The Biota and the World Carbon Budget

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Abstract
Current knowledge of the world C budget with special emphasis on whether the biota is now a source or a sink for CO2 is reviewed. The analysis shows that the biota is not a sink and may be a source of CO2 as large as or larger than the fossil fuel source. The issue is important because of the potential that changes in the CO2 content of air have for changing climate worldwide. Various approaches to estimating the role of the biota in the world C budget are consistent in that none suggests that the biota is or was an appreciable sink for C. The trend throughout the past 2 millennia has been for a more or less steady reduction in the earth''s forests. The evidence for contemporary changes is more equivocal. If current appraisals are correct, the contemporary biotic release of C is approximately equal to the fossil fuel release. The total for both sources may exceed 1016 g annually. Of this about 2.3 .times. 1015 g accumulates in the atmosphere. The remainder probably enters the oceans, but the mechanism for this remains puzzling. The apparently erroneous assumption that the biota is a contemporary sink for CO2 arose because the data on oceanic mixing seem to show that the oceans have a limited capacity for absorbing CO2 in the short run of years or decades and other sinks seemed necessary. The problem is still more puzzling if the biota not only is not a sink for fossil fuel C, but is an additional source of CO2. There appears to be no single major sink for C, except the oceans, equivalent to the releases estimated here.