Climate/CH4 interactions on 10-10,000-year timescales (Invited)

Atmospheric CH4 concentrations have been shown to vary on nearly all timescales throughout the last million years. On glacial/interglacial timescales, CH4 values are low (~375 ppb) during glacial periods and high during interglacial periods (~700ppb). Within glacial periods, CH4 concentration records share a substantial amount of variance with the precession index, presumably in response to the monsoon influence on CH4 emissions in the tropics. A strong covariance between CH4 and the Dansgaard/Oeschger (DO) cycles, first recorded in Greenland ice cores, points to a teleconnection between North Atlantic climate and global CH4 emissions, with the onset of the warming phase of a typical DO cycle being coincident with abrupt increases in atmospheric CH4. These observations suggest that the biogeochemical cycles that ultimately control atmospheric CH4 levels are closely tied to climate. But what is the nature of the climate/CH4 teleconnection? The answer to this question is paramount as we strive to assess future climate change and atmospheric CH4 loading. Recent isotopic evidence has shed some new light on the climate/CH4 teleconnection. During the last deglaciation, both the 13C/12C and D/H ratio of atmospheric CH4 decreased as atmospheric CH4 increased. The enriched nature of glacial atmospheric CH4 is likely to be the result of a heavy isotope enrichment of global sources (e.g., elevated emissions from biomass burning or geologic CH4). Data on the 14C content and the D/H ratio of CH4 at the end of the Younger Dryas period indicate the abrupt increase in atmospheric CH4 was not related to increased emissions of geologic/clathrate CH4.