Records of the δ 13 C of atmospheric CH 4 over the last 2 centuries as recorded in Antarctic snow and ice

Methane is one of the important greenhouse gases accumulating in the atmosphere today. The increased loading over the past 2 centuries is thought to be the result of increased anthropogenic emissions. Here we present records of the δ 13 C of CH 4 in firn air from the South Pole and in trapped bubbles in a short ice core from Siple Dome, Antarctica, that help constrain historical emissions of various sources throughout the last 2 centuries. Using two firn air samplings in 1995 and 2001 we calculate that δ 13 CH 4 has increased by an average of 0.06 ± 0.02‰/yr over the 6 years between samplings. Our ice core results suggest the δ 13 C of atmospheric CH 4 has increased by 1.8 ± 0.2‰ between 1820 A.D. and 2001 AD. The δ 13 CH 4 changes in both data sets are the result of an increase in the relative proportion of CH 4 sources with elevated 13 C/ 12 C isotope ratios. One explanation for observed trends involves a 16 Tg/yr increase in CH 4 emissions associated with biomass burning over the past 2 centuries.