The Human Role
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The Carbon Cycle
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The long-term record of atmospheric carbon dioxide obtained from Antarctic ice cores shows huge fluctuations over the past 150,000 years. Periods of low carbon dioxide concentration correspond to ice ages, while higher carbon dioxide concentrations are linked to warmer periods. The last ice age ended 10,000 to 20,000 years ago, as carbon dioxide levels rose from below 200 parts per million to about 280 parts per million. Current atmospheric carbon dioxide levels are above 370 parts per million because of the burning of fossil fuels. This has raised concern in the scientific community that average global temperatures may rise as a result. (Graph by Robert Simmon, based on data from Lorius, C., J. Jouzel, C. Ritz, L. Merlivat, N.I. Barkov, Y.S. Korotkevitch, and V.M. Kotlyakov. 1995. A 150,000-year climatic record from Antarctic ice. Nature 316:591-596.) Not all of the carbon dioxide that has been emitted by human
activities remains in the atmosphere. The oceans have absorbed some of
it because as the carbon dioxide in the atmosphere increases it drives
diffusion of carbon dioxide into the oceans. However, when we try to
account for sources and sinks for carbon dioxide in the atmosphere we
uncover some mysteries. For example, notice in Figure 1 (schematic of
the carbon cycle) that fossil fuel burning releases roughly 5.5 gigatons
of carbon (GtC [giga=1 billion]) per year into the atmosphere and that land-use
changes such as deforestation contribute roughly 1.6 GtC per year.
Measurements of atmospheric carbon dioxide levels (going on since 1957)
suggest that of the approximate total amount of 7.1 GtC released per
year by human activities, approximately 3.2 GtC remain in the
atmosphere, resulting in an increase in atmospheric carbon dioxide. In
addition, approximately 2 GtC diffuses into the worlds oceans, thus
leaving 1.9 GtC unaccounted for. |
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What happens to the leftover 1.9 GtC? Scientists dont know for sure, but evidence points to the land surface. However, at this time, scientists do not agree on which processes dominate, or in what regions of the Earth this missing carbon flux occurs. Several scenarios could cause the land to take up more carbon dioxide than is released each year. For example, re-growth of forests since the massive deforestation in the Northern Hemisphere over the last century could account for some of the missing carbon while changing climate could also contribute to greater uptake than release. The missing carbon problem illustrates the complexity of biogeochemical cycles, especially those in which living organisms play an important role. It is critically important that we understand the processes that control these sources and sinks so that we can predict their behavior in the future. Will these sinks continue to help soak up the carbon dioxide that we are producing? Or will they stop or even reverse and aggravate the atmospheric increases? With the use of satellites and field studies, NASA scientists will help to obtain crucial information on the carbon cycle. next: NASA Missions to Study the Global Carbon Cycle and Climate
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In any given year, tens of billions of tons of carbon move between the atmosphere, hydrosphere, and geosphere. Human activities add about 5.5 billion tons per year of carbon dioxide to the atmosphere. The illustration above shows total amounts of stored carbon in black, and annual carbon fluxes in purple. (Illustration courtesy NASA Earth Science Enterprise) |
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