The natural geochemical carbon cycle maintained an equilibrium of atmospheric and dissolved CO2 for thousands of years. This equilibrium, however, has been disrupted by the anthropogenic emission of CO2, which has increased CO2 concentrations in the atmosphere and is associated with climate change.

The biological carbon cycle primarily depends on two physiological processes of living things: photosynthesis and respiration. Photosynthesis converts light energy to chemical energy, which then is used to incorporate carbon dioxide (CO2) into organic compounds, a process called carbon fixation. The organic compounds can be used by organisms as a source of energy through the process of respiration, which breaks chemical bonds to release energy for use. This breakdown of organic molecules provides energy to living cells and releases CO2 back to the environment as a by-product. All living organisms respire, but plants do so at a much slower rate than they normally photosynthesize. As a result, overall global net photosynthesis is approximately equal to net respiration, and CO2 levels are in balance.

In terrestrial ecosystems, the atmosphere is a major reservoir of carbon, holding about 750 billion metric tons (gigatons) of carbon in the form of CO2. Nearly as much, about 610 gigatons of carbon, forms the biomass of living plants. Each year, plants convert about 120 gigatons of carbon from CO2 into new organic tissue, with nearly that amount returned to the atmosphere through respiration of plants and animals, as well as through decomposition of dead organisms. A small amount of excess fixed carbon is added to the soil as humus. Over the millennia, this accumulation of organic material has formed soils that provide a reservoir of about 1,500 gigatons of carbon.

The depth and richness of soil are directly related to the photosynthetic productivity of the plants supported in a particular environment. The vegetation of Earth's temperate areas has produced deep, rich soils during the past ten to twenty thousand years that in turn promote high plant productivity in these regions. (Thus, although plants are predicted to migrate poleward in response to increasing temperatures associated with climate change during the twenty-first century, they will be moving into areas of poorer soils that will decrease plant productivity, even though other environmental factors may be positive.) Over a longer period of time during the past 3 to 3.5 million years, organic remains that did not decompose were converted into geological deposits of coal and oil. About 4 trillion metric tons of carbon are sequestered in these fossil fuels.

Scientists calculate that, at the start of the Industrial Revolution in the mid-eighteenth century, the atmospheric reservoir contained about 580 gigatons of carbon. This means that atmospheric CO2 has increased by about 30 percent during the past three hundred years, largely as a result of CO2 being released to the atmosphere by burning fossil fuels. Fossil fuel consumption during the early 21st century adds about 5.5 gigatons of carbon to the atmosphere every year. In addition, changes in land use, particularly deforestation, contribute another 1.6 gigatons of carbon per year. While added CO2 stimulates photosynthesis, particularly by carbon 3 plants, this activity fixes less than 1.9 gigatons of carbon per year. About 2 gigatons of carbon per year diffuse into the oceans, particularly in colder waters, but the majority remains in the atmosphere, increasing CO2 concentrations and promoting global warming.

A direct connection between the terrestrial and marine ecosystems exists in the equilibrium between atmospheric CO2 and dissolved CO2 in the oceans. This equilibrium shifts slightly toward the oceans as CO2 reacts with water (H2O) to form carbonate(CO3) or bicarbonate (HCO3) ions along with hydrogen ions (H+), which lowers pH, increasing the acidity of the oceans. Oceans absorb about 2 gigatons of carbon per year in this way.

The limnetic zone of the oceans supports a biotic carbon cycle similar to that of terrestrial ecosystems. About 1 trillion metric tons of carbon is dissolved in the near-surface waters that serve as a reservoir for marine organisms, which account for about 3 gigatons of carbon. These organisms convert CO2 to calcium carbonate, a building block of shells and other portions of marine life. The dissolved organic compounds produced by marine organisms account for nearly 700 gigatons of carbon.

When marine organisms die, their remains form a detritus of organic materials that slowly sinks to the seafloor. A huge amount of carbon, about 38.1 trillion metric tons, is accounted for by this deep ocean detritus, which eventually forms bottom sediments.

Marine sediments are molded by the pressure of the ocean as well as by volcanic heat, eventually forming sedimentary rocks. By far the largest amount of carbon on Earth is found in the lithosphere, in carbon-containing sediments and sedimentary rocks such as chalk, limestone, and dolomite. Scientists estimate that these calcium-carbonate-rich deposits--formed by corals, shell-producing animals, coralline algae, and marine plankton millions of years ago--contain up to 100 quadrillion metric tons of carbon.

Similarly, coal and oil are the carbon remains of ancient organisms stored in the Earth. As is true of sedimentary rock, these remains are transformed over millions of years by the heat and pressure of the Earth, which compresses them into a new form. Contemporary burning of fossil fuels returns about 5.5 gigatons of carbon to the atmosphere each year.

Initially, it seemed that rising CO2 levels in Earth's atmosphere would be partially compensated for by increased rates of plant photosynthesis and also by increased uptake of CO2 by the oceans. The increased photosynthesis promoted by higher CO2 levels, however, is countered by a decrease resulting from higher temperatures. Moreover, the rising acidity of the oceans resulting from higher levels of dissolved CO2 counters the photosynthetic benefits of higher CO2 levels. The global carbon cycle is thus much more complex than was imagined even a decade ago.

Bibliography:

1)         Fagan, Brian. Floods, Famines, and Emperors: El Nino and the Fate of Civilizations. New York: Basic Books, 1999.

2)         Field, Christopher B., and Michael R. Raupach, eds. The Global Carbon Cycle: Integrating Humans, Climate, and the Natural World. Washington, D.C.: Island Press, 2004.

3)         Odum, Eugene. Ecological Vignettes: Ecological Approaches to Dealing with Human Predicaments. Amsterdam, the Netherlands: Harwood Academic, 1998.

4)         Wigley, T. M. L., and David Steven Schimel, eds. The Carbon Cycle. New York: Cambridge University Press, 2000.

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