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.