Natural marine hydrocarbon seeps on continental margins today represent a small source of methane in the global atmosphere budget, which is dominated by anthropogenic sources and contributions from wetlands in the tropics and northern high latitudes. In glacial times with lowered sea level, exposed seeps must have vented directly to the atmosphere and the portion of methane that was formerly dissolved and oxidized in the ocean contributed to the global atmospheric methane budget. We estimate that during lowered sea level 40–100!10 g/yr of methane were added to the atmosphere from gas seeps on the exposed shelves. This source could account for much of the atmospheric methane during glacial episodes because major wetlands were largely absent prior to the Holocene. q 2005 Elsevier Ltd. All rights reserved.