During the last glacial cycle, greenhouse gas concentrations fluctuated on decadal and longer timescales. Concentrations of methane, as measured in polar ice cores, show a close connection with Northern Hemisphere temperature variability, but the contribution of the various methane sources and sinks to changes in concentration is still a matter of debate. Here we assess changes in methane cycling over the past 160,000 years by measurements of the carbon isotopic composition δ 13 C of methane in Antarctic ice cores from Dronning Maud Land and Vostok. We find that variations in the δ 13 C of methane are not generally correlated with changes in atmospheric methane concentration, but instead more closely correlated to atmospheric CO 2 concentrations. We interpret this to reflect a climatic and CO 2 -related control on the isotopic signature of methane source material, such as ecosystem shifts in the seasonally inundated tropical wetlands that produce methane. In contrast, relatively stable δ 13 C values occurred during intervals of large changes in the atmospheric loading of methane. We suggest that most methane sources - most notably tropical wetlands - must have responded simultaneously to climate changes across these periods. © 2013 Macmillan Publishers Limited.