The evolution of the atmosphere has been intimately linked to the development of life, and there is a con-
stant feedback between the atmosphere and the biosphere. Both CO2 and water are greenhouse gases, and have played an important role in regulating the Earth's surface temperature during the atmosphere's evolution. Water vapor is by far the more abundant, with an average (though highly variable) concentration of one percent by volume. The equilibrium between liquid water and water vapor is crucial in controlling Earth's temperature. On Venus, the high temperature prevented liquid water from forming, which is thought to have led to that planet's runaway greenhouse effect.
Current ice-core data for atmospheric CO2 concentration goes back 650,000 years. These data show that during this time the present ecosystem of land mammals and flowering plants have never experienced CO2 concentrations above 300 ppmv (parts per million by volume) until very recent history. The concentration of CO2 has risen rapidly in the last 300 years, since the beginning of the Industrial Revolution and the dramatic increase in fossil fuel use. Isoto-
pic analysis shows that the rise in CO2 concentration to its present value of more than 370 ppmv is largely due to human activities, chiefly fossil fuel burning.
The widely held scientific consensus is that this rise will lead to significant perturbations in the surface temperature and large scale global climate change, significantly altering the biosphere-atmosphere feedbacks. The unprecedented release of fossil carbon laid down in geological deposits during the life of the ancient Carboniferous Great Forests is perhaps the unfolding storyline in the atmosphere's continuing evolution.
SEE ALSO: Biogeochemical Feedbacks; Earth's Climate History; Gaia Hypothesis; Oxygen Cycle.
BIBLIOGRAPHY. H.D. Holland, The Chemical Evolution of the Atmosphere and Oceans (Princeton University Press, 1884); James Lovelock, The Ages of Gaia (Oxford University Press, 2000); R.P. Wayne, Chemistry of Atmospheres (Oxford University Press, 2000).
Christopher J. Ennis University of Teesside, United Kingdom
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