Abrupt climate change and the carbon isotope composition of atmospheric methane

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Kaplan, J. O. , Melton, J. R. and Köhler, P. (2007): Abrupt climate change and the carbon isotope composition of atmospheric methane , QUEST Open Science Conference (OSC), 26 - 28 March 2007, Paramount Hotel, Oxford, UK. .
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Recent measurement of ice core δ13C-CH4 and deconvolution of the global CH4 budget during abrupt climate change events has led to a widespread speculation regarding the mechanisms behind observed rapid changes in atmospheric CH4 concentrations. Using a paleoclimate scenario from an intermediate complexity climate model and a dynamic global vegetation model, we simulate changes in the magnitude and carbon isotope composition of CH4 emitted from wetlands and fires. In a control scenario, global wetland CH4 emissions are dominated by the tropics. During full glacial climate conditions of low sea level, tropical continental shelves supported wetland areas larger than those at present. Preliminary results indicate that any shift in the carbon isotope ratio of the CH4 forming substrate due to changing vegetation composition may have been offset by the effect of warmer temperatures on the fractionation between substrate and CH4. These results may partially explain why observed δ13C-CH4 appears stable even at times of rapidly changing atmospheric CH4 concentrations, and help in ice-core based deconvolution of the atmospheric CH4 record.

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