Summary

Interest in the carbon cycle has increased because of the observed increase in levels of atmospheric CO2 (from ∼280 ppmv in 1800 to ∼315 ppmv in 1957 to ∼356 ppmv in 1993) and because the signing of the UN Framework Convention on Climate Change has forced nations to assess their contributions to sources and sinks of CO2n, and to evaluate the processes that control CO2 accumulation in the atmosphere. Over the last few years, our knowledge of the carbon cycle has increased, particularly in the quantification and identification of mechanisms for terrestrial exchanges, and in the preliminary quantification of feedbacks.

The Increase in Atmospheric CO2 Concentration Since Pre-Industrial Times

Atmospheric levels of CO2 have been measured directly since 1957. The concentration and isotope records prior to that time consist of evidence from ice cores, moss cores, packrat middens, tree rings, and the isotopic measurements of planktonic and benthic foraminifera. Ice cores serve as the primary data source because they provide a fairly direct and continuous record of past atmospheric composition. The ice cores indicate that an increase in CO2 level of about 80 ppmv paralleled the last interglacial warming. There is uncertainty over whether changes in CO2 levels as rapid as those of the 20th century have occurred in the past.