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.

Carbon Cycle

Introduction

The carbon cycle describes the reservoirs and transformations that the atom carbon undergoes as it cycles through the atmosphere, lithosphere, biosphere, and hydrosphere (Earth's air, land, water, and other areas that support life). The carbon cycle plays a key role in climate change because carbon dioxide is perhaps the most important gas contributing to global warming in the atmosphere. In addition, carbon is one of the most basic molecules used by organisms to store energy in chemical bonds.

Historical Background and Scientific Foundations

One of the largest reservoirs of carbon is carbon dioxide in the atmosphere. Plants on land and algae in water-based ecosystems incorporate carbon dioxide from the atmosphere into carbohydrates during photosynthesis. Some algae sink to the lakebed or seafloor, and over millennia, algal cells are converted into natural gas and oil. Similarly, plant material can be buried and converted into coal. These fossilized plants, or fossil fuels, represent a large carbon reservoir. When fossil fuels and plant biomass are

burned, carbon dioxide is released from the chemical bonds of these materials and returned to the atmosphere.

Chemically mediated transformations of carbon within the larger carbon cycle described above also occur. Carbon dioxide from the atmosphere can be dissolved into water as carbonate and bicarbonate. Similarly, it can be released from its dissolved form back into the atmosphere.

Biologically mediated cycles are also present within the larger carbon cycle. Plants and algae respire carbon dioxide directly into the atmosphere. When an animal consumes a plant, it incorporates the carbon from the plant's carbohydrates into its own biomolecules. During animal respiration, carbon dioxide is released into the atmosphere.

Some marine organisms incorporate dissolved carbon into their shells. When these organisms die, they sink to the bottom of the ocean and leave their shells behind. Over long periods of time, these shells are transformed into limestone rock. When limestone is uplifted, it becomes land. As the limestone weathers, carbon is returned to the atmosphere and dissolved in water.

Impacts and Issues

Because of industrial dependence on fossil fuels, larger concentrations of carbon dioxide are input to the atmosphere than can be taken up by biologically and chemically mediated transformations. In addition, burning large sections of forests without replanting has released carbon dioxide into the atmosphere and decreased the size of a significant carbon reservoir.

WORDS TO KNOW

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

CARBON: Chemical element with atomic number 6. The nucleus of a carbon atom contains 6 protons and from 6 to 8 neutrons. Carbon is present, by definition, in all organic substances; it is essential to life and, in the form of the gaseous compounds CO₂(carbon dioxide) and CH₄(methane), the major driver of climate change.

FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.

HYDROSPHERE: The totality of water encompassing Earth, comprising all the bodies of water, ice, and water vapor in the atmosphere.

LIMESTONE: A carbonate sedimentary rock composed of more than 50% of the mineral calcium carbonate (CaCO₃).

LITHOSPHERE: The rigid, uppermost section of Earth's mantle, especially the outer crust.

PHOTOSYNTHESIS: The process by which green plants use light to synthesize organic compounds from carbon dioxide and water. In the process, oxygen and water are released. Increased levels of carbon dioxide can increase net photo-synthesis in some plants. Plants create a very important reservoir for carbon dioxide.

RESERVOIR: A natural or artificial receptacle that stores a particular substance for a period of time

TRANSFORMATION: The processes involved in the transfer of a substance from one reservoir to another.

Carbon dioxide has a property that allows it to trap heat. Most scientists agree that the increase in carbon dioxide in the atmosphere has caused an increase in the global temperature and will likely continue to do so for quite some time. Such an increase has significant impacts, including the melting of the polar ice sheets contributing to sea level rise, changes in the patterns and intensity of weather systems, the increased spread of insect-born disease, and challenges for agriculture.

See Also Arctic Melting: Greenland Ice Cap; Atmospheric Chemistry; Atmospheric Circulation; Atmospheric Pollution; Atmospheric Structure; Biogeochemical Cycle; Biosphere; Carbon Credits; Carbon Dioxide (CO₂); Carbon Dioxide Concentrations; Carbon Sequestration Issues; Carbon Sinks; Coal; Extreme Weather; Forests and Deforestation; Glacier Retreat; Global Warming; Industry (Private Action and Initiatives); Infectious Disease and Climate Change; Melting; Natural Gas; Petroleum; Petroleum: Economic Uses and Dependency; Polar Ice; Sea Level Rise; Sequestration; Sink; Social Cost of Carbon (SCC).

BIBLIOGRAPHY

Books

Raven, Peter H., and Linda R. Berg. Environment. Hoboken, NJ: John Wiley and Sons, 2006.

Web Sites

“Carbon Cycle.” Environmental Literacy Council, September 25, 2006. < http://www.enviroliteracy.org/article.php/478.html> (accessed October 17, 2007).

“The Carbon Cycle.” NASA's Earth Observatory. < http://earthobservatory.nasa.gov/Library/CarbonCycle/> (accessed October 17,2007).

“Understanding the Global Carbon Cycle.” Woods Hole Research Center, 2007. < http://www.whrc.org/carbon/index.htm> (accessed October 17, 2007).

Juli Berwald

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 acids—all 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 molecules—molecules of which dead organisms are composed—into 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 ]

Carbon cycle

Carbon makes up no more than 0.27% of the mass of all elements in the universe and only 0.0018% by weight of the elements in the earth's crust. Yet, its importance to living organisms is far out of proportion to these figures. In contrast to its relative scarcity in the environment , it makes up 19.4% by weight of the human body. Along with hydrogen , carbon is the only element to appear in every organic molecule in every living organism on earth.

The series of chemical, physical, geological, and biological changes by which carbon moves through the earth's air, land, water, and living organisms is called the carbon cycle.

In the atmosphere , carbon exists almost entirely as gaseous carbon dioxide . The best estimates are that the earth's atmosphere contains 740 billion tons of this gas. Its global concentration is about 350 parts per million (ppm), or 0.035% by volume. That makes carbon dioxide the fourth most abundant gas in the atmosphere after nitrogen , oxygen and argon. Some carbon is also released as carbon monoxide to the atmosphere by natural and human mechanisms. This gas reacts readily with oxygen in the atmosphere, however, converting it to carbon dioxide.

Carbon returns to the hydrosphere when carbon dioxide dissolves in the oceans, as well as in lakes and other bodies of water. The solubility of carbon dioxide in water is not especially high, 88 milliliters of gas in 100 milliliters of water. Still, the earth's oceans are such a vast reservoir that experts estimate that approximately 36,000 billion tons of carbon are stored there. They also estimate that about 93 billion tons of carbon flows from the atmosphere into the hydrosphere each year.

Carbon moves out of the oceans in two ways. Some escapes as carbon dioxide from water solutions and returns to the atmosphere. That amount is estimated to be very nearly equal (90 billion tons) to the amount entering the oceans each year. A smaller quantity of carbon dioxide (about 40 billion tons) is incorporated into aquatic plants.

On land, green plants remove carbon dioxide from the air through the process of photosynthesis—a complex series of chemical reactions in which carbon dioxide is eventually converted to starch, cellulose, and other carbohydrates. About 100 billion tons of carbon are transferred to green plants each year, and a total of 560 billion tons of the element is thought to be stored in land plants alone.

The carbon in green plants is eventually converted into a large variety of organic (carbon-containing) compounds. When green plants are eaten by animals, carbohydrates and other organic compounds are used as raw materials for the manufacture of thousands of new organic substances. The total collection of complex organic compounds stored in all kinds of living organisms represents the reservoir of carbon in the earth's biosphere .

The cycling of carbon through the biosphere involves three major kinds of organisms. Producers are organisms with the ability to manufacture organic compounds such as sugars and starches from inorganic raw materials such as carbon dioxide and water. Green plants are the primary example of producing organisms. Consumers are organisms that obtain their carbon (that is, their food) from producers: all animals are consumers. Finally, decomposers are organisms such as bacteria and fungi that feed on the remains of dead plants and animals. They convert carbon compounds in these organisms to carbon dioxide and other products. The carbon dioxide is then returned to the atmosphere to continue its path through the carbon cycle.

Land plants return carbon dioxide to the atmosphere during the process of respiration . In addition, animals that eat green plants exhale carbon dioxide, contributing to the 50 billion tons of carbon released to the atmosphere by all forms of living organisms each year. Respiration and decomposition both represent, in the most general sense, a reverse of the process of photosynthesis . Complex organic compounds are oxidized with the release of carbon dioxide and water—the raw materials from which they were originally produced.

At some point, land and aquatic plants and animals die and decompose. When they do so, some carbon (about 50 billion tons) returns to the atmosphere as carbon dioxide. The rest remains buried in the earth (up to 1,500 billion tons) or on the ocean bottoms (about 3,000 billion tons). Several hundred million years ago, conditions of burial were such that organisms decayed to form products consisting almost entirely of carbon and hydrocarbons . Those materials exist today as pockets of the fossil fuels—coal, oil, and natural gas . Estimates of the carbon stored in fossil fuels range from 5,000 to 10,000 billion tons.

The processes that make up the carbon cycle have been occurring for millions of years, and for most of this time, the systems involved have been in equilibrium. The total amount of carbon dioxide entering the atmosphere from all sources has been approximately equal to the total amount dissolved in the oceans and removed by photosynthesis. However, a hundred years ago changes in human society began to unbalance the carbon cycle. The Industrial Revolution initiated an era in which the burning of fossil fuels became widespread. In a short amount of time, large amounts of carbon previously stored in the earth as coal , oil, and natural gas were burned up, releasing vast quantities of carbon dioxide into the atmosphere.

Between 1900 and 1992, measured concentrations of carbon dioxide in the atmosphere increased from about 296 ppm to over 350 ppm. Scientists estimate that fossil fuel combustion now released about five billion tons of carbon dioxide into the atmosphere each year. In an equilibrium situation, that additional five billion tons would be absorbed by the oceans or used by green plants in photosynthesis. Yet this appears not to be happening: measurements indicate that about 60% of the carbon dioxide generated by fossil fuel combustion remains in the atmosphere.

The problem is made even more complex because of deforestation . As large tracts of forest are cut down and burned, two effects result: carbon dioxide from forest fires is added to that from other sources, and the loss of trees decreases the worldwide rate of photosynthesis. Overall, it appears that these two factors have resulted in an additional one to two billion tons of carbon dioxide in the atmosphere each year.

No one can be certain about the environmental effects of this disruption of equilibria in the carbon cycle. Some authorities believe that the additional carbon dioxide will augment the earth's natural greenhouse effect , resulting in long-term global warming and climate change. Other argue that we still do not know enough about the way oceans, clouds, and other factors affect climate to allow such predictions.

This controversy involves a difficult choice. Should actions that could potentially cost billions of dollars be taken to reduce the emission of carbon dioxide when evidence for climate change is still uncertain? Or should governments wait until that evidence becomes more clear, with the risk that needed actions may then come too late.

[David E. Newton ]

RESOURCES

BOOKS

McGraw-Hill Encyclopedia of Science & Technology. 7th ed. New York: McGraw-Hill, 1992.

Carbon cycle

The carbon cycle describes the movement of carbon in the atmosphere, where it is in the gaseous form carbon dioxide , through organisms, and then back into the atmosphere and the oceans. Carbon is a central element of the huge diversity of organic chemicals found in living things, such as the many kinds of carbohydrates, proteins , and fats. Energy is contained in the chemical bonds that hold the atoms of carbon and other elements together in these organic compounds. Organisms use chemical energy from organic compounds to carry out all the processes necessary to life.

How carbon is released into the atmosphere

Carbon is released into the atmosphere through three major processes: cellular respiration, the burning of fossil fuels , and volcanic eruptions. In each of these processes, carbon is returned to the atmosphere or to the ocean .

Cellular respiration

Plants convert the carbon in atmospheric carbon dioxide into carbon-containing organic compounds, such as sugars, fats, and proteins. Plants take in carbon dioxide through microscopic openings in their leaves, called stomata. They combine atmospheric carbon with water and manufacture organic compounds, using energy trapped from sunlight in a process called photosynthesis . The by-product of photosynthesis is oxygen , which plants release into the atmosphere through the stomata.

Animals that eat plants, or that eat other animals, incorporate the carbon in the sugars, fats, and proteins derived from the ingested biomass into their bodies. Inside their cells, energy is extracted from the food in a process called cellular respiration. Cellular respiration requires oxygen (which is the by-product of photosynthesis) and it produces carbon dioxide, which is used in photosynthesis. In this way, photosynthesis and cellular respiration are linked in the carbon cycle.

Photosynthesis requires atmospheric carbon, while cellular respiration returns carbon to the atmosphere, and vice versa for oxygen. The global rates of photosynthesis and cellular respiration influence the amount of carbon dioxide in the atmosphere. In the summer, the high rate of photosynthesis uses up much of the carbon dioxide in the atmosphere, and the amount of atmospheric carbon dioxide decreases. In the winter, when the rate of photosynthesis is low, the amount of atmospheric carbon dioxide increases.

Another way that cellular respiration releases carbon into the atmosphere is through the actions of decomposers. Decomposers, such as bacteria and fungi , derive their nutrients by feeding on the remains of plants and animals. The bacteria and fungi use cellular respiration to extract the energy contained in the chemical bonds of the decomposing organic matter , and so release carbon dioxide into the atmosphere.

In some ecosystems, such as tropical rainforests, decomposition is accomplished quickly, and carbon dioxide is returned to the atmosphere at a relatively fast rate. In other ecosystems, such as northern forests and tundra , decomposition proceeds more slowly. In some places, such as bogs and the deep ocean, the organic matter of plants and animals may accumulate in deep sediments, where decomposers cannot function well because of the lack of oxygen. Slowly, over millions of years, the carbon-rich materials are converted into carbon-rich fossil fuels, such as petroleum , natural gas , and coal . Also in marine environments, carbon-containing matter (such as calcium carbonate ) is incorporated into the shells and other hard parts of aquatic organisms. When these organisms die, the carbon-rich hard parts sink to the ocean bed. There they become buried in sediment, and eventually densify into rocks such as limestone and dolomite.

The burning of fossil fuels

When fossil fuels are burned, their organic carbon is released into the atmosphere. During the past 100 years, fossil fuel consumption has increased dramatically, and this has led to a huge amount of carbon dioxide being released into the atmosphere. In addition, the widespread clearing of forests is resulting in the emission of huge quantities of carbon dioxide into the atmosphere (forests store large amounts of organic carbon in their biomass, most of which is emitted through decomposition and fire when deforestation occurs). The increasing concentrations of atmospheric carbon dioxide are a cause for concern, since they may be responsible for global warming and associated climatic and ecological disruptions. During the middle of the nineteenth century, the atmospheric concentration of carbon dioxide was about 270 ppm, but in 2000 it had increased to 365 ppm.

Volcanic eruption

Another way that carbon is released into the atmosphere is through volcanic eruptions. When a volcano erupts, it sends huge amounts of ash and soot high into the atmosphere. Some of this ash and soot is derived from ancient carbon-rich sediments, and the cloud of debris that results from a volcanic eruption returns large amounts of carbon to the atmosphere.

The carbon cycle in land and sea

The cycling of carbon takes place in oceans and other aquatic ecosystems as well as in terrestrial environments. The world's oceans contain about 50% more carbon than does the atmosphere. The oceans are able to absorb some of the carbon dioxide currently being released by the burning of fossil fuels, thus offsetting global warming. However, the oceans cannot do this as quickly as the carbon dioxide is being released to the atmosphere, and this time lag is resulting in increasing concentrations in the atmosphere.

In aquatic environments, carbon cycling is more complex because carbon interacts with water. When carbon dioxide is released by cellular respiration, it combines with water to form carbonic acid and bicarbonate. Therefore, in aquatic environments most inorganic carbon is in the form of bicarbonate rather than carbon dioxide. Carbon dioxide from the atmosphere readily diffuses into water, and it is quickly converted to bicarbonate.

Importance of the carbon cycle

The carbon cycle is important in ecosystems because it moves carbon, a life-sustaining element, from the atmosphere and oceans into organisms and back again to the atmosphere and oceans. If the balance between these latter two reservoirs is upset, serious consequences, such as global warming and climate disruption, may result. Scientists are currently looking into ways in which humans can use other, non-carbon containing fuels for energy. Nuclear power , solar power, wind power, and water power are a few alternative energy sources that are being investigated.

See also Greenhouse effect.

Resources

books

Dunnette, David A., and Robert J. O'Brien, eds. The Science of Global Change: The Impact of Human Activities on the Environment. Washington, DC: American Chemical Society, 1992.

Hamblin, W.K., and E.H. Christiansen. Earth's Dynamic Systems. 9th ed. Upper Saddle River: Prentice Hall, 2001.

Levi, Barbara Gross, David Hafemeister, and Richard Scribner. Global Warming: Physics and Facts. New York: American Institute of Physics, 1992.

Matthews, John A., E. M. Bridges, and Christopher J. Caseldine. The Encyclopaedic Dictionary of Environmental Change. New York: Edward Arnold, 2001.

Tolbert, N. E., and Jack Preiss, eds. Regulation of Atmospheric Carbon by Photosynthetic Carbon Metabolism. New York: Oxford University Press, 1994.

periodicals

Hileman, Bette. "New CO2 Model Shows Whole Earth 'Breathing" ' Chemical and Engineering News 72 (January 10, 1994): 6.

Vitousek, Peter M. "Beyond Global Warming: Ecology and Global Change." Ecology 75 (October 1994): 1861.

Volk, Tyler. "The Soil's Breath." Natural History 103 (November 1994): 48.

Zimmer, Carl. "The War Between Plants and Animals." Discover 14 (July 1993): 16.

Kathleen Scogna