Carbon Cycle
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Carbon cycle
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For the thermonuclear reaction involving carbon that helps power stars, see CNO cycle.
Diagram of the carbon cycle. The black numbers indicate how much carbon is stored in
various reservoirs, in billions of tons ("GtC" stands for GigaTons of Carbon and figures
are circa 2004). The purple numbers indicate how much carbon moves between
reservoirs each year. The sediments, as defined in this diagram, do not include the ~70
million GtC of carbonate rock and kerogen.
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the
biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth.
The cycle is usually thought of as four major reservoirs of carbon interconnected by
pathways of exchange. These reservoirs are:
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The atmosphere.
The terrestrial biosphere, which is usually defined to include fresh water systems
and non-living organic material, such as soil carbon.
The oceans, including dissolved inorganic carbon and living and non-living
marine biota,
The sediments including fossil fuels.
The annual movements of carbon, the carbon exchanges between reservoirs, occur
because of various chemical, physical, geological, and biological processes. The ocean
contains the largest active pool of carbon near the surface of the Earth, but the deep ocean
part of this pool does not rapidly exchange with the atmosphere.
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., atmosphere ↔
biosphere) of the carbon cycle. An examination of the carbon budget of a pool or
reservoir can provide information about whether the pool or reservoir is functioning as a
source or sink for carbon dioxide.
Contents
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1 In the atmosphere
2 Carbon Dioxide
3 In the biosphere
4 In the ocean
5 See also
6 References
o 6.1 Further reading
7 External links
[edit] In the atmosphere
Carbon exists in the Earth's atmosphere primarily as the gas carbon dioxide (CO2).
Although it is a small percentage of the atmosphere (approximately 0.04% on a molar
basis, and increasing), it plays an important role in supporting life. Other gases containing
carbon in the atmosphere are methane and chlorofluorocarbons (the latter is entirely
anthropogenic). The overall atmospheric concentration of these greenhouse gases has
been increasing in recent decades, contributing to global warming.[1]
Carbon is taken from the atmosphere in several ways:
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When the sun is shining, plants perform photosynthesis to convert carbon dioxide
into carbohydrates, releasing oxygen in the process. This process is most prolific
in relatively new forests where tree growth is still rapid. The effect is strongest in
deciduous forests during spring leafing out. This is visible as an annual signal in
the Keeling curve of measured CO2 concentration. Northern hemisphere spring
predominates, as there is far more land in temperate latitudes in that hemisphere
than in the southern.
Forests store 86% of the planet's above-ground carbon and 73% of the planet's
soil carbon.[2]
At the surface of the oceans towards the poles, seawater becomes cooler and more
carbonic acid is formed as CO2 becomes more soluble. This is coupled to the
ocean's thermohaline circulation which transports dense surface water into the
ocean's interior (see the entry on the solubility pump).
In upper ocean areas of high biological productivity, organisms convert reduced
carbon to tissues, or carbonates to hard body parts such as shells and tests. These
are, respectively, oxidized (soft-tissue pump) and redissolved (carbonate pump) at
lower average levels of the ocean than those at which they formed, resulting in a
downward flow of carbon (see entry on the biological pump).
The weathering of silicate rock. Carbonic acid reacts with weathered rock to
produce bicarbonate ions. The bicarbonate ions produced are carried to the ocean,
where they are used to make marine carbonates. Unlike dissolved CO2 in
equilibrium or tissues which decay, weathering does not move the carbon into a
reservoir from which it can readily return to the atmosphere.
Carbon is released into the atmosphere in several ways:
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Through the respiration performed by plants and animals. This is an exothermic
reaction and it involves the breaking down of glucose (or other organic
molecules) into carbon dioxide and water.
Through the decay of animal and plant matter. Fungi and bacteria break down the
carbon compounds in dead animals and plants and convert the carbon to carbon
dioxide if oxygen is present, or methane if not.
Through combustion of organic material which oxidizes the carbon it contains,
producing carbon dioxide (and other things, like water vapor). Burning fossil
fuels such as coal, petroleum products, and natural gas releases carbon that has
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been stored in the geosphere for millions of years. Burning agrofuels also releases
carbon dioxide.
Production of cement. Carbon dioxide is released when limestone (calcium
carbonate) is heated to produce lime (calcium oxide), a component of cement.
At the surface of the oceans where the water becomes warmer, dissolved carbon
dioxide is released back into the atmosphere.
Volcanic eruptions and metamorphism release gases into the atmosphere.
Volcanic gases are primarily water vapor, carbon dioxide and sulfur dioxide. The
carbon dioxide released is roughly equal to the amount removed by silicate
weathering; so the two processes, which are the chemical reverse of each other,
sum to roughly zero, and do not affect the level of atmospheric carbon dioxide on
time scales of less than about 100,000 years.
[edit] Carbon Dioxide
This section overlaps with other sections. It should be combined with the rest of
the article.
Please improve this article if you can.
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In the recent past, transfer rates due to photosynthesis and respiration and
including decay, were just about even. However, more carbon dioxide is being
deposited in the Earth's atmosphere, than is being removed.
In 1850, atmospheric carbon dioxide was about 280 parts per million (ppm), and
today it is about 350 ppm. This increase is believed to be due to the burning of
wood and fossil fuels and the destruction of forests to make way for farmland and
pasture.[3]
[edit] In the biosphere
Around 1,900 gigatons of carbon are present 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.
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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 in a process called chemosynthesis.
The most important autotrophs for the carbon cycle are trees in forests on land
and phytoplankton in the Earth's oceans. Photosynthesis follows the reaction
6CO2 + 6H2O → C6H12O6 + 6O2
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.
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Most carbon leaves the biosphere through respiration. When oxygen is present,
aerobic respiration occurs, which releases carbon dioxide into the surrounding air
or water, following the reaction C6H12O6 + 6O2 → 6CO2 + 6H2O. 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).
Burning of biomass (e.g. forest fires, wood used for heating, anything else
organic) can also transfer substantial amounts of carbon to the atmosphere
Carbon may also be circulated within 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.[4] Because of
their size and composition, these houses are rarely collected in such traps, so most
biogeochemical analyses have erroneously ignored them.
Carbon storage in the biosphere is influenced by a number of processes on different timescales: while net primary productivity follows a diurnal and seasonal cycle, carbon can be
stored up to several hundreds of years in trees and up to thousands of years in soils.
Changes in those long term carbon pools (e.g. through de- or afforestation or through
temperature-related changes in soil respiration) may thus affect global climate change.
[edit] In the ocean
"Present day" (1990s) sea surface dissolved inorganic carbon concentration (from the
GLODAP climatology)
The oceans contain around 36,000 gigatonnes of carbon, mostly in the form of
bicarbonate ion (over 90%, with most of the remainder being carbonate). Extreme storms
such as hurricanes and typhoons bury a lot of carbon, because they wash away so much
sediment. For instance, a team reported in the July 2008 issue of the journal Geology that
a single typhoon in Taiwan buries as much carbon in the ocean -- in the form of sediment
-- as all the other rains in that country all year long combined.[5] Inorganic carbon, that is
carbon compounds with no carbon-carbon 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 downwelling transfer carbon (CO2) from the
atmosphere to the ocean. When CO2 enters the ocean, it participates in a series of
reactions which are locally in equilibrium:
Solution:
CO2(atmospheric) ⇌ CO2(dissolved)
Conversion to carbonic acid:
CO2(dissolved) + H2O ⇌ H2CO3
First ionization:
H2CO3 ⇌ H+ + HCO3− (bicarbonate ion)
Second ionization:
HCO3− ⇌ H+ + CO3−− (carbonate ion)
This set of reactions, each of which has its own equilibrium coefficient determines the
form that inorganic carbon takes in the oceans[6]. The coefficients, which have been
determined empirically for ocean water, are themselves functions of temperature,
pressure, and the presence of other ions (especially borate). In the ocean the equilibria
strongly favor bicarbonate. Since this ion is three steps removed from atmospheric CO2,
the level of inorganic carbon storage in the ocean does not have a proportion of unity to
the atmospheric partial pressure of CO2. The factor for the ocean is about ten: that is, for
a 10% increase in atmospheric CO2, oceanic storage (in equilibrium) increases by about
1%, with the exact factor dependent on local conditions. This buffer factor is often called
the "Revelle Factor", after Roger Revelle.
In the oceans, bicarbonate can combine with calcium to form limestone (calcium
carbonate, CaCO3, with silica), which precipitates to the ocean floor. Limestone is the
largest reservoir of carbon in the carbon cycle. The calcium comes from the weathering
of calcium-silicate rocks, which causes the silicon in the rocks to combine with oxygen to
form sand or quartz (silicon dioxide), leaving calcium ions available to form limestone[7].
[edit] See also
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C4MIP
Carbon cycle re-balancing
Carbon footprint
Global Carbon Project
Carbon diet
Low carbon diet
Ocean acidification
Primary production
Breathing
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Calvin cycle
[edit] References
1. ^ The Intergovernmental Panel on Climate Change (IPCC) which represents wide
consensus of international scientific opinion.
2. ^ Sedjo, Roger.1993. The Carbon Cycle and Global Forest Ecosystem. Water, Air, and
Soil Pollution 70, 295-307. (via Oregon Wild Report on Forests, Carbon, and Global
Warming)
3. ^ Sylvia Madder: Biology, 9th ed. McGraw-Hill, NY, 2007.
4. ^ Monterey Bay Aquarium Research Institute (MBARI) (2005-06-09). ""Sinkers"
provide missing piece in deep-sea puzzle". Press release. Retrieved on 2007-10-07.
5. ^ Typhoons Bury Tons of Carbon in the Oceans Newswise, Retrieved on July 27, 2008.
6. ^ Millero, Frank J. (2005). Chemical Oceanography, 3, CRC Press. ISBN 0849322804.
7. ^ Notes, Lecture. "The Carbon Cycle". Department of Atmospheric Sciences. University
of Washington. Retrieved on 2008-07-08.
[edit] Further reading
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Appenzeller, Tim (2004). "The case of the missing carbon". National Geographic
Magazine. - article about the missing carbon sink
Bolin, Bert; Degens, E. T.; Kempe, S.; Ketner, P. (1979). The global carbon
cycle. Chichester ; New York: Published on behalf of the Scientific Committee on
Problems of the Environment (SCOPE) of the International Council of Scientific
Unions (ICSU) by Wiley. ISBN 0471997102. Retrieved on 2008-07-08.
Houghton, R. A. (2005). "The contemporary carbon cycle", in William H
Schlesinger (editor): Biogeochemistry. Amsterdam: Elsevier Science, 473-513.
ISBN 0080446426.
Janzen, H. H. (2004). "Carbon cycling in earth systems—a soil science
perspective". Agriculture, ecosystems and environment 104 (3): 399–417.
doi:10.1016/j.agee.2004.01.040. ISSN 0167-8809.
Millero, Frank J. (2005). Chemical Oceanography, 3, CRC Press. ISBN
0849322804.
Sundquist, Eric; Broecker, Wallace S.(editors) (1985). The Carbon Cycle and
Atmospheric CO2: Natural variations Archean to Present, Geophysical
Monographs Series 32. American Geophysical Union.
[edit] External links
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Carbon Cycle Science Program - an interagency partnership.
NOAA's Carbon Cycle Greenhouse Gases Group
Global Carbon Project - initiative of the Earth System Science Partnership
UNEP - The present carbon cycle - Climate Change carbon levels and flows
Biogeochemical cycles
Carbon cycle - Hydrogen cycle - Nitrogen cycle
Oxygen cycle - Phosphorus cycle - Sulfur cycle - Water cycle
Retrieved from "http://en.wikipedia.org/wiki/Carbon_cycle"
Categories: Geochemistry | Chemical oceanography | Photosynthesis | Soil biology | Soil
chemistry | Carbon | Numerical climate and weather models | Biogeography
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http://en.wikipedia.org/wiki/Carbon_cycle
The Carbon Cycle
Carbon is an important element to living things. As we learned earlier, the most abundant
substance in organisms is water. The second most abundant substance is carbon. Much of
the solid portions of life forms in this manner. Today, almost 99% of all the carbon on
Earth has been locked up deep within the Earth.
Carbon cycle movie
http://epa.gov/climatechange/kids/ca
rbon_cycle_version2.html
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. That is our
topic.
Carbon exists in the nonliving environment as:
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carbon dioxide (CO2) in the atmosphere and dissolved in water (forming HCO3−)
carbonate rocks (limestone and coral = CaCO3)
deposits of coal, petroleum, and natural gas derived from once-living things
dead organic matter, e.g., humus in the soil
Carbon enters the biotic world through the action of autotrophs:
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primarily photoautotrophs, like plants and algae, that use the energy of light to
convert carbon dioxide to organic matter.
Links to
photosynthesis
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and to a small extent, chemoautotrophs — bacteria and archaea that do the same
but use the energy derived from an oxidation of molecules in their substrate.
Carbon returns to the atmosphere and water by
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respiration (as CO2)
burning
decay (producing CO2 if oxygen is present, methane (CH4) if it is not.
The uptake and return of CO2 are not in balance.
The carbon dioxide content of the atmosphere is
gradually and steadily increasing. The graph shows
the CO2 concentration at the summit of Mauna Loa
in Hawaii from 1958 through 1999. The values are
in parts per million (ppm). The seasonal fluctuation
is caused by the increased uptake of CO2 by plants
in the summer.
The increase in CO2 probably began with the start of the industrial revolution. Samples of
air trapped over the centuries in the glacial ice of Greenland show no change in CO2
content until 300 years ago.
Since measurements of atmospheric CO2 began late in the nineteenth century, its
concentration has risen over 20%. This increase is surely "anthropogenic"; that is, caused
by human activities:
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burning fossil fuels (coal, oil, natural gas) which returns to the atmosphere carbon
that has been locked within the earth for millions of years.
clearing and burning of forests, especially in the tropics. In recent decades, large
areas of the Amazon rain forest have been cleared for agriculture and cattle
grazing.
Where is the missing carbon?
Curiously, the increase in atmospheric CO2 is only about one-half of what would have
been expected from the amount of fossil fuel consumption and forest burning.
Where has the rest gone?
Research has shown that increased CO2 levels lead to increased net production by
photoautotrophs. There is evidence that at least some of the missing CO2 has been
incorporated by
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increased growth of forests, especially in North America;
increased amounts of phytoplankton in the oceans;
uptake by desert soils (mechanism as yet unknown).
The Greenhouse Effect and Global Warming
Despite these "sinks" for our
greatly increased CO2
production, the concentration
of atmospheric CO2 continues
to rise? Should we be worried?
Carbon dioxide is transparent
to light but rather opaque to
heat rays. Therefore, CO2 in
the atmosphere retards the
radiation of heat from the earth
back into space — the "greenhouse effect".
Has the increase in carbon dioxide led to global warming?
Average temperatures do seem to have increased slightly (~0.6°C) in the last century.
Some evidence:
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Careful monitoring of both ocean and land temperatures.
Many glaciers and ice sheets are receding.
Woody shrubs are now growing in areas of northern Alaska that 50 years ago
were barren tundra.
Many angiosperms in temperate climates are flowering earlier in the spring than
they used to.
Many species of birds and butterflies are moving north and breeding earlier in the
spring.
Will continued increase in carbon dioxide lead to more global warming and, if so, how
much?
At this point, the answer depends on what assumptions you plug into your computer
models. But as the different models have been improved, they seem to be converging on
a consensus: a doubling of the CO2 concentration (expected by the end of this century)
will cause the earth to warm somewhere in the range of 2.5–3.5°C.
Other Greenhouse Gases
Although their levels in the atmosphere are much lower than that of CO2,
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methane (CH4) and
chlorofluorocarbons (CFCs)
are also potent greenhouse gases.
Methane
Although methane ("marsh gas") is released by natural processes (e.g. from decay
occurring in swamps), human activities may now account for over two-thirds of the total.
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mining, processing, and use of coal, oil, and natural gas;
growing rice in paddies;
burning forests;
raising cattle (fermentation in their rumens produces methane that is expelled
from their GI tract).
So burning of the tropical rain forest adds to the atmospheric methane budget in two
ways:
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incomplete combustion during burning and
release from the GI tract of the cattle that are later placed on the cleared land.
The methane concentration in the air is presently some 1.8 parts per million (ppm) and is
growing at a rate of 1% per year. Although this concentration is far less than that of CO2,
methane is 21 times as potent a greenhouse gas and may now be responsible for ~20% of
the predicted global warming.
The marked warming of the earth that occurred at the end of the Paleocene epoch is
thought to have been caused by the release of large amounts of methane from the sea
floor.
Chlorofluorocarbons (CFCs)
Chlorofluorocarbons (CFCs) are synthetic gases in which the hydrogen atoms of methane
are replaced by atoms of fluorine and chlorine (e.g., CHF2Cl, CFCl3, CF2Cl2).These gases
are noninflammable, nontoxic, and very stable. They are widely used in industry as
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refrigerants (e.g., in refrigerators and air conditioners)
solvents
propellants in aerosol cans (now banned in some countries)
in the manufacture of plastic foams.
They escape to the air from all of these uses (e.g., from leaky and discarded refrigeration
units).
Their chemical inertness, which makes CFCs so desirable for industry, also makes them a
threat to the atmosphere. Once in the atmosphere, it may take 60–100 years for them to
decompose and disappear. In the meantime, they may contribute to as much as 25% of
the greenhouse effect. But perhaps even more worrisome is the threat they pose to the
ozone shield.
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