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Carbon Cycle

Carbon cycle

The carbon cycle is the process in which carbon atoms are recycled over and over again on Earth. Carbon recycling takes place within Earth's biosphere and between living things and the nonliving environment. Since a continual supply of carbon is essential for all living organisms, the carbon cycle is the name given to the different processes that move carbon from one to another. The complete cycle is made up of "sources" that put carbon back into the environment and "sinks" that absorb and store carbon.

Recycling carbon

Earth's biosphere can be thought of as a sealed container into which nothing new is ever added except the energy from the Sun. Since new matter can never be created, it is essential that living things be able to reuse the existing matter again and again. For the world to work as it does, everything has to be constantly recycled. The carbon cycle is just one of several recycling processes, but it may be the most important process since carbon is known to be a basic building block of life. As the foundation atop which a huge family of chemical substances called organic substances are formed, carbon is the basis of carbohydrates, proteins, lipids, and nucleic acidsall of which form the basis of life on Earth.

Since all living things contain the element carbon, it is one of the most abundant elements on Earth. The total amount of carbon on Earth, whether we are able to measure it accurately or not, always remains the same, although the carbon regularly changes its form. A particular carbon atom located in someone's eyelash may have at one time been part of some now-extinct species, like a dinosaur. Since the dinosaur died and decomposed millions of years ago, its carbon atoms have seen many forms before ending up as part of a human being. It may have been part of several plants and trees, free-floating in the air as carbon dioxide, locked away in the shell of a sea creature and then buried at the ocean bottom, or even part of a volcanic eruption. Carbon is found in great quantities in Earth's crust, its surface waters, the atmosphere, and the mass of green plants. It is also found in many different chemical combinations, including carbon dioxide (CO2) and calcium carbonate (CaCO3), as well as in a huge variety of organic compounds such as hydrocarbons (like coal, petroleum, and natural gas).

Words to Know

Biosphere: The sum total of all life-forms on Earth and the interaction among those life-forms.

Decomposition: The breakdown of complex moleculesmolecules of which dead organisms are composedinto simple nutrients that can be reutilized by living organisms.

Fossil fuel: A fuel such as coal, oil, or natural gas that is formed over millions of years from the remains of plants and animals.

Greenhouse effect: The warming of Earth's atmosphere due to water vapor, carbon dioxide, and other gases in the atmosphere that trap heat radiated from Earth's surface.

Hydrocarbons: Molecules composed solely of hydrogen and carbon atoms.

Photosynthesis: Chemical process by which plants containing chlorophyll use sunlight to manufacture their own food by converting carbon dioxide and water to carbohydrates, releasing oxygen as a by-product.

Respiration: The process in which oxygen is used to break down organic compounds into carbon dioxide and water.

Carbon cycle processes

If a diagram were drawn showing the different processes that move carbon from one form to another, its main processes would be photosynthesis, respiration, decomposition, natural weathering of rocks, and the combustion of fossil fuels.

Photosynthesis. Carbon exists in the atmosphere as the compound carbon dioxide. It first enters the ecological food web (the connected network of producers and consumers) when photosynthetic organisms, such as plants and certain algae, absorb carbon dioxide through tiny pores in their leaves. The plants then "fix" or capture the carbon dioxide and are able to convert it into simple sugars like glucose through the biochemical process known as photosynthesis. Plants store and use this sugar to grow and to reproduce. Thus, by their very nature as makers of their own food, plants remove carbon dioxide from the atmosphere. When plants are eaten by animals, their carbon is passed on to those animals. Since animals cannot

make their own food, they must get their carbon either directly by eating plants or indirectly by eating animals that have eaten plants.

Respiration. Respiration is the next step in the cycle, and unlike photosynthesis, it occurs in plants, animals, and even decomposers. Although we usually think only of breathing oxygen when we hear the word "respiration," it has a broader meaning that involves oxygen. To a biologist, respiration is the process in which oxygen is used to break down organic compounds into carbon dioxide (CO2) and water (H2O). For an animal then, respiration is both taking in oxygen (and releasing carbon dioxide) and oxidizing its food (or burning it with oxygen) in order to release the energy the food contains. In both cases, carbon is returned to the atmosphere as carbon dioxide. Carbon atoms that started out as components of carbon dioxide molecules have passed through the body of living organisms and been returned to the atmosphere, ready to be recycled again.

Decomposition. Decomposition is the largest source through which carbon is returned to the atmosphere as carbon dioxide. Decomposers are microorganisms that live mostly in the soil but also in water, and which feed on the rotting remains of plants and animals. It is their job to consume both waste products and dead matter, during which they also return carbon dioxide to the atmosphere by respiration. Decomposers not only play a key role in the carbon cycle, but also break down, remove, and recycle what might be called nature's garbage.

Weathering of rocks. Not all carbon atoms are always moving somewhere in the carbon cycle. Often, many become trapped in limerock, a type of stone formed on the ocean floor by the shells of marine plankton. Sometimes after millions of years, the waters recede and the limerock is eventually exposed to the elements. When limerock is exposed to the natural process of weathering, it slowly releases the carbon atoms it contains, and they become an active part of the carbon cycle once again

Human-caused increase of carbon dioxide in the atmosphere. In recent history, humans have added to the carbon cycle by burning fossil fuels. Ever since the rapid growth of the Industrial Revolution in the nineteenth century when people first harnessed steam to power their engines, human beings have been burning carbon-containing fuels like coal and oil (called fossil fuels) for artificial power. This constant burning produces massive amounts of carbon dioxide, which are released into Earth's atmosphere. Over the last 150 years, the burning of coal, oil, and natural gas has released some 270 billion tons (245 billion metric tons) of carbon into the air in the form of carbon dioxide.

Luckily, more than half of the carbon dioxide emitted by the burning of fossil fuels is absorbed by the oceans, by plants, and by soils. Regardless, scientists feel fossil fuel consumption could be an example of a human activity that affects and possibly alters the natural processes (photosynthesis, respiration, decomposition) that nature had previously kept in balance. Many scientists believe that carbon dioxide is a "greenhouse gas." This means that it traps heat and prevents it from escaping from Earth. As a result, this trapped gas leads to a global temperature rise, a natural phenomenon known as the greenhouse effect, which can have disastrous effects on Earth's environment.

[See also Greenhouse effect ]

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carbon cycle

carbon cycle, in biology, the exchange of carbon between living organisms and the nonliving environment. Inorganic carbon dioxide in the atmosphere is converted by plants into simple carbohydrates, which are then used to produce more complex substances. Animals eat the plants and are then eaten by other animals. When these life forms die, they decay, breaking down into, among many other things, carbon dioxide, which returns to the atmosphere. Plants and animals also release carbon dioxide during respiration. Animals and some microorganisms require the carbon-containing substances from plants in order to produce energy and as a source of materials for many of their own biochemical reactions; this cycle is vital to them. The process of incorporating carbon dioxide into the molecules of living matter is called fixation. Nearly all carbon dioxide fixation is accomplished by means of photosynthesis, in which green plants form carbohydrates from carbon dioxide and water, using the energy of sunlight to drive the chemical reactions involved. Green plants use carbohydrates to build the other organic molecules that make up their cells, such as cellulose, fats, proteins, and nucleic acids. Some of these compounds require the incorporation of nitrogen (see nitrogen cycle). When carbohydrates are oxidized in cells they release the energy stored in their chemical bonds, and some of that energy is also used by the cell to drive other reactions. In the process of oxidation, or respiration, oxygen from the atmosphere (or from water) is combined with portions of the carbohydrate molecule, producing carbon dioxide and water, the compounds from which the carbohydrates were originally formed. However, not all of the carbon atoms incorporated by the plant can be returned to the atmosphere by its own respiration; some remain fixed in the organic materials that make up its cells. When the plant dies, its tissues are consumed by bacteria and other microorganisms, a process called decay. These microorganisms break down the organic molecules of the plant and use them for their own cell-building and energy needs; by their respiration more of the carbon is returned to the atmosphere. The carbon-containing molecules that an animal derives from consuming other organisms are reorganized to build its own cells or oxidized for energy by respiration, releasing carbon dioxide and water. When the animal dies it too is decayed by microorganisms, resulting in the return of more carbon to the atmosphere. Carbon-containing molecules in wood (or other dry, slow-decaying organic materials) may be oxidized by burning, or combustion, also producing carbon dioxide and water. Under conditions prevailing on earth at certain times, green plants have decayed only partially and have been transformed into fossil fuels—coal, peat, and oil. These materials are made of organic compounds formed by the plants; when burned, they too restore carbon dioxide to the atmosphere.

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Carbon Cycle

Carbon Cycle

The carbon cycle involves the circulation of carbon dioxide (CO2) from the atmosphere into plants and other living organisms; the transfer of carbon from these organisms into other temporary storage pools, living or nonliving, containing organic and inorganic carbon compounds; and the return of CO2 to the atmosphere through respiration or combustion processes. The carbon cycle provides a unifying framework for examining exchanges or storage of carbon associated with photosynthesis and energy assimilation by organisms, respiration and metabolism , productivity and biomass accumulation, and the decay and recycling of organic matter at the level of a single organism, an ecosystem , or the global biosphere.

Analysis of the carbon cycle in a forest ecosystem, for example, requires the estimation of pools of carbon in live biomass, dead wood, decaying litter (branches and leaves), and soil organic matter. This information is combined with estimates of major transfers within the cycle such as carbon fixation via photosynthesis, CO2 release by respiration, carbon flow to the soil as litterfall and root turnover, and carbon flow through grazing and decomposer food chains.

On a global scale, the primary carbon storage pools are the oceans and marine sediments, fossil fuels and shale deposits, terrestrial plants and soils, and the atmosphere. The global carbon cycle is characterized by large exchanges of carbon between Earth and its atmosphere. Photosynthesis and ocean uptake processes remove CO2 from the atmospheric carbon pool, whereas CO2 is returned to the atmosphere by biological respiration, deforestation and land clearing, forest fires, and fossil fuel combustion associated with human activities. As of 2001, the atmosphere is experiencing a net gain of 3 billion tons of carbon per year from CO2 emissions derived from human combustion of coal, oil, and gas, as well as from deforestation and land clearing activities. This imbalance in the global carbon cycle is reflected in the rising concentration of atmospheric CO2, which has increased 15 percent from 320 ppm (parts per million) to 368 ppm since the mid-1960s.

see also Biogeochemical Cycles; Ecosystem; Global Climate Change; Plankton

Christopher S. Cronan

Bibliography

Botkin, Daniel, and Edward Keller. Environmental Science. New York: John Wiley & Sons, 1995.

Schlesinger, William H. Biogeochemistry: An Analysis of Global Change. New York: Academic Press, 1991.

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carbon cycle

carbon cycle The movement of carbon through the surface, interior, and atmosphere of the Earth. Carbon exists in atmospheric gases, in dissolved ions in the hydrosphere, and in solids as a major component of organic matter and sedimentary rocks, and is widely distributed. Inorganic exchange is mainly between the atmosphere and hydrosphere. The major movement of carbon results from photosynthesis and respiration, with exchange between the biosphere, atmosphere, and hydrosphere. Rates of exchange are very small, but over geologic time they have concentrated large amounts of carbon in the lithosphere, mainly as limestones and fossil fuels. This carbon was probably present as CO2 in the primordial atmosphere. The burning of fossil fuels and the release of CO2 from soil air through the clearance of tropical forests may eventually change the balance of the carbon cycle, although the climatic effects may be partly mitigated by the buffering action of the oceans; it is estimated that about 200 billion tonnes of CO2 have been added to the atmosphere in this way since 1850. See ‘greenhouse effect’.

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carbon cycle

carbon cycle The movement of carbon through the surface, interior, and atmosphere of the Earth. Carbon exists in atmospheric gases, in dissolved ions in the hydrosphere, and in solids as a major component of organic matter and sedimentary rocks, and is widely distributed. Inorganic exchange is mainly between the atmosphere and hydrosphere. The major movement of carbon results from photosynthesis and respiration, with exchange between the biosphere, atmosphere and hydrosphere. Rates of exchange are very small, but over geologic time they have concentrated large amounts of carbon in the lithosphere, mainly as limestones and fossil fuels. This carbon was probably present as CO2 in the primordial atmosphere. The burning of fossil fuels and the release of CO2 from soil air through the clearance of tropical forests may eventually change the balance of the carbon cycle, although the climatic effects may be partly mitigated by the buffering action of the oceans; it is estimated that about 200 billion tonnes of CO2 have been added to the atmosphere in this way since 1850. See GREENHOUSE EFFECT.

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Carbon Cycle

Carbon Cycle

All life on Earth is based on carbon, the sixth element of the periodic table. The term carbon cycle refers to the movement of carbon in various forms between Earth's biogeochemical reservoirs: the oceans, the atmosphere, plants, animals and soils on land (the land biosphere), and the geosphere (rocks). Carbon dioxide (CO2) in the air traps heat, contributing to warming of Earth's surface (called the greenhouse effect) and thereby influencing the climate. Human activities such as burning fossil fuels and clearing forests are causing the amount of CO2 in the atmosphere to increase rapidly. Concern that global climate change may result has led to a pressing need for scientific research to better understand the global carbon cycle.

The Path of Carbon

To illustrate some of the important processes of the carbon cycle one can follow a carbon atom as it moves through the biogeochemical reservoirs of the cycle. Begin with a carbon atom that is in the atmosphere in the form of CO2. In the atmosphere CO2 is the fifth most abundant gas, behind nitrogen (N2), oxygen (O2), argon (Ar), and water vapor (H2 O). Nevertheless, of every million molecules of air, fewer than four hundred are CO2.

The CO2 molecule contacts the leaf of an apple tree. It is removed from the air by the process of photosynthesis, also called carbon fixation, whereby plants use light energy from the Sun and water from the soil to convert CO2 to carbohydrate (sugar) and O2 gas. The carbohydrate may be converted to other compounds that the plant needs to grow and reproduce. The carbon atom may be used by the plant to grow an apple, which may be picked and eaten. The body uses the carbohydrate in the apple for fuel, converting the carbon back into CO2, which is breathed out to the air. Or, perhaps the apple falls to the ground and gradually rots, meaning that the carbon is converted to CO2 by decomposers in the soil, including insects, worms, fungi, and bacteria. Either way, this process of converting the carbon in the apple to CO2 consumes O2 from the air and is called respiration. About one-tenth of all the CO2 in the atmosphere is taken up by photosynthesis on land each year, and very nearly the same amount is converted back to CO2 by respiration. Most of the carbon fixed each year on land is used by plants to make new leaves, which eventually die and fall to the ground where they are decomposed, just like the apple. The rich, dark brown material in the top several centimeters of most soil is mainly decomposing plant material.

The CO2 molecule rides on the wind out over the ocean. It crashes into the ocean surface and dissolves, like sugar dissolving in a glass of water. Since CO2 is very soluble in water the oceans contain about fifty times as much carbon as the atmosphere. About one-eighth of all the CO2 in the atmosphere dissolves into the ocean waters each year, but nearly the same amount returns to the atmosphere because the total amount of CO2 in the ocean is approximately in equilibrium with the amount in the air, and CO2 is constantly moving into and out of the seawater. The CO2 molecule, dissolved in the water, is taken up by a single-celled marine plant called a coccolithophore. The carbon is used by the coccolithophore to add to its hard protective coating, which is made of calcium carbonate (CaCO3). When the coccolithophore dies its coating sinks to the bottom of the ocean and becomes part of the marine sediment. Most of the carbon in the sediment is recycled rapidly by respiration or dissolution, but a small amount remains in the sediment and eventually (over millions of years) becomes sedimentary rock.

Being trapped in a sedimentary rock is not the end of the cycle for the carbon atom. If that were the case eventually all of the carbon in the atmosphere, the plants and soils, and the oceans would have ended up in rocks, and the carbon cycle would have stopped long ago. Fortunately, a little of this carbon is returned to the atmosphere each year, mainly by volcanism. The amount of CO2 that is emitted by volcanoes and geothermal vents is small, but it is enough to have kept the carbon cycle turning for billions of years.

Following a carbon atom through some pathways of the carbon cycle touches on many important processes. The balance between photosynthesis and respiration on land, the transfer of CO2 into and out of the oceans, and the incorporation of carbon into sedimentary rocks and return to the atmosphere via volcanic activity all represent recycling of carbon atoms. It is important to understand that the carbon cycle is a dynamic process, it is constantly changing and an adjustment or change in one carbon cycle process will cause changes in many other parts of the cycle. For example, if the amount of CO2 in the atmosphere increases for some reason, more CO2 will dissolve into the oceans. Also, since plants require CO2 as a nutrient, a larger amount of CO2 in the air will increase plant growth, a process called CO2 fertilization.

Carbon and Climate

A very important part of the carbon cycle is the influence of CO2 on Earth's climate. Carbon dioxide is one of several gases in the air (water vapor is the most important one) that trap heat near the surface, causing the surface to be warmed. This process is known as the greenhouse effect. If there were no greenhouse gases in the atmosphere the surface temperature would be about 35°C colder on average than it is, and life on Earth would be very different. More CO2 means more warming, that is, higher average surface temperature. That means that the amount of CO2 and other greenhouse gases in the air has a strong influence on the climate of Earth. Furthermore, since many parts of the carbon cycle, such as the plants and soils on land, and the chemistry of the oceans, are sensitive to climate, a change in climate can cause a change in the carbon cycle. For example, in the temperate zone during a warm spring, leaves will come out on the trees earlier than in a cool spring. With a longer growing season the plants can remove more CO2 from the air, and will grow faster.

Human Influences on the Carbon Cycle

Humans are causing large changes in the carbon cycle. First, humans have altered the land biosphere by cutting forests to clear land for agriculture; for lumber, pulp, and fuel wood; and to make room for cities. Natural grasslands have also been plowed for agriculture. In the early 1990s about 38 percent of Earth's land surface was used for agriculture including crop-lands and pastures, according to United Nations statistics. When land is cleared, most of the carbon stored in the plants and much of that stored in the soils is converted to CO2 and lost to the atmosphere. Second, since the mid-1800s humans have learned to harness the energy stored in fossil fuels, mainly coal, oil, and natural gas. The term fossil fuels refers to the fact that these materials are composed of the fossil remains of ancient plants. When fossil fuels are burned, energy that can be used to light and heat our homes, drive our cars, and manufacture all the goods that we use from day to day is released. Burning fossil fuels also consumes O2 and releases CO2 to the air. In 1996, 6.5 billion metric tons of carbon were released to the atmosphere from fossil fuels. That's a little more than 1 ton of carbon per person per year worldwide. The use of fossil fuel, however, is not evenly distributed. The United States, with less than 5 percent of Earth's population, used 22 percent of the fossil fuels in 1996, and on a per-person basis residents of the United States used about nineteen times as much fossil fuel as the residents of Africa. The use of fossil fuels is growing rapidly, particularly in developing countries such as China.

Carbon dioxide from fossil fuels and land clearing caused a 25 percent increase in CO2 in the atmosphere between the eighteenth century and the 1990s. Only about one-half of the CO2 that has been emitted into the atmosphere has remained there, the rest has been taken up by the oceans and the land biosphere. Scientists do not know exactly how much of the added CO2 has gone into the oceans and how much has gone into the land biosphere, nor do they understand precisely why the land biosphere is taking up a lot of CO2. One reason that the land biosphere may be taking up carbon is the CO2 fertilization effect mentioned above. Another explanation is that forests that were cleared for agriculture and lumber in the 1800s and early 1900s may be regrowing. These are important questions for future research. The answers will affect our ability to regulate the amount of CO2 in the atmosphere in order to lessen climate change.

Burning fossil fuels represents a huge increase in the transfer of carbon into the atmosphere from sedimentary rocks in Earth's crust. Unless an alternative source of energy is found, it is likely that in a few hundred years humans could burn all of the coal, oil, and gas that is believed to exist on Earth, and that took many millions of years to form. If this occurs the amount of CO2 in the atmosphere will be several times the preindustrial amount, and the oceans will become completely saturated with CO2, which would drastically alter their chemical composition. Also, the increased greenhouse effect would cause very substantial but currently unpredictable changes in climate. Because the leak of carbon out of the oceans and atmosphere into the sediments and eventually into the sedimentary rocks is very slow, the added carbon would take thousands of years to dissipate from the oceans and atmosphere.

see also Biogeochemical Cycles; Decomposers; Global Warming; Human Impacts; Photosynthesis, Carbon Fixation and.

Peter S. Bakwin

Bibliography

Tans, P. P. "Why Carbon Dioxide from Fossil Fuel Burning Won't Go Away." InPerspectives in Environmental Chemistry. Ed. D. Macalady. New York: Oxford University Press, 1998.

Vitousek, P. M., H. A. Mooney, J. Lubchenco, and J. M. Melillo. "Human Domination of Earth's Ecosystems." Science 277 (1997): 494-99.

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carbon cycle

carbon cycle One of the major cycles of chemical elements in the environment (see biogeochemical cycle). Carbon (as carbon dioxide) is taken up from the atmosphere and incorporated into the tissues of plants in photosynthesis. It may then pass into the bodies of animals as the plants are eaten (see food chain). During the respiration of plants, animals, and organisms that bring about decomposition, carbon dioxide is returned to the atmosphere. The combustion of fossil fuels (e.g. coal and peat) also releases carbon dioxide into the atmosphere. See illustration.

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carbon cycle

carbon cycle Circulation of carbon in the biosphere. It is a complex chain of events. The most important elements are the taking up of carbon dioxide (CO2) by green plants during photosynthesis, and the return of CO2 to the atmosphere by the respiration and eventual decomposition of animals which eat the plants. The burning of fossil fuels has also, over the years, released CO2 back into the atmosphere.

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