The sizes of major sources and sinks of atmospheric methane (CH 4 ), an important greenhouse gas, are poorly known. CH 4 from north Siberian lakes contributes ∼1.5 teragrams CH 4 year −1 to observed winter increases in atmospheric CH 4 concentration at high northern latitudes. CH 4 emitted from these lakes in winter had a radiocarbon age of 27,200 years and was derived largely from Pleistocene-aged carbon.

Over three years, we found a consistent CO2 efflux from forest tundra of the Russian North throughout the year, including a large (89 g C m -2 yr -1 ) efflux during winter. Our results provide one explanation for the observations that the highest atmospheric CO2 concentration and greatest seasonal amplitude occur at high latitudes rather than over the mid-latitudes, where fossil fuel sources are large, and where high summer productivity offset by winter respiration should give large seasonal oscillations in atmospheric CO2. Winter respiration probably contributed substantially to the boreal winter CO2 efflux. Respiration is an exothermic process that produces enough heat to warm soils and promote further decomposition. We suggest that, as a result of this positive feedback, small changes in surface heat flux, associated with human activities in the North or with regional or global warming, could release large quantities of organic carbon that are presently stored in permafrost.