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Climate Science for Today’s World
You are about to embark on a systematic study of
climate, climate variability, and climate change. Earth
is a mosaic of many climate types, each featuring a
unique combination of physical, chemical, and
biological characteristics.
Differences in climate
distinguish, for example, deserts from rainforests,
temperate regions from glacier-bound polar localities,
and treeless tundra from subtropical savanna. We will
come to understand the spatial and temporal (time)
variations in climate as a response to many interacting
forcing agents or mechanisms both internal and
external to the planetary system. At the same time we
will become familiar with the scientific principles and
basic understandings that underlie the operations and
interactions of those forcing agents and mechanisms.
This is climate science, the systematic study of the
mean state of the atmosphere at a specified location
and time period as governed by natural laws.
Our study of climate science provides
valuable insights into one of the most pressing
environmental issues of our time: global climate
change. We explore the many possible causes of
climate change with special emphasis on the role
played by human activity (e.g., burning fossil fuels,
clearing vegetation). A thorough grounding in climate
science enables us to comprehend the implications of
anthropogenic climate change, how each of us
contributes to the problem, and how each of us can be
part of the solution to the problem.
The essential value in studying climate
science stems from the ecological and societal impacts
of climate and climate change. Climate is the ultimate
environmental control that governs our lives; for
example, what crops can be cultivated, the supply of
fresh water, and the average heating and cooling
requirements for homes.
By its very nature, climate science is
interdisciplinary, drawing on principles and basic
understandings of many scientific disciplines. We
recognize climate as a system in which Earth’s major
subsystems (i.e., atmosphere, hydrosphere, cryosphere,
geosphere, and biosphere) individually and in concert
function as controls of climate.
Linking these
subsystems are biogeochemical cycles (e.g., global
carbon cycle, global water cycle), pathways for transfer
of climate-sensitive materials (e.g., greenhouse gases)
and energy among Earth-bound reservoirs.
An easy and popular way of summarizing
local or regional climate is in terms of the averages of
weather elements, such as temperature and
precipitation, derived from observations taken over a
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span of many years. In this empirically-based context,
climate is defined as weather (the state of the
atmosphere) at some locality averaged over a specified
time interval. Climate must be specified for a
particular place and period because, like weather,
climate varies both spatially and temporally. Thus, for
example, the climate of Chicago differs from that of
New Orleans, and winters in Chicago were somewhat
milder in the 1980s and 1990s than in the 1880s and
1890s.
In addition to average values of weather
elements, the climate record includes extremes in
weather.
Climatic summaries typically tabulate
extremes such as the coldest, warmest, driest, wettest,
snowiest, or windiest day, month or year on record for
some locality. Extremes are useful aspects of the
climate record if only because what has happened in
the past can happen again. For this reason, for
example, farmers are interested in not only the average
rainfall during the growing season but also the
frequency of exceptionally wet or dry growing seasons.
In essence, records of weather extremes provide a
perspective on the variability of local or regional
climate.
Selection of an internationally agreed 30-year
period for averaging weather data may be inappropriate
for some applications because climate varies over a
broad range of time scales and can change significantly
in periods much shorter than 30 years. For example, El
Niño refers to an inter-annual variation in climate
involving air/sea interactions in the tropical Pacific and
weather extremes in various parts of the world. The
phenomenon typically lasts for 12 to 18 months and
occurs about every 3 to 7 years. For some purposes, a
30-year period is a short-sighted view of climate
variability. Compared to the long-term climate record,
for example, the current 1971-2000 averaging period
was unusually mild over much of the nation.
In the United States, 30-year averages are
computed for temperature, precipitation (rain plus
melted snow and ice), and degree days and identified
as normals. Averages of other climate elements such
as wind speed and humidity are derived from the entire
period of record or at least the period when
observations were made at the same location. Other
useful climate elements include average seasonal
snowfall, length of growing season, percent of possible
sunshine, and number of days with dense fog.
Tabulation of extreme values of weather elements is
usually also drawn from the entire period of the
observational record.
While the empirical definition of climate (in
terms of statistical summaries) is informative and
useful, the dynamic definition of climate is more
fundamental. It addresses the nature and controls of
Earth’s climate together with the causes of climate
variability and change operating on all time scales.
Climate differs from season to season and with those
variations in climate, the array of weather patterns that
characterize one season differs from the array of
characteristic weather patterns of another season. The
status of the planetary system (that is, the Earthatmosphere-land-ocean system) determines (or selects)
the array of possible weather patterns for any season.
In essence, this status constitutes boundary conditions
(i.e., forcing agents and mechanisms) such as incoming
solar radiation and the albedo (reflectivity) of Earth’s
surface. Hence, in a dynamic context, climate is
defined by the boundary conditions in the planetary
system coupled with the associated typical weather
patterns that vary with the seasons. For example, the
higher Sun’s path across the local sky and the longer
daylight length in Bismarck, ND during July increase
the chance of warm weather and possible
thunderstorms, whereas lower Sun angles and shorter
daylight duration during January would mean colder
weather and possible snow.
Climatology is the study of climate, its
controls, and spatial and temporal variability.
Climatology is primarily a field science rather than a
laboratory science. The field is the atmosphere and
Earth’s surface where data are obtained by direct (in
situ) measurement by instruments and remote sensing,
mostly by sensors flown aboard Earth-orbiting
satellites.
The only scientific experiments routinely
conducted by climate scientists involve manipulation
of numerical climate models. Usually these global or
regional models are used to predict the climatic
consequences of change in the boundary conditions of
Earth’s climate system. Furthermore, climatology is an
interdisciplinary science that reveals how the various
components of the natural world are interconnected.
For example, the composition of the atmosphere is the
end product of many processes where gases are emitted
(e.g., via volcanic eruptions) or absorbed (e.g., gases
dissolving in the ocean). The composition of the
atmosphere, in turn, affects the ocean, living
organisms, geological processes, and climate.
Climate and Society
Probably the single most important reason for studying
climate science is the many linkages between climate
and society. For one, climate imposes constraints on
social and economic development. For example, the
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abject poverty of North Africa’s Sahel in large measure
is due to the region’s subtropical climate that is
plagued by multi-decadal droughts. In other regions,
climate provides resources that are exploited to the
advantage of society. For example, some climates
favor winter or summer recreational activities (e.g.,
skiing, boating) that attract vacationers and feed the
local economy. Severe weather (e.g., tornadoes,
hurricanes, floods, heat waves, cold waves, and
drought) can cause deaths and injuries, considerable
long-term disruption of communities, property damage,
and economic loss. The impact of Hurricane Katrina
on the Gulf Coast is still being felt many years after
that weather system made landfall (August 2005).
Regardless of a nation’s status as developed or
developing, it is not possible to weather- or climateproof society to prevent damage to life and property.
In the agricultural sector, for example, the prevailing
strategy is to depend on technology to circumvent
climate constraints. Where water supply is limited,
farmers and ranchers routinely rely on irrigation water
usually pumped from subsurface aquifers (e.g., the
High Plains Aquifer in the central U.S.) or transferred
via aqueducts and canals from other watersheds.
Because of consumers’ food preferences and for
economic reasons, this strategy is preferred to
matching crops to the local or regional climate (e.g.,
dry land farming).
Other strategies include
construction of dams and reservoirs to control runoff
and genetic manipulation to breed drought resistant
crops. Although these strategies have some success,
they have limitations and often require tradeoffs. For
example, many rivers around the world lose so much of
their flow to diversions (mostly for irrigation) that they
are reduced to a trickle or completely dry up prior to
reaching the sea at least during part of the year.
Compounding the constraints of climate on
society is the prospect of global climate change. The
scientific evidence is now convincing that human
activity is influencing climate on a global scale with
significant consequences for society. Burning of fossil
fuels (coal, oil, natural gas) and clearing of vegetation
is responsible for a steady build-up of atmospheric
carbon dioxide (CO2) and enhancement of Earth’s
greenhouse effect. This enhancement is exacerbated
by other human activities that are increasing the
concentration of methane (CH4) and nitrous oxide
(N2O), also greenhouse gases.
Our understanding of the potential impact of
climate and climate change on society requires
knowledge of (1) the structure and function of Earth’s
climate system, (2) interactions of the various
components of that system, and (3) how human
activities influence and are influenced by these
systems.
The Climate System
What is the climate system and, more fundamentally,
what is a system? A system is an entity whose
components interact in an orderly manner according to
the laws of physics, chemistry, and biology. A familiar
example of a system is the human body, which consists
of various identifiable subsystems including the
nervous, respiratory, and reproductive systems, plus the
input/output of energy and matter. In a healthy person,
these subsystems function internally and interact with
one another in regular and predictable ways that can be
studied based upon analysis of the energy and mass
budgets for the systems. Extensive observations and
knowledge of a system enable scientists to predict how
the system and its components are likely to respond to
changing internal and external conditions. The ability
to predict the future state(s) of a system is important,
for example, in dealing with the complexities of global
climate change and its potential impacts on Earth’s
subsystems and society.
The 1992 United Nations Framework
Convention on Climate Change defines Earth’s
climate system as the totality of the atmosphere,
hydrosphere (including the cryosphere), biosphere and
geosphere and their interactions. The view of Planet
Earth in Figure 1, resembling a “blue marble,” shows
all the major subsystems of the climate system. The
ocean, the most prominent feature covering more than
two-thirds of Earth’s surface, appears blue. Clouds
obscure most of the ice sheets (the major part of the
cryosphere) that cover much of Greenland and
Antarctica. The atmosphere is made visible by
swirling storm clouds over the Pacific Ocean near
Mexico and the middle of the Atlantic Ocean. Viewed
edgewise, the atmosphere appears as a thin, bluish
layer. Land (part of the geosphere) is mostly green
because of vegetative cover (biosphere).
Figure 1. Planet Earth, viewed from space by satellite, appears as a “blue marble” with its surface mostly ocean
water and partially obscured by swirling masses of clouds. [Courtesy of NASA, Goddard Space Flight Center]
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ATMOSPHERE
Earth’s atmosphere is a relatively thin
envelope of gases and tiny suspended particles
surrounding the planet. But the thin atmospheric skin
is essential for life and the orderly functioning of
physical, chemical and biological processes on Earth.
Nitrogen (N2) and oxygen (O2), the chief atmospheric
gases, make up a uniform 78.08% and 20.95% by
volume, respectively through most of the atmosphere.
Not counting water vapor (with its highly variable
concentration), the next most abundant gases are argon
(0.93%) and carbon dioxide (0.038%). Many other
gases occur in the atmosphere in trace concentrations,
including ozone (O3) and methane (CH4) (Table 1).
Unlike nitrogen and oxygen, the percent volume of
some of these trace gases varies with time and location.
TABLE 1
Some Gases Composing Dry Air in the Lower
Atmosphere
Gas
% by volume
78.08
Nitrogen (N2)
20.95
Oxygen (O2)
Argon (Ar)
0.93
Carbon Dioxide (CO2) 0.0388
0.00014
Methane (CH4)
0.00005
Nitrous Oxide (N2O)
0.000007
Ozone (O3)
Parts per million
780,840.0
209,460.0
9,340.0
388.0
1.4
0.5
0.07
Aerosols, minute solid and liquid particles,
suspended mainly in the lower atmosphere derive from
wind erosion of soil, ocean spray, forest fires, volcanic
eruptions, industrial chimneys, and the exhaust of
motor vehicles. Although aerosol concentrations are
relatively small, they participate in some important
processes. Aerosols are nuclei for cloud formation,
interact with incoming solar radiation and dust blown
out over the tropical Atlantic Ocean from North Africa
may affect the development of tropical cyclones
(hurricanes and tropical storms).
The significance of an atmospheric gas is not
necessarily related to its concentration.
Some
atmospheric components that are essential for life
occur in very low concentrations. For example, most
water vapor is confined to the lowest kilometer or so of
the atmosphere and is never more than about 4% by
volume even in the most humid places on Earth (e.g.,
over tropical rainforests and seas). But without water
vapor, the planet would have no water cycle, no rain or
snow, no ocean, and no fresh water. Also, without
water vapor, Earth would be much too cold for most
forms of life to exist. Water vapor is the main
greenhouse gas, one that interacts with infrared
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radiation.
Although comprising only 0.038% of the
lower atmosphere, carbon dioxide is essential for photosynthesis. Without carbon dioxide, green plants and
the food webs they support could not exist. While the
atmospheric concentration of ozone (O3) is minute, the
chemical reactions responsible for its formation (from
oxygen) and dissociation (to oxygen) in the
stratosphere (mostly at altitudes between 30 and 50
km) shield organisms on Earth’s surface from
potentially lethal levels of solar UV radiation. Carbon
dioxide and ozone are also greenhouse gases.
The atmosphere is dynamic; the atmosphere
continually circulates in response to different rates of
heating and cooling within the rotating planetary
system. Heat is conveyed from warmer locations to
colder locations, from Earth’s surface to the
atmosphere and from the tropics to higher latitudes.
The global water cycle and accompanying phase
changes of water play an important role in this
planetary-scale transport of heat energy.
HYDROSPHERE
The hydrosphere is the water component of
the climate system. Water continually cycles among
reservoirs within the climate system. The ocean, by far
the largest reservoir of water in the hydrosphere, covers
about 70.8% of the planet’s surface and has an average
depth of about 3.8 km (2.4 mi). About 96.4% of the
hydrosphere is ocean salt water. The next largest
reservoir in the hydrosphere is glacial ice (also
considered the cryosphere), most of which covers
much of Antarctica and Greenland. Ice and snow make
up 2.1% of water in the hydrosphere. Considerably
smaller quantities of water occur on the land surface
(lakes, rivers), in the subsurface (soil moisture,
groundwater), the atmosphere (water vapor, clouds,
precipitation), and biosphere (plants, animals).
The ocean and atmosphere are coupled such
that the wind drives surface ocean currents. Winddriven currents are restricted to a surface ocean layer
typically about 100 m (300 ft) deep and take a few
months to years to cross an ocean basin. Ocean
currents at much greater depths are more sluggish and
more challenging to study than surface currents
because of greater difficulty in taking measurements.
Movements of deep-ocean waters are caused primarily
by small differences in water density (mass per unit
volume) arising from small differences in water temperature and salinity (a measure of dissolved salt
content). Cold sea water, being denser than warm
water, tends to sink whereas warm water, being less
dense, is buoyed upward by (or floats on) colder water.
Likewise, saltier water is denser than less salty water
and tends to sink, whereas less salty water is buoyed
upward. The combination of temperature and salinity
determines whether a water mass remains at its original
depth or sinks to the ocean bottom. Even though deep
currents are relatively slow, they keep ocean waters
well mixed so that the ocean has a nearly uniform
chemical composition.
The densest ocean waters form in polar or
nearby subpolar regions. Salty waters become even
saltier where sea ice forms at high latitudes because
growing ice crystals exclude dissolved salts. Chilling
of this salty water near Greenland and Iceland and in
the Norwegian and Labrador Seas further increases its
density so that surface waters sink and form a bottom
current that flows southward under equatorial surface
waters and into the South Atlantic as far south as
Antarctica. Here, deep water from the North Atlantic
mixes with deep water around Antarctica. Branches of
that cold bottom current then spread northward into the
Atlantic, Indian, and Pacific basins. Eventually, the
water slowly diffuses to the surface, mainly in the
Pacific, and then begins its journey on the surface
through the islands of Indonesia, across the Indian
Ocean, around South Africa, and into the tropical
Atlantic. There, intense heating and evaporation make
the water hot and salty. This surface water is then
transported northward in the Gulf Stream thereby
completing the cycle. This meridional overturning
circulation (MOC) and its transport of heat energy and
salt is an important control of climate.
The hydrosphere is dynamic; water moves
continually through different parts of Earth’s landatmosphere-ocean system and the ocean is the ultimate
destination of all moving water. Water flowing in river
or stream channels may take a few weeks to reach the
ocean. Groundwater typically moves at a very slow
pace through sediment, and the fractures and tiny
openings in bedrock, and feeds into rivers, lakes, or
directly into the ocean. The water of large, deep lakes
moves even more slowly, in some cases taking
centuries to reach the ocean via groundwater flow.
CRYOSPHERE
The frozen portion of the hydrosphere, the
cryosphere, encompasses massive continental (glacial)
ice sheets, much smaller ice caps and mountain
glaciers, ice in permanently frozen ground (permafrost), and the pack ice and ice bergs floating at sea.
All of these ice types except pack ice (frozen sea
water) and undersea permafrost are fresh water. A
glacier is a mass of ice that flows internally under the
influence of gravity. The Greenland and Antarctic ice
sheets in places are up to 3 km (1.8 mi) thick. The
Antarctic ice sheet contains 90% of all ice on Earth.
Much smaller glaciers (tens to hundreds of meters
thick) primarily occupy the highest mountain valleys
on all continents. At present, glacial ice covers about
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10% of the planet’s land area but at times during the
past 1.7 million years, glacial ice expanded over as
much as 30% of the land surface, primarily in the
Northern Hemisphere.
As snow accumulates, the pressure exerted by
the new snow converts underlying snow to ice. As the
ice forms, it preserves traces of the original seasonal
layering of snow and traps air bubbles. Chemical
analysis of the ice layers and air bubbles in the ice
provides clues to climatic conditions at the time the
original snow fell. Ice cores extracted from the
Greenland and Antarctic ice sheets yield information
on changes in Earth’s climate and atmospheric
composition extending as far back as hundreds of
thousands of years—to 800,000 years or more in
Antarctica.
Under the influence of gravity, glacial ice
flows slowly from sources at higher latitudes and
higher elevations (where some winter snow survives
the summer) to lower latitudes and lower elevations,
where the ice either melts or flows into the nearby
ocean. Around Antarctica, streams of glacial ice flow
out to the ocean. Ice, being less dense than seawater,
floats, forming ice shelves (typically about 500 m or
1600 ft thick). Thick masses of ice eventually break
off the shelf edge, forming flat-topped icebergs that are
carried by surface ocean currents around Antarctica.
Likewise, irregularly shaped icebergs break off the
glacial ice streams of Greenland and flow out into the
North Atlantic Ocean, posing a hazard to navigation.
Most sea ice surrounding Antarctica forms
each winter through freezing of surface seawater.
During summer most of the sea ice around Antarctica
melts, whereas in the Arctic Ocean sea ice can persist
for several years before flowing out through Fram
Strait into the Greenland Sea, and eventually melting.
This “multi-year” ice loses salt content with age as
brine, trapped between ice crystals, melts downward,
so that Eskimos can harvest this older, less salty ice for
drinking water.
How long is water frozen into glaciers?
Glaciers normally grow (thicken and advance) and
shrink (thin and retreat) slowly in response to changes
in climate. Mountain glaciers respond to climate
change on time scales of a decade. Until recently,
scientists had assumed that the response time for the
Greenland and Antarctic ice sheets is measured in
millennia; however, two Greenland glaciers have
exhibited significant changes in discharge in only a few
years. Changes in ice surface elevation were detected
by sensors onboard NASA’s Ice, Cloud, and Land
Elevation Satellite (ICESat). Hence, ice sheet glaciers
may behave more like mountain glaciers, raising
questions regarding the long-term stability of polar ice
sheets and their response to global climate change.
GEOSPHERE
The geosphere is the solid portion of the
planet consisting of rocks, minerals, soil, and
sediments. Surface geological processes encompass
weathering and erosion occurring at the interface
between Earth’s crust and the other Earth subsystems.
Weathering entails the physical disintegration,
chemical decomposition, or solution of exposed rock.
Rock fragments produced by weathering become
sediments.
Water plays an important role in
weathering by dissolving soluble rock and minerals,
and participating in chemical reactions that decompose
rock. Water’s unusual physical property of expanding
while freezing can fragment rock when the water
saturates tiny cracks and pore spaces. Often the water
is not as confined and fragmentation is due to stress
caused by the growth of ice lenses within the rock.
The ultimate weathering product is soil, a
mixture of organic (humus) and inorganic matter
(sediment) on Earth’s surface that supports plants, also
supplying nutrients and water. Soils derive from the
weathering of bedrock or sediment, and vary widely in
texture (particle size). Typical soil is 50% open space
(pores), roughly equal proportions of air and water.
Plants also participate in weathering via the physical
action of their growing roots and the carbon dioxide
they release to the soil.
Erosion refers to the removal and transport of
sediments by gravity, moving water, glaciers, and
wind. Running water and glaciers are pathways in the
global water cycle. Erosive agents transport sediments
from source regions (usually highlands) to low-lying
depositional areas (e.g., ocean, lakes). Weathering aids
erosion by reducing massive rock to particles that are
sufficiently small to be transported by agents of
erosion.
Erosion aids weathering by removing
sediment and exposing fresh surfaces of rock to the
atmosphere and weathering processes.
Together,
weathering and erosion work to reduce the elevation of
the land.
Internal geological processes counter surface
geological processes by uplifting land through tectonic
activity, including volcanism and mountain building.
Most tectonic activity occurs at the boundaries between
crustal plates. The overlying crust and rigid mantle is
broken into a dozen massive plates (and many smaller
ones) that are slowly driven (typically less than 20 cm
per year) across the face of the globe by huge
convection currents in Earth’s mantle. Continents are
carried on the moving plates and ocean basins are
formed by seafloor spreading.
Plate tectonics probably has operated on the
planet for at least 3 billion years, with continents
periodically assembling into supercontinents and then
splitting apart. The most recent supercontinent, called
Pangaea (Greek for “all land”), broke apart about 200
6
million years ago and its constituent landmasses, the
continents of today, slowly moved to their present locations.
Plate tectonics explains such seemingly
anomalous discoveries as glacial sediments in the
Sahara and fossil coral reefs, indicative of tropical
climates, in northern Wisconsin. Such discoveries
reflect climatic conditions hundreds of millions of
years ago when the continents were at different
latitudes than they are today.
Geological processes occurring at boundaries
between plates produce large-scale landscape and
ocean bottom features, including mountain ranges,
volcanoes, deep-sea trenches, as well as the ocean
basins themselves. Enormous stresses develop at plate
boundaries, bending and fracturing bedrock over broad
areas. Hot molten rock material, known as magma,
wells up from deep in the crust or upper mantle and
migrates along rock fractures. Some magma pushes
into the upper portion of the crust where it cools and
solidifies into massive bodies of rock, forming the core
of mountain ranges (e.g., Sierra Nevada). Some
magma feeds volcanoes or flows through fractures in
the crust and spreads over Earth’s surface as lava flows
(flood basalts) that cool and slowly solidify (e.g.,
Columbia River Plateau in the Pacific Northwest and
the massive Siberian Traps). At spreading plate
boundaries on the sea floor, upward flowing magma
solidifies into new oceanic crust. Plate tectonics and
associated volcanism are important in geochemical
cycling, releasing to the atmosphere water vapor,
carbon dioxide, and other gases that impact climate.
BIOSPHERE
All living plants and animals on Earth are
components of the biosphere. They range in size from
microscopic single-celled bacteria to the largest
organisms (e.g., redwood trees and blue whales).
Bacteria and other single-celled organisms dominate
the biosphere, both on land and in the ocean.
Organisms on land or in the atmosphere live close to
Earth’s surface. However, marine organisms occur
throughout the ocean depths and even inhabit rock
fractures, volcanic vents, and the ocean floor. Certain
organisms live in extreme environments at
temperatures and pressures once considered impossible
to support life. In fact, some scientists estimate that the
mass of organisms living in fractured rocks on and
below the ocean floor may vastly exceed the mass of
organisms living on or above it.
Photosynthesis and cellular respiration are
essential for life near the surface of the Earth, and
exemplify how the biosphere interacts with the other
subsystems of the climate system. Photosynthesis is
the process whereby green plants convert light energy
from the Sun, carbon dioxide from the atmosphere, and
water to sugars and oxygen (O2). The sugars, which
contain a relatively large amount of energy and
oxygen, are essential for cellular respiration. Through
cellular respiration, an organism processes food and
liberates energy for maintenance, growth, and reproduction, also releasing carbon dioxide, water, and heat
energy to the environment. With few exceptions,
sunlight is the originating source of energy for most organisms living on land and in the ocean’s surface
waters.
Dependency between organisms on one
another (e.g., as a source of food) and on their physical
and chemical environment (e.g., for water, oxygen,
carbon dioxide, and habitat) is embodied in the concept
of ecosystem. Ecosystems consist of plants and
animals that interact with one another, together with
the physical conditions and chemical substances in a
specific geographical area. An ecosystem is home to
producers (plants) which take nutrients to produce
foods, consumers (animals) which consume the food to
grow, and decomposers (bacteria, fungi) which return
nutrients to the environment.
Feeding relationships among organisms,
called a food chain, can be quite simple or more
complex as in a food web. In a food chain, each stage,
a trophic (or feeding) level transfers only about 10% of
the energy available to the next higher level, i.e.
producers to consumers to decomposers. Because
mass transfers are more easily measured than energy,
biomass, the total weight or mass of organisms, is
generally tracked through food chains or webs.
Climate is the principal ecological control,
largely governing the location and species composition
of natural ecosystems such as deserts, rain forests, and
tundra. A warmer climate would likely mean fewer
days of arctic air and a northward shift of the boreal
forest. What actually happens to the forest, however,
could hinge on the rate of climate change. Relatively
rapid warming may not only shift an ecosystem
northward but also alter the ecosystem’s species
composition and disturb the orderly internal operation
of the ecosystem. For example, rapid climate change
could
disrupt
long-established
predator/prey
relationships with implications for the stability of
populations of plants and animals.
Similar observations of close relationships
between vegetation and climate variables on a global
basis were made by the noted German climatologist
Wladimir Köppen (1846-1940) in the early 20th
century. This is a central aspect of his widely used
climate classification system.
Subsystem Interactions: Biogeochemical
Cycles
Biogeochemical cycles are the pathways along which
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solids, liquids, and gases move among the various
reservoirs on Earth, often involving physical or
chemical changes to these substances. Accompanying
these flows of materials are transfers and
transformations of energy. Reservoirs in these cycles
are found within the subsystems of the overall
planetary
system
(atmosphere,
hydrosphere,
cryosphere, geosphere, and biosphere). Examples of
biogeochemical cycles are the water cycle, carbon
cycle, oxygen cycle, and nitrogen cycle.
Earth is an open (or flow-through) system for
energy, where energy is defined as the capacity for
doing work. Earth receives energy from the Sun
primarily and some from its own interior while
emitting energy in the form of invisible infrared
radiation to space. Along the way, energy is neither
created nor destroyed, although it is converted from
one form to another. This is the law of energy
conservation (also known as the first law of
thermodynamics).
The Earth system is essentially closed for
matter; that is, it neither gains nor loses matter over
time (except for meteorites and asteroids).
All
biogeochemical cycles obey the law of conservation of
matter, which states that matter can be neither created
nor destroyed, but can change in chemical or physical
form. When a log burns in a fireplace, a portion of the
log is converted to ash and heat energy, while the rest
goes up the chimney as carbon dioxide, water vapor,
creosote and heat. In terms of accountability, all losses
from one reservoir in a cycle can be accounted for as
gains in other reservoirs of the cycle.
Stated
succinctly, for any reservoir:
Input = Output + Storage
The quantity of a substance stored in a
reservoir depends on the rates at which the material is
cycled into and out of the reservoir. This cycling will
include gains to or losses from a reservoir through
chemical reactions within the reservoir. If the input
rate exceeds the output rate, the amount of material
stored in the reservoir increases. If the input rate is less
than the output rate, the amount stored decreases. Over
the long term, the cycling rates of materials among the
various global reservoirs are relatively stable; that is,
equilibrium tends to prevail between the rates of input
and output.
Consider the global cycling of carbon as an
illustration of a biogeochemical cycle that has
important implications for climate (Figure 1.16).
Through photosynthesis, carbon dioxide cycles from
the atmosphere to green plants where carbon is
incorporated into sugar (C6 H12O6). Plants use sugar to
manufacture other organic compounds including fats,
proteins, and other carbohydrates. As a byproduct of
cellular respiration, plants and animals transform a portion of the carbon in these organic compounds into CO2
that is released to the atmosphere. In the ocean, CO2 is
cycled into and out of marine organisms through
photosynthesis and respiration. In addition to the
uptake of CO2 via photosynthesis, marine organisms
also use carbon for calcium carbonate (CaCO3) to make
hard, protective shells. Furthermore, decomposer
organisms (e.g., bacteria) act on the remains of dead
plants and animals, releasing CO2 to the atmosphere
and ocean through cellular respiration.
When marine organisms die, their remains
(shells and skeletons) slowly settle downward through
ocean waters. In time, these organic materials reach
the sea floor, accumulate, are compressed by their own
weight and the weight of other sediments, and
gradually transform into solid, carbonate rock.
Common carbonate rocks are limestone (CaCO3) and
Subsequently, tectonic
dolostone (CaMg(CO3)2).
processes uplift these marine rocks and expose them to
the atmosphere and weathering processes. Rainwater
contains dissolved atmospheric CO2 producing
carbonic acid (H2CO3) that, in turn, dissolves carbonate
rock releasing CO2. As part of the global water cycle,
rivers and streams transport these weathering products
to the sea where they settle out of suspension or
precipitate as sediments that accumulate on the ocean
floor. Over the millions of years that constitute
geologic time, the formation and ultimate weathering
and erosion of carbon-containing rocks have
significantly altered the concentration of carbon
dioxide in the atmosphere thereby changing the
climate.
From about 280 to 345 million years ago, the
geologic time interval known as the Carboniferous
period, trillions of metric tons of organic remains
8
(detritus) accumulated on the ocean bottom and in lowlying swampy terrain on land. The supply of detritus
was so great that decomposer organisms could not keep
pace. In some marine environments, plant and animal
remains were converted to oil and natural gas. In
swampy terrain, heat and pressure from accumulating
organic debris concentrated carbon, converting the
remains of luxuriant swamp forests into thick layers of
coal. Today, when we burn coal, oil, and natural gas,
collectively called fossil fuels, we are tapping energy
that was originally locked in vegetation through
photosynthesis hundreds of millions of years ago.
During combustion, carbon from these fossil fuels
combines with oxygen in the air to form carbon dioxide
which escapes to the atmosphere.
Another important biogeochemical cycle
operating in the Earth system is the global water cycle
(Chapter 5), which is closely linked to all other
biogeochemical cycles. Reservoirs in the water cycle
(hydrosphere, atmosphere, geosphere, biosphere) are
also reservoirs in other cycles, for which water is an
essential mode of transport. In the nitrogen cycle, for
example, intense heating of air caused by lightning
combines atmospheric nitrogen (N2), oxygen (O2), and
moisture to form droplets of extremely dilute nitric
acid (HNO3) that are washed by rain to the soil. In the
process, nitric acid converts to nitrate (NO3-), an
important plant nutrient that is taken up by plants via
their root systems. Plants convert nitrate to ammonia
(NH3), which is incorporated into a variety of
compounds, including amino acids, proteins, and DNA.
On the other hand, both nitrate and ammonia readily
dissolve in water so that heavy rains can deplete soil of
these important nutrients and wash them into
waterways.