Atmospheric Composition and Biogeochemical Cycles over the Last Million Years

Ice core analysis provides the most direct evidence of changes in some major greenhouse gases (CO2, CH4 and N2O) over the climatic cycle covering approximately the last 150,000 years. A remarkable overall correlation is observed between the CO2 or CH4 record and the climatic changes in the high latitudes of the Southern Hemisphere, with lowest greenhouse gas concentrations found under full glacial conditions. In terms of phase relationship, CO2 and CH4 are roughly in phase with the climatic signal during the deglaciation periods; when entering the glaciation, CH4 appears to decrease in phase with the Antarctic cooling but CO2 lags strikingly behind. The CH4 record exhibits a marked signal which is most likely associated with the abrupt cooling of the Younger Dryas. Existing differences between CO2 and CH4 records in comparison with climate reflect differences in sources which are mainly oceanic in the case of CO2 and continetal in the case of CH4. For N2O only few data are available suggesting that the N2O concentrations may also have been lower during the Last Glacial Maximum than during the Holocene. Greenhouse gases are likely to have played an important climatic role in amplifying, together with continental ice, the initial orbital forcing of the glacial-interglacial climatic changes. © 1992.