Download Chapter 1: Overview

Global Change
Instruction Program
Chapter 1: Overview
The Importance of the Carbon Cycle
and the movement of CO2 across the air-sea interface. Other processes, such as the transformation
of carbon into limestone and its subsequent
release as the rock is weathered, occur very slowly (thousands to millions of years). Scientists call
these slower transfers of carbon among reservoirs
the long-term carbon cycle.
The carbon cycle is inextricably linked to
other chemical cycles, including those of nitrogen,
phosphorus, and sulfur, as well as to the global
hydrological cycle. Please see the Global Change
Instruction Program module Global Biogeochemical
Cycles and the Physical Climate System or other
works for information on these important topics.
Carbon is an element found in all living substances as well as in many inorganic materials.
Both diamond and coal are nearly pure carbon,
but with different structures. Carbon is a key element for life, composing almost half of the dry
mass of the earth’s plants (that is, the mass when
all water is removed). The carbon cycle is the
exchange of carbon among three reservoirs or
storage places: the land, the oceans, and the
atmosphere. The amount of carbon in these reservoirs is so large that it is expressed in gigatons
(Gt): 109 metric tons (1 metric ton equals 1,000
kilograms or 2,200 pounds).
The atmosphere is the smallest pool of actively cycling carbon, that is, carbon that stays in a
reservoir less than a thousand years or so. The
land and its plants and animals, which scientists
call the terrestrial biosphere, is the next largest
reservoir of carbon. The oceans are the earth’s
largest active carbon reservoir by far (Figure 1).
The carbon budget is the balance of carbon among
the three reservoirs.
The carbon cycle is vitally important to life
on earth. Through photosynthesis and respiration
(see p. 3), it is the way the earth produces food
and other renewable resources. Through decomposition, it serves as the earth’s waste disposal
system. In addition, the carbon cycle is important
because carbon-containing gases in the atmosphere affect the earth’s climate (see “The
Greenhouse Effect,” p.2). Increased carbon
dioxide (CO2) in the atmosphere has been
responsible for more than half of the climate
warming observed in recent decades.
The processes by which carbon moves
through the earth’s reservoirs take place on very
different time scales. The short-term carbon cycle
includes processes that transfer carbon from one
reservoir to another in a matter of years, including photosynthesis, plant and animal respiration,
Carbon Reservoirs
Actively cycling carbon in its three reservoirs
affects human life every day. Carbon in the
atmosphere serves as food for plants (in the presence of sunlight); carbon in the soil serves as
energy for the growth of miniature animals called
microbes; carbon in plants feeds humans and
Figure 1. Magnitudes of the reservoirs of actively cycling CO2 in
gigatons of carbon. Of the ocean pool, roughly 1,000 Gt are in
contact with the atmosphere in any given decade. The terrestrial
biosphere consists of soil (ca. 1,600 Gt) and vegetation (ca. 600
Gt). Data from Schimel et al. (1995).(after Christensen, 1991).
754
2,190
Atmosphere
Oceans
Terrestrial Biosphere
39,120
1
Understanding Global Change: Earth Science and Human Impacts
The Greenhouse Effect
Some heat energy escapes
outgoing into space.
other animals; and carbon in the
ocean makes up the homes of many
marine animals. Obviously, carbon
is critically important in all of the
reservoirs where it is found.
The Atmosphere
Incoming solar
energy
warms the earth's
surface.
Some heat energy is absorbed by
greenhouse gases in the atmosphere
and is reradiated
back
toward
earth...
...keeping the surface comfortably warm.
Warmed surface
radiates heat energy
back out toward space.
Figure 2. A simplified diagram of the greenhouse effect. Net input of solar radiation
must be balanced by net output of radiation from the earth’s surface. About a third of
incoming solar radiation is reflected, and the remainder is mostly absorbed by the surface. Some of the outgoing heat radiation is absorbed by greenhouse gases and by
clouds, thereby keeping the surface about 33°C warmer than it otherwise would be.
Carbon composes much less
than 1% of the atmosphere. Even
this small percentage adds up to
750 Gt of carbon, which is a lot
(although, as noted earlier, much
less than in the other two reservoirs). Carbon in the atmosphere
occurs almost entirely as CO2; small
amounts of methane (CH4), carbon
monoxide (CO), and chlorofluorocarbons (CFCs) are also present.
These gases are all radiatively
active, that is, they trap heat near
the surface of the earth. So they also
are considered greenhouse gases.
(See “The Greenhouse Effect,”
below.)
The Greenhouse Effect
Like the glass walls of a greenhouse, certain gases trap heat. How do they do this? Shortwave radiation from the sun
passes through the earth’s atmosphere and is absorbed by the surface, thereby warming it. Some of this absorbed energy is reradiated back toward space in a different form: as infrared radiation or heat. The greenhouse gases in the atmosphere absorb the heat energy and keep it from escaping back to space (Figure 2). Thus the heat stays in the atmosphere
and warms the earth. The greenhouse effect is a natural process that has gone on for millions of years. Without it, life on
earth would be impossible, because the earth’s temperature would be about 33°C (60°F) colder than it is today.
Whereas the greenhouse effect is a good thing, an enhanced greenhouse effect might not be. Scientists are concerned that the rapid increase in the amount of CO2 and many other greenhouse gases in the atmosphere has implications for global climate. Figure 3 shows the relationship between atmospheric CO2 concentration and temperature over
the last 160,000 years. The strong connection suggests—although it alone does not prove—that the level of CO2 in the
atmosphere plays an important role in determining the earth’s temperature. However, there is not a simple linear relationship between the amount of greenhouse gases in the atmosphere and the temperature at the surface of the earth. Complex
mathematical models of the earth’s climate system are needed to study potential future consequences of our continued
dependence on fossil-fuel energy.
Carbon dioxide, water vapor, methane, nitrous oxide, and ozone are the greenhouse gases that occur naturally. The
concentrations of all of these gases in the atmosphere are increasing as a result of human activities. The chlorofluorocarbons (CFCs) are also greenhouse gases, but they were created by humans; they did not exist in the atmosphere before the
1950s. The CFCs were manufactured for use as aerosol propellants, refrigerants, and solvents. Although their manufacture has been banned, their replacements are also very powerful greenhouse gases.
2
The Carbon Cycle
Photosynthesis and respiration
In the short-term carbon cycle, photosynthesis
and respiration are the primary processes that
involve the atmosphere. Photosynthesis is the
process by which green plants make their food.
In this process, plants combine CO2 and water
using light energy to make carbon-containing
compounds (sugars and starches, called carbohydrates). Oxygen is produced during the reaction and released to the atmosphere. On land,
plants use CO2 from the atmosphere (Figure 4). In
the oceans, phytoplankton use CO2 dissolved in
seawater; much of this dissolved CO2 also originally came from the atmosphere. Photosynthesis
also releases oxygen to the atmosphere.
Thus, thanks to the activity of photosynthetic
organisms, all other forms of life on the earth have
oxygen to breathe and food to eat—since even
carnivores feed on animals that eat plants.
Respiration is the chemical process by which
carbon-containing compounds are broken down
within cells. It is essentially the opposite of photosynthesis: oxygen and carbohydrates react to produce CO2 and water, releasing energy during the
process. Living organisms use the energy released
by respiration to power everything they do. For
example, reading this page requires energy, and
that energy is supplied by respiration.
260
Carbon Dioxide
240
+2.5
220
200
0
180
-2.5
-5.0
-7.5
Temperature Change
Departure of Temperature
from Current Level (°C)
Concentration of Carbon Dioxide
in the Atmosphere (ppm)
280
-10.0
40
0
160
120
80
Thousands of Years Before Present
Figure 3. Variations in global temperature (bottom line) and atmospheric CO2 concentration (top line) over the last 160,000 years.
The strong pattern of higher temperatures when atmospheric CO2
levels are high and cooler temperatures when concentrations are
lower has been used to suggest that future increases in atmospheric CO2 concentration could lead to warmer temperatures.
From Barnola et al. (1987), p. 410.
Figure 4. The flow of CO2 into plants via photosynthesis and out
of plants and animals via respiration. Photosynthesis removes
approximately as much CO2 from the atmosphere as respiration
adds to it. This cycle keeps the atmospheric level of CO2 fairly
constant.
3
Understanding Global Change: Earth Science and Human Impacts
Globally, photosynthesis and respiration tend
to be in balance in nature. In other words, photosynthesis takes up about as much CO2 from the
atmosphere each year as is released by respiration. However, extreme weather events—such as
prolonged flooding or drought over large areas—
or extended periods of unusually hot or cold temperatures can temporarily offset the delicate balance and lead to either a release of CO2 to the
atmosphere or a high uptake of CO2 from the
atmosphere.
Antarctica, Australia, several maritime islands,
and high northern latitudes. (At present, there are
no continuous measurement sites in Africa or
South America.)
Figure 5 shows the records from these stations. Atmospheric CO2 has increased about 25%
just since 1957, from 315 parts per million in 1957
to 367 ppm in 1999. The increase in atmospheric
CO2 since 1957 is actually greater than the estimated increase over the preceding 200-plus years
from 1750 to 1957.
Increases in atmospheric carbon since 1750
As Table 1 shows, the carbon-containing compounds in the atmosphere have been increasing
since the Industrial Revolution (about 1750 AD).
At that time, humans began releasing more of
these gases into the atmosphere by burning fossil
fuels (e.g., gas, coal, oil), cutting down or burning
forests to make agricultural land, and other
processes. Although scientists have only been able
to measure the atmospheric concentrations of
these gases for about 40 years, they can estimate
the concentrations in earlier periods from growth
rings in trees, air trapped in polar ice cores, and
other sources. Since 1957, the amount of carbon
dioxide in the atmosphere has been measured
directly at the South Pole; since 1958, at Mauna
Loa, Hawaii; and starting later, at over 20 other
locations around the world, including sites in
Atmospheric CO2 records of the past
Measurements of CO2 concentrations in air
extracted from polar ice cores are currently the
best means to extend the CO2 record through the
geologically “recent” past (last 200,000 years). The
transformation of snow into ice traps air bubbles
that are used to record the atmospheric CO2
concentration. Provided that the cores are extracted carefully from well-chosen sites and the
analysis is done correctly, the ice record provides
reliable information on past atmospheric CO2
concentrations.
The ice-core record reveals an average value
of 280 ppm in the level of atmospheric CO2 over
the 1,000 years before the Industrial Revolution,
with fluctuations up to 10 ppm. The largest of
Figure 5. The atmospheric CO2 increase between 1958 and 1994,
recorded on Mauna Loa, Hawaii. The figure shows both the
increase in atmospheric CO2 concentration since the start of
measurements in 1958 and the seasonal cycle (zigzag pattern) of
change in atmospheric CO2 concentration (see “Why the
Zigzags?,” next page). Data from C.D. Keeling.
Table 1
Atmospheric concentrations of important
carbon-containing gases
Carbon dioxide
Methane
280 ppm*
367 ppm
(773 Gt)
31%
0.7 ppm
1.8 ppm
(5.1 Gt)
157%
0.05 ppm
(0.1 Gt)
——
Carbon monoxide ~0.05 ppm
370
360
CO2 Concentration (ppm)
Gas
Estimated
pre–Industrial
Revolution
Current
concentration concentration Percentage
(ca. 1750)
(1999)
of increase
350
340
330
320
*ppm = parts per million by volume; out of 1 million molecules in the atmosphere before the Industrial Revolution, 280
molecules were carbon dioxide.
310
1950
1960
1970
1980
Year
4
1990
2000
The Carbon Cycle
these fluctuations, which occurred between
roughly 1200 and 1400 AD, was small compared
to the 75 ppm increase the earth has experienced
since the Industrial Revolution. Figure 6 illustrates how well the ice-core record of atmospheric
CO2 concentrations matches the direct atmospheric record. It also shows both how stable the level
of atmospheric CO2 was before 1850 and how
rapidly that level has risen since then.
Why the Zigzags?
The zigzag pattern of increase shown in Figure 5
reflects the seasonal uptake and release of CO2 by
plants. Levels of CO2 are high when plants are respiring
more than they are photosynthesizing (winter) and are
low when photosynthesis exceeds respiration (summer).
Since Northern and Southern Hemisphere winters and
summers occur at opposite times of the year, you might
expect the opposing patterns from the two hemispheres
to cancel each other out in the global record. However,
the pattern for the midlatitude Northern Hemisphere
prevails because its larger land masses support more
seasonally active photosynthesis. In the seasonless
tropics, photosynthesis goes on year round.
The size of the zigzag, that is, the height of each
peak (its amplitude), has generally increased over the
past few decades. We are not sure why this is, because
the trend is not well correlated with the increase in
atmospheric CO2 concentration. The change could reflect
either increased photosynthesis or increased respiration.
The Oceans
The oceans absorb and store about 39,000 Gt
of carbon—more than 60 times as much as the
atmosphere (see Figure 1). Over 95% of oceanic
carbon is in the form of dissolved inorganic carbon (dissolved CO2 and bicarbonate and carbonate ions); the remainder comprises various forms
of organic carbon (living organic matter and particulate and dissolved organic carbon). Scientists
differentiate between the inorganic and organic
forms of carbon because the two forms go
380
CO2 concentration (ppm)
CO2 concentration (ppm)
380
360
340
320
Fossil
300
6
360
4
340
2
320
0
300
280
1850
1900
1950
Fossil CO2 emissions (Gt C/yr)
Figure 6. Historical levels of CO2 in the atmosphere. This figure combines information from Keeling’s direct atmospheric record (see Figure 4)
and from air bubbles trapped in Antarctic ice cores. For comparison, the figure also shows the amount of CO2 released by human use of
fossil fuels. Note that for a thousand years before 1850 the amount of CO2 in the atmosphere was quite constant, but that it began to
increase rapidly after the Industrial Revolution, following the increased use of fossil fuels. From IPCC (1996), p. 16.
2000
Year
Ice core record
Direct atmospheric record
280
260
800
1000
1200
1400
Year
5
1600
1800
2000
Understanding Global Change: Earth Science and Human Impacts
through different chemical reactions and also are
affected in different ways by physical ocean
processes. The physical, chemical, and biological
processes that affect carbon in the oceans are
described in Chapter 2.
be thought of as the most active part of the global
carbon cycle. It is difficult, however, to pin down
the role of the terrestrial biosphere in the global
carbon cycle because of the complex biology
underlying carbon storage, the great heterogeneity
of vegetation and soils, and the somewhat unpredictable nature of human land use and land management. We will discuss some of these concerns
in the next chapter.
The Terrestrial Biosphere
Within this reservoir, the largest pool of carbon (roughly 65.5 million Gt) is in sedimentary
rocks—mostly limestone, a carbonate rock, and
also organic sediments that contain fossil fuels.
However, aside from human use of these fuels
(which will be discussed in the next chapter),
rocks do not play a role in the short-term carbon
cycle. Carbon storage in plants and soils is more
important. Plants and soils store and release
carbon on time scales ranging from seasons to
thousands of years, and they contain about
2,200 Gt carbon (C)—almost three times as much
total carbon as the atmosphere and more organic
(but not more total) carbon than in the oceans
(Figure 7).
Not only is the amount of carbon in the terrestrial biosphere large, but it is readily modified
by human activity. Therefore this reservoir may
Figure 7. Comparison of the land and oceanic carbon pools that are
important in the short-term carbon cycle.
Oceanic Biomass
3 Gt C
Oceanic
Organic
Carbon
1,020 Gt C
Land
Biomass
610 Gt C
Soil Organic
Carbon
1,580 Gt C
6