The Carbon Cycle
The concentration of carbon in living matter (18%) is
almost 100 times greater than its concentration in the
earth (0.19%). So living things extract carbon from their
nonliving environment. For life to continue, this carbon
must be recycled.
What is the Carbon Cycle?
The Carbon Cycle is a complex series of processes through
which all of the carbon atoms in existence rotate. The same
carbon atoms in your body today have been used in
countless other molecules since time began. The wood
burned just a few decades ago could have produced carbon
dioxide which through photosynthesis became part of a
plant. When you eat that plant, the same carbon from the
wood which was burnt can become part of you. The carbon
cycle is the great natural recycler of carbon atoms.
Unfortunately, the extent of its importance is rarely stressed
enough. Without the proper functioning of the carbon cycle,
every aspect of life could be changed dramatically.
Sample carbon cycle and how carbon atoms move through our
natural world
Plants, animals, and soil interact to make up the basic cycles of nature.
In the carbon cycle, plants absorb carbon dioxide from the
atmosphere and use it, combined with water they get from the soil, to
make the substances they need for growth. The process of
photosynthesis incorporates the carbon atoms from carbon dioxide
into sugars. Animals, such as the rabbit pictured here, eat the plants
and use the carbon to build their own tissues. Other animals, such as
the fox, eat the rabbit and then use the carbon for their own needs.
These animals return carbon dioxide into the air when they breathe,
and when they die, since the carbon is returned to the soil during
decomposition. The carbon atoms in soil may then be used in a new
plant or small microorganisms. Ultimately, the same carbon atom can
move through many organisms and even end in the same place where
it began.
Sample Carbon Cycle Diagram
The carbon cycle is the biogeochemical cycle by which
carbon is exchanged between the biosphere, geosphere,
hydrosphere, and atmosphere of the earth. (Other
planetary bodies may have carbon cycles, but little is known
about them.)
The cycle is usually thought of as four main reservoirs of
carbon, interconnected by pathways of exchange. These
reservoirs are the atmosphere, terrestrial biosphere,
oceans, carbonate rocks, and sediments (as organic matter,
including fossil fuels). The movement of carbon—the
carbon exchanges between reservoirs—occurs because of
various chemical, physical, geological, and biological
processes. Overall, the carbon cycle reveals the harmonious
coordination between different biotic and abiotic elements
on Earth.
The global carbon budget is the balance of the exchanges
(incomes and losses) of carbon between the carbon
reservoirs or between one specific loop (e.g., atmospherebiosphere) of the carbon cycle. An examination of the
carbon budget of a pool or reservoir can provide
information about whether that pool or reservoir is
functioning as a source or sink for carbon over different
time scales.
The carbon cycle is central to understanding issues related
to climate change. In resolving the divergent positions with
respect to increases of carbon dioxide in the atmosphere
and global warming, it is important that scientists maintain
integrity in collecting, analyzing, and presenting data in the
face of often strong political, commercial, and
environmental agendas.
Carbon in the atmosphere
Carbon exists in the Earth's atmosphere primarily as the
gas carbon dioxide (CO2). Although it comprises a very
small part of the atmosphere overall (approximately 0.04
percent), it plays an important role in supporting life.
Other gases containing carbon in the atmosphere are
methane and chlorofluorocarbons (the latter are entirely
artificial and are now strictly prohibited under the
Montreal Protocol).
Carbon exchange with the atmosphere, biosphere, and
oceans
Photosynthesis
Utilizing light from the sun, plants and algae perform
photosynthesis to convert carbon dioxide, water, and
sunlight into carbohydrates (C6H12O6, releasing oxygen in
the process. This process removes carbon dioxide from the
atmosphere and stores it in plant biomass, which may
eventually get buried in sediments after the plant dies.
Respiration
Respiration occurs when the biomass from
photosynthetic plants and algae is consumed by animals,
fungi, or bacteria, either while the plant is alive, or after
it has died.
This is essentially the reverse process of photosynthesis,
releasing CO2 back into the atmosphere. However, more
material is photosynthesized than is respired (since a
portion of the organic matter is buried in the sediments),
thus more oxygen enters the atmosphere than does
carbon dioxide as a result of these two processes.
Outgassing
Outgassing of volcanoes and mid-ocean ridges is the largest
source of carbon dioxide in the atmosphere, releasing
carbon dioxide from deep within the Earth that had been
trapped there since the planet's creation. CO2 is released
from subduction zones through metamorphism of
carbonate rocks subducting with the ocean crust. Not all of
this CO2 enters the atmosphere. Some of it dissolves in the
oceans and some remains in biomass of organisms.
Weathering
Weathering is a mechanism that removes carbon from the
atmosphere. When carbon dioxide dissolves in water, it
forms carbonic acid. This acid is used to weather rocks,
yielding bicarbonate ions in addition to other ions
(depending on the mineral content of the rock). The
bicarbonate ion enters oceans through fresh water
systems, and in the ocean, the bicarbonate ion combines
with a calcium ion to form calcium carbonate and a by
product of carbon dioxide and water. The calcium
carbonate is used by marine organisms to form calcareous
shells, and corals use it in their exoskeletons.
Solubility pump
The solubility pump is a physico-chemical process that transports
carbon (as dissolved inorganic carbon) from the ocean's surface to its
interior.
The solubility pump is driven by the coincidence of two processes in
the ocean:
• The solubility of carbon dioxide is a strong inverse function of
seawater temperature (i.e. solubility is greater in cooler water)
• The thermohaline circulation, ocean circulation driven by density
differences in salinity and temperature, is driven by the formation
of deep water at high latitudes where seawater is usually cooler
and more dense
Since deep water (that is, seawater in the ocean's interior) is
formed under the same surface conditions that promote
carbon dioxide solubility, it contains a higher concentration
of dissolved inorganic carbon than one might otherwise
expect. Consequently, these two processes act together to
pump carbon from the atmosphere into the ocean's interior.
One consequence of this is that when deep water upwells in
warmer, equatorial latitudes, it strongly outgasses carbon
dioxide to the atmosphere because of the reduced solubility
of the gas.
Carbon in the biosphere
Carbon is an essential part of life on Earth. It plays an
important role in the structure, biochemistry, and nutrition
of all living cells. And life plays an important role in the
carbon cycle:
Autotrophs are organisms that produce their own organic
compounds using carbon dioxide from the air or water in
which they live. To do this they require an external source of
energy. Almost all autotrophs use solar radiation to provide
this, and their production process is called photosynthesis. A
small number of autotrophs exploit chemical energy sources,
chemosynthesis. The most important autotrophs for the
carbon cycle are trees in forests on land and phytoplankton
in the Earth's oceans.
Carbon is transferred within the biosphere as heterotrophs
feed on other organisms or their parts (e.g., fruits). This
includes the uptake of dead organic material (detritus) by
fungi and bacteria for fermentation or decay.
Most carbon leaves the biosphere through respiration.
When oxygen is present, aerobic respiration occurs, which
releases carbon dioxide into the surrounding air or water.
Otherwise, anaerobic respiration occurs and releases
methane into the surrounding environment, which
eventually makes its way into the atmosphere or
hydrosphere (e.g., as marsh gas or flatulence).
Carbon may also leave the biosphere when dead organic
matter (such as peat) becomes incorporated in the
geosphere. Animal shells of calcium carbonate, in particular,
may eventually become limestone through the process of
sedimentation.
Much remains to be learned about the cycling of carbon in
the deep ocean. For example, a recent discovery is that
larvacean mucus houses (commonly known as "sinkers") are
created in such large numbers that they can deliver as much
carbon to the deep ocean as has been previously detected by
sediment traps (Bennett 2005). Because of their size and
composition, these houses are rarely collected in such traps,
so most biogeochemical analyses have erroneously ignored
them.
Carbon in the oceans
Inorganic carbon, that is, carbon compounds with no carboncarbon or carbon-hydrogen bonds, is important in its
reactions within water. This carbon exchange becomes
important in controlling pH in the ocean and can also vary as
a source or sink for carbon. Carbon is readily exchanged
between the atmosphere and ocean. In regions of oceanic
upwelling, carbon is released to the atmosphere. Conversely,
regions of down welling transfer carbon (CO2) from the
atmosphere to the ocean. When CO2 enters the ocean,
carbonic acid is formed: CO2 + H2O <—> H2CO3
This reaction has a forward and reverse rate; that is it
achieves a chemical equilibrium.
Another reaction important in controlling oceanic pH levels
is the release of hydrogen ions and bicarbonate.
The Carbon Cycle and the Climate
Carbon dioxide and methane are two carbon compounds that act as
greenhouse gases in Earth's atmosphere, insulating the planet and
making it a comfortable place for organisms to survive.
The carbon cycle responds to perturbations through a series of
feedbacks so that temperatures never get too hot or too cold, within
certain bounds. For example, if CO2 outgassing from volcanoes and
mid-ocean ridges increases as a result of increased tectonic activity,
atmospheric temperatures will rise. Rising temperatures and increased
amounts of dissolved CO2 will result in increased rates of weathering
of crustal rocks, which will use up the surplus CO2, decrease
atmospheric CO2 levels, and bring temperatures back down. On the
other hand, if global cooling occurred, weathering would slow down
and CO2 would build up in the atmosphere and temperatures would
rise again.
The recent debate about anthropogenic (human-induced)
climate change has been centered around the release of
thousands of tons of carbon dioxide from the burning of
fossil fuels and its effect on global climate. Some scientists,
using carbon cycle climate models, argue that with the
"business as usual" scenario, atmospheric temperatures
will rise over the next century (Cox et al. 2000). Other
studies suggest that ocean uptake of CO2 will slow because
of increased stratification of the ocean (less deep mixing)
(Sarmiento et al. 1998). In addition, increased global
temperatures would warm the oceans, decreasing the
solubility of CO2 in ocean water. All of these factors are
considered to cause a build up of CO2 in the atmosphere.