Human Influences on the Carbon Cycle

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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 croplands 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,

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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

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