 in the surface ocean?
C. What was the percentage change in atmospheric CO2 concentration between 1700 and 1996?
6. The primary energy sources for the earth are:
• solar radiation, 0.5 cal/cm2/min (about 343 W/m2)
• heat flow from the interior of the earth, 0.9 x 10-4 cal/cm2/min
• tidal energy, 0.9 x 10-5 cal/cm2/min.
About 49% of the solar radiation is absorbed by earth’s surface and reradiated to space as longwave,
infrared radiation.
A. What percentage of the total comes from the combination of heat from the interior of the earth
and tidal energy?
B. In units of W/m2, how much solar radiation reaches the earth’s surface and is absorbed there?
What does this energy do?
C. Of the 388 W/m2 of longwave radiation emitted to space by the earth’s surface, 326 W/m2 are
absorbed by water vapor, CO2, and other greenhouse gases in the atmosphere and reradiated
back to the earth. What happens to this energy, and what effect does it have on the earth?
7. The area of land today is 150 x 1012 m2, and the mean elevation of the continents is 0.84 km. The continents are being eroded at a rate of 200 x 1014 g/yr. The average density of rock is 2.7 g/cm3.
A. At the current rate of erosion, how long would it take to wear the continents down to sea level?
B. Your answer to question 7A was a period of time much less than the age of the earth of 4.6 billion
years. In that case, why are there any continents?
8. The total mass of sedimentary rock younger than 600 x 106 years is 1.8 x 1018 metric tons. Its mean
residence time is 400 x 106 years.
A. In an unchanging system, what is the flux of sediment in and out of the sedimentary rock reservoir?
B. How does sediment get into the sedimentary rock reservoir and how does it get out?
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Understanding Global Change: Earth Science and Human Impacts
9. What chemical species of nitrogen, sulfur, and carbon might you expect to find in a water-logged
(anoxic) soil? In a well-aerated soil?
10. What does the weathering of silicate minerals subtract from or add to the atmosphere?
Ecology
1. Tropical rain forests have a total area of 17 x 1012 m2, and estuaries have an area of 1.4 x 1012 m2.
Their mean net primary production is 2,000 g dry plant matter/m2/yr and 1,800 g dry plant
matter/m2/yr, respectively. Their mean plant biomasses in kg C/m2 are 20 and 0.45, respectively.
Forty-five percent of dry plant matter is carbon.
A. What is the total net primary production of tropical rain forests and estuaries in metric tons of dry
plant matter and carbon per year?
B. What is the total plant mass of these ecosystems in metric tons of dry plant matter and carbon?
C. The tropical rain forests of the world lost 9% of their area due to cutting in the 1980s. How much
dry plant matter does this cutting represent? How much carbon?
D. If all of the carbon in the cut trees of question 1C were emitted to the atmosphere by burning and
slow oxidation, how many grams of CO2 would this represent? What fraction of the atmospheric
CO2 reservoir is this (see Figure 11)?
E. Estuaries receive 1.5 x 1012 g of pollutant dissolved phosphorus annually (see Figure 18). This
amount of P could support how much additional plant productivity as dry matter and as C/m2
per year?
F. If all the additional plant matter in 1A were buried in the sediments of the estuaries, would this
flux qualify as a biological feedback to the accumulation of CO2 in the atmosphere? Explain. What
percentage of the atmospheric flux of 6 x 1015 g C from fossil fuel burning in 1995 does the additional total plant production in estuaries represent?
2. The net primary production of the earth’s surface is 0.37 kg dry matter/m2/yr.
A. With a total area of 510 x 1012 m2, what is the total production of dry matter?
B. How much carbon does the production in 2A represent?
C. Total production on land exceeds that in the ocean by perhaps as much as two times. What is the
annual total production on land and in the ocean?
3. What is the difference between autotrophy and heterotrophy?
4. What are three biogeochemical processes performed by the prokaryotes?
5. What is cultural eutrophication? Why may this phenomenon qualify as a negative feedback to the
accumulation of CO2 in the atmosphere?
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Global Biogeochemical Cycles and the Physical Climate System
Biogeochemical Cycles
1. Construct a diagram showing the reservoirs and fluxes in the global biogeochemical cycle of water.
Use the information in Figure 21 and Table 3, and the reservoirs of atmosphere, land, and ocean.
A. What is the residence time of water in the ocean with respect to the flux of water via the rivers to
the ocean?
B. If the concentration of dissolved calcium is 400 ppm in the ocean and 15 ppm in average river
water, what is the residence time of calcium in the ocean?
2. A nearly rectangular-shaped lake is 5 km long, 2 km wide, and 100 m deep and contains 0.001 mg/L
of dissolved mercury. A river discharges 2 x 1012 L/yr of water with a concentration of mercury of
0.0005 mg/L into the lake.
A. What is the water volume in the lake (in liters)?
B. What is the total mass of mercury in the lake?
C. What is the residence time of mercury in the lake?
D. An industrial plant near the mouth of the river accidentally discharges mercury into the lake. How
long will it take for most (95%) of the mercury contamination to work its way out of the lake?
3. The terrestrial living biomass contains 600 x 109 tons of carbon. It is argued that during the decade of
the 1980s, about 135 x 1012 moles of anthropogenic CO2 were taken up annually by the terrestrial
biomass.
A. What would be the average annual percentage increase in the mass of carbon in the terrestrial
biosphere?
B. Do you believe such an increase would be detectable by doing field studies to determine the
increase in biomass?
4. The mixed layer of the ocean is the vertical layer that is well stirred by winds blowing across the surface of the sea. Chemical and physical characteristics of the water column are rather uniform over
the depth of the mixed layer. The average thickness of the mixed layer throughout the world’s
oceans is about 100 meters but varies from 50 to 300 meters. The average total dissolved inorganic
carbon (DIC, HCO3- + CO32- + CO2) content of the mixed layer is 2.2 micromoles (mmol) per kg.
A. What is the total mass (reservoir) of DIC in the mixed layer of the ocean in moles of carbon? (The
area of the ocean is 360 x 1012 m2.)
B. The ocean takes up about 2 billion tons of carbon annually from the human activities of fossil fuel
combustion, cement manufacturing, and deforestation. What is the annual increase in dissolved
carbon in the mixed layer of the ocean in mmol/kg due to this absorption of anthropogenic CO2?
C. Use the annual increase you derived from 4B and assume the DIC content of the mixed layer was
2.2 mmol/kg in 1975. What would be the percentage increase in the DIC content of the mixed
layer from 1975 to 1996?
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Understanding Global Change: Earth Science and Human Impacts
5. Total evaporation is 496 x 103 km3 H2O/yr over the earth’s surface.
A. What is the residence time of water in the atmosphere?
B. If a pollutant is susceptible to being rained out (washed out) of the atmosphere, would you expect
it to mix evenly throughout the troposphere?
6. What are the three major processes controlling atmospheric CO2 concentration levels on a long time
scale? Write a balanced set of chemical equations demonstrating these processes.
7. What is the connection between organic matter and atmospheric O2?
8. What is the major difference between the biogeochemical cycle of phosphorus and those of carbon,
nitrogen, and sulfur?
9. What is the major difference in the reactivity of DMS and OCS in the atmosphere?
10. The mean atmospheric lifetime (residence time) of NH3 is 14 days; that of CO is 60 days. How do the
rates of destruction of these gases in the atmosphere by OH* compare qualitatively?
11. Referring to Figures 9–11, what is the major reaction in the atmosphere that couples the biogeochemical
cycles of CH4, CO, and CO2? What percentage of the enhanced greenhouse forcing on climate is due to
CO2? To CH4? What are the sources of emissions of CH4 to the atmosphere from human activities? Of
CO2 and CO? If the concentration of CO2 in the atmosphere were doubled, what would be the potential increase in temperature? How much more effective is CH4 than CO2 as a greenhouse gas?
12. Referring to Figure 12, what is the ratio of anthropogenic N fluxes on land involving fixation of
atmospheric N to natural biological fixation? What is the minimum percentage of the N fixed by
human activities that is discharged to the ocean by rivers? What is the important environmental
problem related to this additional N flux to the ocean?
13. Referring to Figure 13, what is the ratio of these two fluxes: the upwelling flux of N to the coastal zone and
the dissolved N flux to the ocean via rivers? Which flux would you expect to change during the next
century?
14. Referring to Figure 14, based on the summation of the high and low estimates of fluxes of N2O from
earth’s surface to the atmosphere, what is the range in residence time estimates for N2O in the
atmosphere? What percentage of the enhanced greenhouse forcing of climate is due to N2O?
15. Referring to Figure 16, of the total flux of NOx to the atmosphere, what percentage is from human
activities? What is the principal type of reaction that destroys NOx in the atmosphere? Write the
reaction for the destruction of NO2 and its subsequent removal as HNO3.
16. Referring to Figure 18, the total mass of P in land plants is 1,800 million tons and that in marine
plankton is 73 million tons. What is the ratio of the internal recycling flux in the ocean to that on land?
What is the residence time of P in the land biota relative to the recycling flux? In the marine plankton?
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Global Biogeochemical Cycles and the Physical Climate System
Answers
Chemistry
1. This question is designed to enable the student to review basic knowledge of the meaning of a chemical equation. Many biogeochemical processes are simply described by chemical reactions. In this
reaction, there are four pure substances that can be decomposed by a chemical change, that is, four
chemical compounds: CO2(gas), H2O(liquid), CH2O(solid), and O2(gas). The atoms in these compounds
may separate from one another and rearrange themselves during a chemical reaction. In this case, in
the presence of light and available nutrients, plant material has formed from CO2 and H2O, and the
carbon atom in CO2 has been rearranged into the carbon atom of CH2O. This reaction further shows
that all chemical equations must balance. The total number of atoms of an element on the left-hand
side of an equation must equal the total number of atoms of that element on the right-hand side of
the equation.
A. The atomic weights of the elements in these compounds are C = 12, H = 1, O = 16. Thus, the summation of these weights, or the gram molecular weight, for CH2O is 12 + (2 x 1) + 16 = 30 g; for
O2, 2 x 16 = 32 g; for CO2, 12 + (2 x 16) = 44 g; and for H2O, (2 x 1) + 16 = 18 g.
B. The weight of one mole of these compounds is CH2O = 30 g; O2 = 32 g; CO2 = 44 g; and H2O = 18 g.
C. The relationships between the compounds as shown in the equation must always be preserved;
thus one unit—that is, one mole of the reactant compound CO2—must react with an equivalent
number of moles of the reactant compound H2O to give you a ratio of product compounds of
CH2O:O2 = 1:1. Thus, the formation of 10 moles of plant material requires 10 moles, or 10 moles x
44 g/mole = 440 g of CO2.
D. At a molar ratio (that is, a ratio of moles) of C:N = 106:16, the moles of nitrogen are 16/106 x 10
moles = 1.51 moles.
E. The atomic weight of N is 14. The reaction required 1.51 moles of N or 1.51 moles of NO3-. (There
is 1 mole of N in 1 mole of NO3-.) The molecular weight of NO3- is 14 + (3 x 16) = 62 g. Thus, the
grams of nitrate consumed were 1.51 moles NO3- x 62 g NO3-/mole = 93.6 g NO3-.
2. This question again deals with chemical reactions. The equation to be balanced represents the weathering of a common mineral found at the surface of the earth. The student not only learns to balance a
reaction on the basis of atoms but also on the basis of charge. The reason for starting the balance on
aluminum is that solids containing aluminum are very insoluble at the temperature and pressure of
weathering. Thus, it is assumed that all the aluminum from the weathering of orthoclase feldspar is
simply transferred to the solid weathering product kaolinite. Of course, this is not true, but it is a
reasonable approximation.
2KAlSi3O8 + 2CO2 + 11H2O = Al2Si2O5(OH)4 + 2 K+ + 2HCO3- + 4H4SiO40
3. Dissolved potassium is the sixth most important dissolved constituent in seawater. Its concentration is
important to biological processes, although it is a minor essential element for phytoplankton productivity.
A. The concentration of K+ in the ocean in moles/kg = 390 x 10-3 g/kg seawater ÷ 39 g/mole = 10 x
10-3 moles/kg seawater or 10 millimoles.
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Understanding Global Change: Earth Science and Human Impacts
B. The average density of seawater is 1.027 g/cm3. Thus, 1 kg of seawater is equivalent to 1,000 g of
seawater ÷ 1.027 g/cm3 = 973.7 cm3 of seawater. The concentration in parts per million by volume
of seawater is 390 x 10-3 g/kg seawater x 1.027 x 10-3 kg/cm3 x 1,000 cm3/L seawater = 400.5 x
10-3 g/L = 400.5 mg/L = 400.5 ppmv of seawater.
4. The atmosphere is an important reservoir that exchanges materials with the earth’s surface. Its composition has changed on a geological time scale and on the human scale of generations because of the activities of human society. This problem illustrates the fact that although atmospheric CO2 accounts for 60% of
the enhanced greenhouse effect, it represents a relatively small part of the total atmospheric mass.
A. The mass of the atmosphere can be obtained from the weight of air above 1 cm2 of the earth’s surface and the total area of the earth: 1,031 g/cm2 x 5.1 x 1018 cm2 = 52 x 1020 g. The total number of
moles of gases in the atmosphere is 52 x 1020 g ÷ 29 g/mole = 1.8 x 1020 moles.
B. The number of moles of CO2 is 0.00036 x 1.8 x 1020 moles = 64.8 x 1015 moles. The mass of CO2 in
grams is 64.8 x 1015 moles x 44 g/mole = 2,850 x 1015 g = 2,850 x 109 metric tons = 2,850 gigatons
x (12 ÷ 44) = 777 gigatons of C ÷ 12 g/mole = 64.8 x 1015 moles C.
5. Water vapor is the most important greenhouse gas, yet as the following calculation shows, it is a
small portion of the mass of the atmosphere.
A. The total mass of water vapor in the atmosphere is 0.002 x 52 x 1020 g = 10.4 x 1018 g.
B. There are 10.4 x 1018 g ÷ 18 g/mole = 57.8 x 1016 moles of water vapor.
The Oceans, Atmosphere, Sediments, and Rocks
1. Upwelling is an important process in the ocean. It involves the upward movement of water and dissolved constituents from depth in the ocean to the surface. Upwelling occurs in the coastal regions of
offshore Peru, California, Namibia, Mauritania, and Somalia, and in open ocean equatorial regions
and the high latitudes of the Southern Hemisphere.
A. To upwell the volume of the ocean with an average depth of 3,800 m at the mean upwelling rate
of 4 m/yr would take 3,800 m ÷ 4 m/yr = 950 years.
B. 400 cm of water rises 1 cm2/yr. With an ocean area of 3.6 x 1018 cm2, this is equivalent to a volume of water of 1,440 x 1018 cm3/yr or 1,440 x 1015 L/yr x 40 x 10-6 moles N/L = 57.6 x 1012
moles N/yr x 14 g/mole = 806 x 1012 g N/yr.
C. 806 x 1012 g N/yr ÷ 14 g/mole = 57.6 x 1012 moles of N/yr. At a C:N ratio of 106:16 = 382 x 1012
moles C/yr x 12 g/mole = 4.6 x 1015 g C/yr ÷ 360 x 1012 m2 = 13 g C/m2/yr. This productivity is
only about 10% of global marine net primary productivity (Figure 11). This calculation illustrates
the fact that much of the nitrogen used in biological productivity in the euphotic zone of the
ocean comes from recycling of the N within this zone.
2. The formation of the deep water of the ocean is part of the conveyor belt circulation pattern of the
world oceans. This pattern is not well known from observations; our understanding of it is based
mainly on theoretical models. The North Atlantic deep water (NADW) flows southward at depth
from its source in the high latitudes of the North Atlantic Ocean and meets a northward-flowing current (the Antarctic Bottom Water, ABW) whose water originated by sinking in the Weddell Sea near
Antarctica. The currents merge and part of the water flows into the deep Indian Ocean and Pacific
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Global Biogeochemical Cycles and the Physical Climate System
Ocean. Water upwells to the surface in all basins of the ocean. In addition, warm water returns from
the Pacific and Indian Oceans into the Atlantic Ocean at about the depth of the thermocline. We
believe these return flows to the Atlantic Ocean occur through the Drake Passage between Antarctica
and South America and through the Bering Sea into the Arctic Ocean and thence to the Atlantic
Ocean. The return flows in the high latitudes of the North Atlantic sink as the NADW.
A. Because of this pattern of circulation, the deep water of the world’s oceans generally gets older
from the Atlantic Ocean to the Indian Ocean to the Pacific Ocean. The time the water stays out of
contact with the atmosphere, that is, its residence time, increases toward the Pacific Ocean. The
residence time of the bottom waters of the Atlantic Ocean is 200–500 years; that of the Pacific
Ocean is 1,000–2,000 years.
B. By upwelling as described above.
C. This is a difficult question and one that is asked by a number of scientists today. The answer is of
concern to the world’s people because of the link to climate. It is likely that any substantial melting of sea ice and the continental glacier of Greenland would add fresh water to the surface of the
ocean in the high latitudes of the North Atlantic for some period of time. This would affect the
rate of deep water formation because of the change in the salt content of surface ocean water. In
turn, the pattern of the conveyor belt circulation of the ocean could be altered. One suggestion is
that there would be a less intense flow of warm water northward by the Gulf Stream, and the climate of Europe would cool.
3. A. Because of the short residence time of CO in the atmosphere (about 70 days) and the fact that
the upwelling of air near the equator effectively separates air exchange in the troposphere
between the two hemispheres, the gas would not be evenly distributed. Higher concentrations
would be found in the Northern Hemisphere troposphere than in the Southern Hemisphere. In
fact, observations show a strong gradient in the concentration of CO between the two hemispheres, with higher concentrations in the North.
B. The barrier is the convergence of the North Equatorial Current and the South Equatorial Current
near the equator and their westward flows. This converging pattern inhibits exchange of surface
waters between the Northern Hemisphere and the Southern Hemisphere. Floating tar balls in the
North Atlantic would be caught in the North Equatorial Current and transported westward to the
northward-moving Gulf Stream.
C. No, the NADW moves southward at depth in the North Atlantic Ocean into the South Atlantic
Ocean. The ABW moves north at depth. Neither current is involved with surface ocean currents.
4. The fine dust particles would take 4 km x 105 cm/km = 4 x 105 cm ÷ 500 cm/yr = 800 years. In actual fact, they settle much faster because they are encapsulated in the fecal pellets of animal plankton
(zooplankton) in the ocean. During feeding the zooplankton inadvertently pass them through their
guts and excrete them contained in bigger mucilaginous fecal particles that sink at rates of 350 m/day.
5. To answer this question requires an understanding of some basic chemistry. The equilibrium
between a gas and a solution is normally given by Henry’s Law, which states that the concentration
of the gas in the solution equals a constant (known as the Henry’s Law constant) times the partial
pressure of the gas. For CO2, the expression is
[CO2] = KHPCO2
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Understanding Global Change: Earth Science and Human Impacts
The brackets around CO2 denote its concentration in moles/L. The partial pressure of CO2 (PCO2) is
in atmospheres. The value of KH varies with the composition of the solution and the temperature.
For seawater at 25°C, KH equals 10-1.54.
A. Carbon dioxide dissolves in seawater to an extent determined by the CO2 concentration in the
atmosphere and the reactions that occur in the seawater. At 25°C and with an atmospheric CO2
concentration of PCO2 = 10-3.45 atm, the relationship is
[CO2] = 10-1.54 x 10-3.45 = 10-4.99 mole/L
B. In 1700 for a PCO2 = 10-3.55 atm, we have
[CO2] = 10-1.54 x 10-3.55 = 10-5.09 mole/L
C. The percentage change is [(10-4.99 mole/L – 10-5.09 mole/L) ÷ 10-5.09 mole/L] x 100 = 26%. Thus,
the dissolved CO2 concentration in the surface ocean has changed by this percentage over the
past 300 years because of fossil fuel combustion and biomass burning.
6. A. The percentage is [(0.9 x 10-4 cal/cm2/min + 0.9 x 10-5 cal/cm2/min) ÷ 0.5 cal/cm2/min + 0.9 x
10-4 cal/cm2/min + 0.9 x 10-5 cal/cm2/min] x 100 = 0.02%.
B. The amount of energy reaching the earth’s surface and absorbed is 343 W/m2 x 0.49 = 168 W/m2.
This energy is used to heat the atmosphere and surface of the earth; to evaporate water; to generate rising air masses (thermals); to drive wind, waves, and currents; and for photosynthesis.
C. The energy is reabsorbed, keeping the earth warm.
7. A. Provided there were no processes restoring the continents, they would be reduced to sea level by
erosion in 150 x 1012 m2 x 840 m = 126 x 1015 m3 x 106 cm3/m3 = 126 x 1021 cm3 x 2.7 g/cm3 =
340 x 1021 g ÷ 200 x 1014 g/yr = 17 x 106 years.
B. This is a question that has plagued geologists for two centuries. There must be processes that add
mass to the continents to keep them above sea level. Certainly lavas originating in the interior of
the earth add material to the continents. In subduction zones, not all the rock of the oceanic crust
is transported down toward the interior of the earth. Some of it is added to the continents and
increases their area and thickness. Finally, continents often collide during their movement about
the earth’s surface. In the collisions, the continents act as a great vise, squeezing sediments originally derived from their erosion and other sources into high mountain ranges. This action adds
volume back to the continents. The collision of India with Asia followed by the formation of the
Himalayan Mountains is an example of such an event. Thus, there is a great rock cycle at work in
which the continents are eroded and their materials deposited in the ocean. The sediments of the
oceans are buried to great depths or transported to subduction zones. In either case, the sedimentary material is eventually returned to the continents to be uplifted to their surfaces and eroded,
completing the cycle.
8. A. This question simply uses the concept of residence time, where λ = mass ÷ flux; therefore, flux =
mass ÷ λ = 1.8 x 1018 metric tons ÷ 600 x 106 yr = 3 x 109 metric tons/yr = 30 x 1014 g/yr.
Interestingly, this flux is much less than that of erosion and thus deposition in the oceans today. This
implies that today is somewhat unusual in terms of geologic history. We know that to be the case from
other geological information.
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Global Biogeochemical Cycles and the Physical Climate System
B. Sediment (clay, mud, silt, sand, gravel, and skeletal and organic materials) enters the sedimentary
rock reservoir by erosion of rocks, transportation of the eroded debris, and subsequent deposition
on the seafloor, followed by burial in some cases to depths of 12 km. The sediment “gets out” by
uplift due to plate tectonic movements and re-erosion of the sedimentary mass.
9. The chemical species in anoxic soils are chiefly reduced forms of the compounds CH4, CO, NH3, and H2S
(see Figures 9, 10, 15, and 19). CO2 occurs as well, although it is a relatively oxidized form of carbon. In a
well-aerated soil, we might expect O2, CO2, N2, and SO2.
10. Your answer to Question 2 in the Chemistry section shows that the weathering of silicate minerals
subtracts CO2 from the atmosphere. This subtraction must be balanced by an addition of CO2 from
other processes or the atmosphere would run out of CO2 in about 6,000 years. Those processes
involve hydrothermal reactions at midocean ridges and the subduction and/or burial of carbonate
minerals to realms of higher pressure and temperature, where carbonates are converted to silicates,
and CO2 is released back to the atmosphere by volcanism and other processes.
Ecology
1. This question is designed to give the student a feeling for some of the characteristics of two important ecosystems that are being severely impacted by human activities. These activities are the deforestation of tropical rain forests and their conversion to pasture and urban areas and the eutrophication of coastal marine environments.
A. The total net primary production of tropical rain forests and estuaries is:
Rain forests: 17 x 1012 m2 x 2,000 g dry matter/m2/yr = 34 x 1015 g dry matter x 0.45 = 15.3 x 1015 g C.
Dividing by 106 g/metric ton, we get 34 x 109 metric tons dry matter and 15.3 x 109 metric tons C.
Estuaries: 1.4 x 1012 m2 x 1,800 g dry matter/m2/yr = 2.52 x 1015 g dry matter x 0.45 = 1.13 x 1015 g C.
Dividing by 106 g/metric ton, we get 2.52 x 109 metric tons dry matter and 1.13 x 109 metric tons C.
Notice that although the NPP of tropical rain forests and estuaries is similar, the smaller area of estuaries leads to more than an order of magnitude difference between the total net primary production
of the two ecosystems.
B. The total plant mass (biomass) of these ecosystems is:
Rain forests: 17 x 1012 m2 x 20 kg C/m2 = 340 x 1012 kg C ÷ 103 kg/metric ton = 340 x 109 metric
tons C ÷ 0.45 = 755 x 109 metric tons dry matter.
Estuaries: 1.4 x 1012 m2 x 0.45 kg C/m2 = 630 x 109 kg C ÷ 103 kg/metric ton = 6.3 x 108 metric
tons C ÷ 0.45 = 1.4 x 109 metric tons dry matter. Notice the orders-of-magnitude difference
between the biomass of tropical rain forests and estuaries.
C. In ten years, 9% of the area of rain forests was lost; this represents 755 x 109 metric tons dry
matter x 0.09 = 68 x 109 metric tons dry matter x 0.45 = 30.6 x 109 metric tons C.
D. The grams of CO2 emitted to the atmosphere in this ten-year period would be 30.6 x 109 metric
tons C x 106 g/metric ton = 30.6 x 1015 g C x (44 ÷ 12) = 112 x 1015 g CO2. This flux represents
(Figure 11) the size of the atmospheric CO2 reservoir of 744 x 1015 g C x (44 ÷ 12) = 2,730 x 1015 g
CO2. 112 x 1015 g CO2 ÷ 2,730 x 1015 g CO2 = 0.041, or 4% of the atmospheric reservoir.
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Understanding Global Change: Earth Science and Human Impacts
The flux would represent about 3 x 1015 g C/yr for the decade of the 1980s. The magnitude of the
flux is too high, meaning that all the woody plant material was not burned to CO2. Some remains
on the ground as waste from the cutting, and some has gone into lumber. The flux of CO2 to the
atmosphere from all land-use activities in tropical rain forests for the 1980s was on the order of
1–1.6 x 1015 g C/yr.
E. At a C:P ratio of 106:1, the additional plant productivity supported by the pollutant P would be
1.5 x 1012 g P/yr ÷ 31 g/mole = 48.4 x 109 moles P/yr x (106 ÷ 1) = 5.13 x 1012 moles C/yr.
Divided by the estuary area of 1.4 x 1012 m2 = 3.7 moles C/m2/yr = 44 g C/m2/yr ÷ 0.45 = 98 g
dry matter/m2/yr, or about a 5% increase in the productivity of the world’s estuaries.
F. In fact, because the pollutant P will be used more than once in plant productivity in estuaries, the
total organic carbon that could be buried in the sediments of estuaries amounts to 140 x 1012 g
C/yr (Figure 18). This flux does qualify as a negative biological feedback because the additional P
has been added to the earth’s surface by the human activities of fertilizer application to croplands
and sewage discharge. Some of this P makes its way to lakes and coastal marine environments
and increases the productivity of such aquatic systems. The flux represents (140 x 1012 g C/yr ÷ 6
x 1015 g C/yr) x 100 = 2% of the fossil fuel flux in 1995. Not much!
2. A. The total production of dry matter is 510 x 1012 m2 x 0.37 kg dry matter/m2/yr = 188.7 x 1012 kg
dry matter/yr.
B. 188.7 x 1012 kg dry matter/yr x 0.45 = 84.9 x 1012 kg C/yr.
C. Let total production in the ocean = X; then production on land = 2X. Thus X + 2X = 188.7 x 1012
kg dry matter/yr, i.e., 3X = 188.7 x 1012 kg dry matter/yr; X = ocean production = 62.9 x 1012 kg
dry matter/yr and 2X = land production = 125.8 x 1012 kg dry matter/yr.
3. Autotrophy is the biochemical pathway by which an organism uses CO2 as a source of carbon and
simple inorganic nutrient compounds of N and P for synthesis of organic matter. In heterotrophy,
more complex organic materials are used as the source of carbon for metabolic processes.
4. Prokaryotes—the Kingdom Monera, including the bacteria and cyanobacteria—take part in a variety
of biogeochemical processes (see Table 1). We often forget the fact that the bacteria are responsible
for the decay of organic matter both on land and in the ocean. In other words, it is their metabolic
activity that returns CO2 and other gases and nutrients back to the environment. The cyanobacteria
are photoautotrophic and produce oxygen as a metabolic byproduct. These organisms were responsible for the initial growth of oxygen in the earth’s atmosphere. The processes include CO2 fixation,
nitrogen fixation, and oxidation of sulfur as examples.
5. Eutrophication is the set of processes leading to the overnourishment of a lake, river, or marine environment; consequent rapid plant growth and death; and oxygen deficiency of the system. This is a
natural set of processes in certain environments. When it occurs because the excess nutrients come
from fertilizers, sewage, detergents, etc., it is called cultural eutrophication. This term distinguishes
the natural situation from that produced by the activities of people.
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Global Biogeochemical Cycles and the Physical Climate System
Biogeochemical Cycles
1. See Figure 22.
A. The residence time = λ = mass of
water in the ocean divided by the
flux of water to the ocean by rivers:
1,370 x 106 km3 ÷ 40 x 103 km3/yr =
34,250 years.
B. λ of Ca in the ocean is (1,370 x 106
km3 x 1012 L/km3 x 400 mg/L) ÷ (40
x 103 km3/yr x 1012 L/km3 x 15
mg/L) = 913,000 years.
LAND
Plants
ATMOSPHERE
Animals
Soil
OCEAN
Water
Figure 22.
Biota
Sediment
2. Here we apply the concept of residence time in the context of how long it takes for a lake to recover
from a single input of a chemical into it. This is an environmental problem often encountered in
developing, as well as developed, countries.
A. The volume of the lake is 5 km x 2 km x 0.1 km = 1 km3 x 1012 L/km3 = 1 x 1012 L.
B. The mass of mercury (Hg) is 1 x 1012 L x 1 x 10-6 g Hg/L = 1 x 106 g Hg.
C. The flux of mercury by the river to the lake is 2 x 1012 L/yr x 5 x 10-7 g Hg/L = 1 x 106 g Hg/yr;
thus the residence time of Hg (λHg) in the lake is 1 x 106 g Hg ÷ 1 x 106 g Hg/yr = 1 year.
D. The time to reach a new equilibrium—that is, for the Hg input to work its way through the lake—
is 3 x λHg = 3 x 1 yr = 3 yr. In this period of time, 95% of the Hg input would be gone.
3. This question provides the student with a feeling for the problem of “looking for a needle in a
haystack” that scientists have when observing the change in the size of the terrestrial living biomass
due to uptake and storage of anthropogenic CO2.
A. 600 x 109 tons C x 106 g/ton = 600 x 1015 g C. 135 x 1012 moles C/yr x 12 g/mole = 16.2 x 1014 g
C/yr. The annual % change would be (16.2 x 1014 g C/yr ÷ 600 x 1015 g C) x 100 = 0.27% per year.
B. Most unlikely. For the decade of the 1980s, this would be only a 2.7% change in the mass of living
biomass globally. This amount of change would be difficult to measure by field studies, even if
they were aimed at seeing a change in the amount of organic carbon stored in terrestrial vegetation. However, if the storage continues, such measurements could provide information in a few
years.
4. A. The total DIC in the mixed layer is 360 x 1012 m2 x 102 m = 360 x 1014 m3 x 106 cm3/m3 = 360 x
1020 cm3 x 1.027 g/cm3 = 37 x 1021 g ÷ 103 g/kg = 37 x 1018 kg seawater x 2.2 x 10-3 moles C/kg
seawater = 81.4 x 1015 moles C.
B. 2 x 109 tons C/yr = 2 x 1015 g/yr ÷ 12 g/mole = 166.7 x 1012 moles C/yr ÷ 37 x 1018 kg seawater = 4.5 x
10-6 moles C/kg seawater/yr = 4.5 micromoles C/kg seawater/yr. Actual measurements of the change
in the DIC content of seawater over time show an increase of about 1 micromole per year. The difference is due to the fact that the anthropogenic carbon taken up by the ocean mixes deeper in the ocean
than the average depth of the mixed layer used in the problem, on average about 300–400 meters.
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C. Using 1 micromole per year as the average carbon uptake since 1975, we get 1 x 10-6 mole C/kg/yr
x 21 yr = 21 x 10-6 moles C/kg ÷ 2.2 x 10-3 moles C/kg x 100 = 0.95%. It was not until the late 1980s
that scientists were able to measure these small changes in seawater DIC accurately and precisely.
5. Referring to Table 3, the total amount of water in the atmosphere as water vapor is 0.013 x 106 km3; thus
the residence time of water vapor in the atmosphere calculated with respect to total evaporation (must
equal precipitation) is 0.013 x 106 km3 H2O ÷ 496 x 103 km3 H2O/yr = 0.026 yr x 365 days/yr = 9.6 days.
B. No, the pollutant would not mix evenly throughout the troposphere because of water’s very short
residence time in the atmosphere. In general, the dust and aerosol content of the troposphere
exhibits a regional pattern and is most concentrated near sources, downwind from sources, and in
regions of relatively dry climate. However, the dust plume in the troposphere derived from the
Sahara and Sahel areas of Africa can be seen in satellite images extending all the way across the
Atlantic Ocean.
6. The three major processes are (1) inputs of volcanic CO2 derived from the metamorphism of CaCO3 to
CaSiO3, (2) weathering of silicate minerals like CaSiO3 and then deposition of calcium carbonate and
silica in the ocean, and (3) the evolution of plants and their effect on weathering. The reactions are
1. CaCO3 + SiO2 ⇒ CaSiO3 + CO2
2. CaSiO3 + 2CO2 + 3H2O ⇒ Ca2+ + 2HCO3- + H4SiO40, and then
Ca2+ + 2HCO3- + H4SiO40 ⇒ CaCO3 + SiO2 + 3H2O + CO2
3. CO2 + H2O ⇔ CH2O + O2
7. The connection is simply the fact that when organic matter is buried in sediments of the ocean, oxygen not used to oxidize the organic matter is left in the atmosphere. With the continuous burial of
organic matter on the seafloor, the oxygen content would increase in a few millions of years to levels
that would lead to burning of forests and grasslands. This does not happen because the buried organic matter is returned to the earth’s surface through uplift by plate tectonic processes. On exposure to
the atmosphere, the organic matter is oxidized, and the oxygen is removed from the atmosphere.
8. The major difference is that there is not a major gas of phosphorus that resides in the atmosphere or
is transported through it (see Figure 18). This statement is not true of carbon, nitrogen, and sulfur.
All of these elements have important gaseous compounds in the atmosphere that exchange with the
earth’s surface. (See the text sections on the cycles of C, N, and S.)
9. DMS in the troposphere reacts fairly rapidly with hydroxyl radical in the presence of light, water,
and oxygen to make sulfate aerosol. On the contrary, OCS is inert in the troposphere but is converted
in the stratosphere to sulfate aerosol (see Figure 19).
10. The difference in residence time implies that ammonia reacts more rapidly than carbon monoxide in
the atmosphere (see Figures 10 and 16). This is simply a consequence of the fact that the shorter the
residence time (lifetime) of a chemical compound in a reservoir, the more reactive the substance.
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11. This question and the following five questions relate to relationships depicted in the figures of the
biogenic trace gases. They are designed to help the student to study the figures and interpret them.
The major reaction in the atmosphere coupling the biogeochemical cycles of CH4, CO, and CO2 is the
oxidation of the reduced carbon gas CH4 to CO and then on to CO2 by OH*. About 60% of the
enhanced greenhouse forcing is due to CO2, and about 20% is due to CH4. The major anthropogenic
sources of emissions of CH4 to the atmosphere are fossil fuel burning and leakage from gas transmission pipelines, biomass burning, landfills, rice paddies, and enteric fermentation in domesticated
animals. CO emissions come from fossil fuel and biomass burning. Land-use activities and fossil fuel
burning are the major anthropogenic sources of CO2 to the atmosphere. A doubling of atmospheric
CO2 concentration could lead to a 2.5°C increase in mean global temperature. Molecule for molecule,
CH4 is about 20 times more effective as a greenhouse gas than CO2. (Compare the amount of temperature change per ppbv for the two gases.)
12. The ratio is (42 + 20 + 78) x 106 metric tons N/yr ÷ 126 x 106 metric tons N/yr = 1.1:1. The anthropogenic nitrogen fluxes on land slightly exceed the natural biological fixation flux! The minimum
percentage is (21 x 106 metric tons N/yr ÷ 140 x 106 metric tons N/yr) x 100 = 15%. The additional
nitrogen flux to the ocean is a cause of eutrophication of coastal marine environments.
13. The ratio is 206 x 106 metric tons N/yr ÷ 62 x 106 metric tons N/yr = 3.3:1. This is a difficult question
to answer. On the time scale of a century, it is very likely that the flux of N to the ocean from rivers
and groundwaters will increase because of continuous use of industrial fertilizers, atmospheric
deposition of anthropogenic nitrogen, and disposal of sewage. If there is climatic change on this time
scale, it is uncertain whether the upwelling rate of the world’s oceans will change. However, a
warming of the global surface layer of the ocean would most likely lead to a slowing of upwelling
and thus delivery of nutrients to the surface ocean.
14. The range is 143 to 339 years. 73 x 106 tons N ÷ (2.9 + 0.1 + 0.02 + 0.01 + 1.4) x 106 tons N/yr = 339
years. 1,500 x 106 tons N ÷ (5.2 + 0.3 + 0.2 + 2.2 + 2.6) = 143 years. N2O accounts for about 9% of the
enhanced greenhouse effect.
15. The percentage from human activities is [(21 + 3) x 106 tons N/yr ÷ (21 + 3 + 20) x 106 tons N/yr] x
100 = 54.5%. Human activities substantially interfere with the fluxes of NOx. Environmental problems associated with the anthropogenic fluxes are acid deposition, photochemical smog, and
increased tropospheric ozone, a greenhouse gas. The principal reaction leading to destruction of NOx
in the atmosphere is photochemical. The products of the reaction are nitric acid, peroxylacetyl
nitrate, and organic nitrates. The reaction is NO2 + OH*+ light ⇒ HNO3.
16. The ratio is 1,085 x 106 tons P/yr ÷ 186 x 106 tons P/yr = 5.8:1.
The residence time of P in the marine biota relative to the recycling flux is 73 x 106 tons P ÷ 1,085 x106
tons P/yr = 0.07 years. That for P in the land biota is 1,800 x 106 tons P ÷ 186 x 106 tons P/yr = 9.6 years.
The phytoplanktyon of the ocean “turn over” much more rapidly than do terrestrial plants. There are
more generations of death and birth for marine plankton than for most plants growing on land.
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Glossary
Acid deposition—the fallout of acidic substances, primarily of nitrogen and sulfur, from the atmosphere
on to the earth’s surface in rain, snow, or other forms. Acid rain has a pH generally less than 5 (averaged over a year).
Aerosol—the suspension of very fine, generally micrometer-sized, solid and liquid particles in the atmosphere.
Albedo—the amount of incident radiation that is reflected by a surface and thus does not contribute to
the heating of the surface. The albedo of the whole earth is approximately 30%. The albedo of clean
snow is about 90% and that of water is about 10%.
Anaerobic—an organism that does not need oxygen to carry on its metabolism or an environment without oxygen.
Anoxic—without oxygen.
Anthropogenic—of, relating to, or influenced by the impact of humans on nature.
Atom—the smallest component of an element having the chemical properties of the element. An atom
consists of a nucleus of neutrons and protons and one or more electrons bound to the nucleus by electrical attraction.
Autotrophy—the biochemical pathway by which an organism uses carbon dioxide as a source of carbon
and simple nutrient compounds for synthesis of organic matter.
Autotrophic system—an environment in which the difference between gross photosynthesis and gross
respiration is positive. In such a terrestrial or aquatic environment, the net transfer of carbon dioxide is
into the system.
Bacteria—one-celled organisms having a spherical, spiral, or rod shape belonging to the Kingdom Monera.
Benthic—of, relating to, or occurring at the bottom of a body of water.
Bioessential—required by virtually all living organisms. The major bioessential elements are oxygen,
carbon, nitrogen, phosphorus, sulfur, potassium, magnesium, and calcium. Minor or trace quantities of
iron, manganese, copper, zinc, boron, silicon, molybdenum, chlorine, vanadium, cobalt, and sodium are
also required by organisms.
Biogenic gas—a gas whose production or consumption on earth is accomplished by biological reactions.
Biogeochemical cycle—representation of biological, geological, and chemical processes that involve the
movement of an element or compound about the surface of the earth.
Biogeochemical system—the interactive system of biogeochemical processes and cycles of elements and
compounds.
Biogeochemistry—the discipline that links various aspects of biology, geology, and chemistry to investigate the surface environment of the earth.
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Biological productivity—the rate of production per unit area of organic matter by producer organisms.
For example, the rate may be given as grams of carbon per square meter per year for a marine grass
community. There are several kinds of productivity. Gross primary production (GPP) refers to the total
amount of plant material produced by photosynthesis in a defined area in an interval of time. Net primary production (NPP) is the net amount of plant material produced per unit area per unit time and is
the difference between GPP and cell respiration. Net ecosystem production (NEP) is the difference
between GPP and cell respiration plus heterotrophic processes of decay.
Biological pump—the set of processes by which organic carbon is exported from the surface ocean to
the deep sea.
Biomass—the amount of living matter in a unit area or volume of habitat. For example, the total biomass of the world’s tropical rain forests is 42 kilograms of dry matter per square meter of forest, or a
total of 420 billion tons of dry matter (equivalent to approximately 170 billion tons of carbon).
Biosphere—the living and dead organic components of the earth. Sometimes this term is used in the
same way as the term ecosphere in this module, and sometimes for only the living animals and plants.
Climate—the characteristic long-term environmental conditions of temperature, precipitation, winds,
etc., in a region or for the globe at present or in the past (paleoclimate).
Cloud condensation nuclei—airborne particles of very small size, generally less than one micrometer in
diameter, that serve as sites on which liquid cloud droplets condense when an air mass is supersaturated with water vapor. The particles are commonly composed of water-soluble material.
Coccolithophoridae—a family of planktonic algae that build skeletons of micrometer-sized disc-shaped
plates of calcite, called coccoliths.
Concentration—the fraction of the total of a substance made up of one component. For example, seawater contains 400 parts per million by weight of calcium. Concentration is also expressed in moles per
liter or kilogram or in percent (that is, parts per hundred), parts per thousand (°/°°), per million (ppm),
per billion (ppb), and so forth, either by weight or by volume.
Coupled—the condition in which information from one part of the system is provided to, and influences
the behavior of, other parts. The biogeochemical cycles of the elements necessary for life are coupled
through processes that are essential for life, e.g., photosynthesis and respiration.
Crust—the outer layer of the earth, enriched in silicon, sodium, and potassium and having a thickness
of 35 kilometers beneath the continents and 10 kilometers beneath the oceans.
Cryosphere—the icy part of the earth; its continental and mountain glaciers, ice sheets, and ice shelves;
a reservoir in the earth’s surface system.
Decay—the oxidative process of conversion of organic tissue to simpler organic and inorganic compounds. The oxidizing agent may be diatomic oxygen (O2), nitrate (NO3-), or other chemical compounds.
Denitrification—the conversion, principally by bacteria, of compounds of nitrogen in soils and aquatic
systems to nitrogen gas (N2) and nitrous oxide gas (N2O) and the eventual release of these gases to
the atmosphere.
Diagenesis—the collection of physical, chemical, and biological processes that operate on a sediment
after deposition.
Diatom—planktonic and benthic freshwater and marine algae that commonly use silicon to build a
skeleton of opal.
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Understanding Global Change: Earth Science and Human Impacts
Dry deposition—the deposition of materials from the atmosphere on to the earth’s surface in the form
of solid particles. Such particles also may be “washed out” of the atmosphere by rain.
Ecosphere—the system that includes the biosphere and its interactions with the air, water, and soils and
sediments of the earth.
Ecosystem—complex of a community or group of communities and the environment that functions as
an ecological unit in nature.
Electromagnetic spectrum—the entire range of radiation. The wavelengths (distances between adjacent
peaks) of the electromagnetic waves within the spectrum range from kilometers for radio waves to billionths of a meter (nanometers) for X rays.
Entropy—a scientific measure of the degree of disorder in a system. The greater the disorder the greater is
the entropy of the system. The Second Law of Thermodynamics states that entropy is always increasing.
Equilibrium—stable, balanced state in which all influences on a system are countered by others.
Erosion—the set of processes by which the surface of the earth is worn away by the action of water,
wind, glacial ice, etc.
Euphotic zone—the upper, lighted zone of the ocean or a lake in which most of the productivity of
plants occurs. In the ocean, the euphotic zone extends from the surface to a depth where the light intensity is reduced to about 0.1–1.0% of that available at the surface. The depth of the euphotic zone
depends on season and latitude.
Eutrophication—the set of processes leading to overnourishment of an aquatic system in nutrients,
rapid plant growth and death, and oxygen consumption and deficiency in the system. These processes
occur naturally in some aquatic systems but may be speeded up by additions of nutrients from human
activities (e.g., fertilizer application) to the systems. Human-induced eutrophication is often called cultural eutrophication.
Evaporation—the physical process by which water is converted from liquid to vapor and is transported
into the atmosphere.
Evapotranspiration—the combined processes of evaporation and transpiration.
Evasion—the escape or release of a gas from the surface of the ocean or land to the atmosphere.
Evolution—the pattern of development and change in a variable from one state to another. Biological
evolution describes the pattern of emergence, development, and extinction of organic species through
geologic time.
Feedback—a process or mechanism in which some fraction of the output is returned or “fed back” to
the input. Feedback loops may either stabilize (negative feedback) or destabilize (positive feedback) a
system undergoing a perturbation. These feedback loops exist in both the biogeochemical cycles and the
climate system.
Fermentation—the bacterial process of conversion of sugars to carbon dioxide.
Fixation—see Nitrogen fixation.
Flux—the movement of a variable or a substance into or out of a reservoir.
Foraminifera—animal plankton (zooplankton) in the ocean belonging to the Phylum Protozoa that commonly have a shell of calcium carbonate.
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Forcing—the ability of a variable, like the concentration of a greenhouse gas in the atmosphere, to
induce a change in a system. A forcing function controls the behavior of a system and often makes it
regular and predictable.
General circulation model (GCM)—a simulation, usually performed on a large computer, of the largescale, or general, wind and ocean systems on earth to calculate climate and its changes.
Geologic time scale—a calendar of earth history. The time scale is divided into variable time units of
eon, era, period, and epoch (see Figure 5).
Glacial stage—an extended cold interval of time within the Pleistocene Epoch in which continental glaciers covered much of the Northern Hemisphere continents, atmospheric CO2 concentrations were low,
and sea level was low.
Greenhouse effect—the warming of the earth’s atmosphere and surface by the atmospheric greenhouse
gases. These gases absorb and reradiate longwave radiation from the earth, keeping it in the atmosphere
and thus warming the global temperature. Without the natural greenhouse effect, the planet would be
about 33°C cooler than its global mean annual temperature of 15°C, that is, –18°C. Because of inputs
from human activities, these gases are increasing in concentration in the atmosphere. This may lead to
an enhanced greenhouse effect and warming of the planet.
Greenhouse gas—an atmospheric gas that absorbs and radiates energy in the infrared part of the electromagnetic spectrum. Such gases include water vapor, carbon dioxide, methane, nitrous oxide, tropospheric ozone, and the synthetic chlorofluorocarbon gases. These gases warm the atmosphere and the
earth’s surface below.
Groundwater—the water beneath the ground, largely formed by the seepage of surface water downward.
Heterotrophic system—an environment in which the difference between gross photosynthesis and gross
respiration is negative. In such a terrestrial or aquatic environment, the net transfer of carbon dioxide is
out of the system.
Heterotrophy—a biochemical pathway in which organic substrates are used by organisms to make
organic matter.
Hothouse—an extended period of geologic time during which the earth was warm.
Hydrosphere—the watery envelope surrounding the earth; a reservoir in the earth’s surface system.
Hydrothermal reaction—a chemical reaction involving hot water and minerals in a rock.
Hydroxyl radical (OH*)—the excited chemical compound of hydrogen and oxygen in the atmosphere
with an imbalance of electric charge. The hydroxyl radical is responsible for the oxidation of many
chemically reduced gases emitted from the surface of the earth.
Ice age—a glacial stage, especially within the Pleistocene Epoch, beginning about 1.8 million years ago.
Ice house—an extended period of geologic time in which the earth was cool.
Infrared radiation—the region of the electromagnetic spectrum with wavelengths longer than visible
light (about 1 micrometer) but shorter than microwaves (about 1 millimeter). Commonly known as heat.
Radiation emitted from the earth back to space is predominantly infrared radiation.
Interglacial stage—an extended warm interval of time within the Pleistocene Epoch in which the continental glaciers retreated and atmospheric CO2 concentrations and sea level were low.
Ion—an electrically charged atom or group of atoms formed by the loss or gain of one or more electrons.
A positive ion, the cation, is created by an electron loss, and a negative ion, the anion, is created by an
electron gain.
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Irreversible process—a process in which the entropy change is greater than zero. After the process is
complete, the system is more disordered than and different from its initial state.
Kerogen—fossil organic matter dispersed throughout a rock.
Leaching—the selective removal of substances from a substrate, usually with water. For example, rainwater percolating through a soil can dissolve nitrogen and transport it to the groundwater. This is leaching.
Lifetime—a measure of the reactivity of an atmospheric chemical compound. The more reactive the
compound, the shorter its atmospheric lifetime. Analogous to residence time.
Limestone—a sedimentary rock consisting predominantly of calcium carbonate minerals.
Limiting nutrient—the chemical compound, generally inorganic, that limits productivity in a terrestrial
or aquatic environment. Examples are nitrate, phosphate, and iron.
Lithosphere—the dynamic subdivision of earth on the order of 100 kilometers in thickness forming the
outer, rigid part of the planet. Also, the solid portion of the earth, composed of minerals, rocks, and
soils; a reservoir in the earth’s surface system.
Mantle—the portion of earth between its crust and its innermost zone (the core). The mantle is enriched
in magnesium and iron and has a thickness of about 2,900 kilometers.
Metamorphism—the set of processes that lead to a change in the structure or composition of a rock due
to pressure and temperature. A metamorphic rock is formed from a preexisting rock by an increase in
pressure and temperature.
Methanogenesis—the conversion of organic material to methane, principally by bacteria.
Methanotrophy—the conversion of methane to carbon dioxide, principally by bacteria.
Midocean ridge—any of several seismically active, submarine mountain ranges that are found in the
Atlantic, Indian, and Pacific Oceans. These ridges are regions where the seafloor originates and are the
source of the lithospheric plates.
Mixotrophy—the use of both organic and inorganic materials to make organic matter.
Mole—one gram atomic weight of an element or one gram molecular weight of a compound. One gram
atomic weight of an element is its atomic weight expressed in grams (i.e., the atomic weight of oxygen
is 16; its gram atomic weight is 16 grams). One gram molecular weight of a compound is its molecular
weight expressed in grams (i.e., the molecular weight of carbon dioxide is 44; its gram molecular weight
is 44 grams).
Molecule—the smallest physical unit of an atom or compound, consisting of one or more similar atoms
in an element and two or more different atoms in a compound.
Negative feedback—a process or mechanism that relieves or subtracts from an initial perturbation to a system.
Net primary production—see Biological productivity.
Nitrification—the conversion of ammonium to nitrite and nitrate by nitrifying bacteria.
Nitrogen fixation—the conversion of diatomic nitrogen gas (N2) to ammonium by bacteria. Also, the industrial conversion of free nitrogen into combined forms used as starting materials for fertilizers and explosives.
Nutrient—a substance that supplies nutrition to a living organism, like phosphorus and nitrogen.
Organic—pertaining to a class of chemical compounds that include carbon as a component; characteristic of or derived from living organisms.
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Oxidation—the removal of electrons from an atom or molecule. Oxidizing capacity is the intrinsic ability of a system to oxidize reduced substances.
pH—the negative logarithm of the effective hydrogen ion concentration, used in expressing both acidity
and alkalinity on a scale whose values run from 0 to 14, with 7 representing neutrality. Numbers less
than 7 denote increasing acidity, and numbers greater than 7 increasing alkaline (basic) conditions.
Photoautotrophy—the conversion of inorganic carbon into organic matter in the presence of light.
Photochemical—pertaining to chemical reactions involving chemical compounds in the presence of
light. Urban smog is the result of a complex series of photochemical reactions involving ozone, nitrogen
and sulfur oxides, and hydrocarbons.
Photolysis (photodissociation)—pertaining to chemical reactions triggered by light that convert a complex compound to more simple products. The photolysis of ammonia is an example: 2NH3 ⇒ N2 + 3H2.
Photosynthesis—the synthesis of complex organic materials (e.g., carbohydrates) from carbon dioxide, water,
and nutrients, using sunlight as a source of energy with the aid of chlorophyll and associated pigments.
Phytoplankton—minute plant life that passively floats in a body of water. The phytoplankton are at the
base of the food chain in the ocean.
Plankton—minute plant and animal life of the ocean ranging in size from 5 micrometers to 3 centimeters. The plant plankton are the phytoplankton; the animal plankton are the zooplankton.
Plate tectonics—the theory of global tectonics in which the lithosphere is divided into a number of
crustal plates that move on the underlying plastic asthenosphere. These plates may collide with adjacent
plates, slide under or over them, or move past them in a nearly horizontal direction. The sources of the
plates are the great midocean ridges of the world’s oceans, where hot molten material upwells from
within the earth. The plates are destroyed at subduction zones, like that along the western margin of the
Pacific Ocean, where they sink down into the underlying asthenosphere.
Positive feedback—a process or mechanism that reinforces or adds to an initial perturbation of a system.
Precipitation—the removal of water from the atmosphere and its deposition on the earth’s surface in the
form of rain, ice, or snow.
Prokaryote—any cellular organism that has no membrane about its nucleus and no organelles in the
cytoplasm except ribosomes. Prokaryotic genetic material is in the form of single, continuous strands
forming coils or loops, characteristic of all organisms of the Kingdom Monera, such as bacteria or
cyanobacteria.
Protozoan—eukaryotic organism of the Kingdom Protoctista, Phylum Protozoa, with a membranebound nucleus and organelles within a mass of protoplasm. Planktonic foraminifera and radiolarians
which secrete shells of calcium carbonate and opal, respectively, are members of the group.
Radical—an electronically excited compound with an imbalance of electric charge, which enables it to
react rapidly with another molecule.
Redfield ratio—the relatively constant ratio of 106:16:1 of the bioessential elements carbon, nitrogen,
and phosphorus in marine plankton. The concept of the Redfield ratio has been applied to the terrestrial
realm as well as to organic matter in soils and sediments.
Reduction—the chemical process by which an atom or a molecule gains electrons.
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Understanding Global Change: Earth Science and Human Impacts
Reservoir or stock—a part of a system that can store or accumulate and be a source of one of the substances that compose the system. For example, the atmosphere is a reservoir of the surface system of the
earth (the ecosphere). It can store water vapor released to it from the land by evapotranspiration and
from the ocean by evaporation, and return the water to the earth’s surface as precipitation.
Residence time—the total mass of a substance in a reservoir divided by its rate of inflow or outflow.
The residence time is a measure of the reactivity of the substance in the reservoir. For example, the residence time of sodium in the ocean is very long (55 million years). Therefore, sodium does not enter into
chemical or biochemical reactions that remove it very rapidly from the ocean. In contrast, the residence
time of dissolved silica in the ocean is about 20,000 years. This compound is readily taken up by certain
types of plankton to build their skeletons.
Respiration—the physical and chemical processes by which an organism supplies its cells and tissues with
the oxygen needed for metabolism and releases carbon dioxide formed in the energy-producing reactions.
Reversible process—a process in which the change in entropy is zero. In general, after the process is
complete, the state of the system is as it was initially.
Saturation—the degree to which a solution or a gas is at equilibrium with one of its components. It is
measured in several different ways. For example, a humidity of 125% would be a supersaturation of 25%
with respect to water vapor in the air. Saturation of seawater with respect to the mineral calcite (CaCO3)
of 50% would mean that the seawater was 50% undersaturated with respect to calcite. If a lake water
contained exactly enough dissolved CO2 to be in equilibrium with the atmosphere, it would have a saturation of 100% with respect to CO2.
Sedimentary rock—a rock formed from the erosion of preexisting rocks and the deposition of the eroded materials as sediment. Sedimentary rocks are also formed by inorganic or biological precipitation of
minerals from natural waters.
Shortwave radiation—generally, the region of the electromagnetic spectrum with wavelengths shorter than
0.5 micrometers. Solar radiation has an important component of shortwave radiation of varying intensity.
Solar radiation—the electromagnetic radiation emitted by the sun. It includes energy wavelengths from
the very short ultraviolet (<0.2 micrometers) to about 3 micrometers.
Stratosphere—the region of the upper atmosphere extending upward from the troposphere to about 30
kilometers above the earth’s surface. This region is characterized by an increase in temperature as altitude increases.
Subduction zone—the juncture of two lithospheric plates where the collision of the plates results in one
plate’s being drawn down or overridden by another plate. This region is the sink of the crustal plates of
the earth.
System—a selected set of interactive components. An example of a simple system is an air conditioning
unit. A biogeochemical system consists of reservoirs, processes and mechanisms, and associated fluxes
involving material transport. The global climate system is very complex and involves all the physical,
chemical, and biological interactions that control the long-term environmental conditions of the world.
Thermocline—the depth range in the ocean where the temperature decreases rapidly with increasing
depth. The thermocline is about one kilometer thick and extends from the base of the surface layer of the
ocean at a depth of 50–300 meters to a depth of about 800–1,000 meters.
Trace gas—a gas present in the atmosphere in a very low concentration (less than 1% of the composition of
the atmosphere). For example, methane, nitrous oxide, and carbon monoxide are considered trace gases.
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Transitional phenomenon—a feature of a system that changes from one state to another. Such a change
may be relatively slow or more generally abrupt. In the boiling of water, the change from the state of
little water motion to that of turbulence is a transitional phenomenon.
Transpiration—the process by which water in plants is excreted through a plant membrane as water vapor.
Troposphere—the lowest level of the atmosphere, up to 8–13 kilometers high, within which there is a
steady drop in temperature with increasing altitude. It is the region where most cloud formations occur
and weather conditions manifest themselves.
Ultraviolet radiation—the region of the electromagnetic spectrum with wavelengths longer than 0.5
nanometers but shorter than 0.5 micrometers. Solar radiation has an important component of ultraviolet
radiation of varying intensity.
Uptake—generally, the incorporation of a substance into a solid or liquid. For example, the invasion of
CO2 into the ocean represents the uptake of CO2 from the atmosphere.
Upwelling—the upward movement of water from depths of typically 50–150 meters at speeds of
approximately 1–3 meters per day. The upwelling of water generally results from the lateral movement
of surface water. Upwelling zones in the ocean are found along the western margins of the continents, in
equatorial regions, and at high latitudes of the Southern Hemisphere.
Vascular plant—either a plant with seeds that are not enclosed in a fruit or seed case, such as pine, fir,
spruce, and other cone-bearing trees or shrubs (gymnosperm), or a flowering plant that produces
encased seeds, such as oak, maple, and eucalyptus trees (angiosperm).
Volatilization—the conversion of a substance into the gas or vapor state and its emission into the environment.
Washout—the scavenging of particles from the atmosphere by rainfall and their subsequent deposition
on the surface of the earth.
Weathering—the set of chemical, physical, and biological processes that lead to the disintegration of
minerals, kerogen, and rocks.
Wet deposition—the deposition on the earth’s surface of solid particles and dissolved chemical compounds in rain.
Zooplankton—minute animal life in a body of water that generally drift passively or swim very weakly.
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Supplementary Reading
Berner, E.K., and R.A. Berner, 1996, Global Environment: Water, Air, and Geochemical Cycles. Prentice Hall,
Upper Saddle River, New Jersey.
Broecker, W.S., 1983, The ocean. Scientific American, vol. 1249, no. 3, 100–112.
Chiras, D.D., 1988, Environmental Science. Benjamin Cumming Publishing Co., Redwood City, California.
Ehrlich, P.R., A.H. Ehrlich, and J.P. Holden, 1977, Ecoscience: Population, Resources, Environment. W.H.
Freeman and Co., San Francisco, California.
Garrels, R.M., F.T. Mackenzie, and C.H. Hunt, 1975, Chemical Cycles and the Global Environment: Assessing
Human Influences. William Kaufmann, Inc., Los Altos, California.
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