The sizes of major sources and sinks of atmospheric methane (CH4), an important greenhouse gas, are poorly known. CH4 from north Siberian lakes contributes ~1.5 teragrams CH4 year+1 to observed winter increases in atmospheric CH4 concentration at high northern latitudes. CH4 emitted from these lakes in winter had a radiocarbon age of 27,200 years and was derived largely from Pleistocene-aged carbon.