Methane bubbling from northern lakes: present and future contributions to the global methane budget

Large uncertainties in the budget of atmospheric methane (CH 4 ) limit the accuracy of climate change projections. Here we describe and quantify an important source of CH 4 —point-source ebullition (bubbling) from northern lakes—that has not been incorporated in previous regional or global methane budgets. Employing a method recently introduced to measure ebullition more accurately by taking into account its spatial patchiness in lakes, we estimate point-source ebullition for 16 lakes in Alaska and Siberia that represent several common northern lake types: glacial, alluvial floodplain, peatland and thermokarst (thaw) lakes. Extrapolation of measured fluxes from these 16 sites to all lakes north of 45° N using circumpolar databases of lake and permafrost distributions suggests that northern lakes are a globally significant source of atmospheric CH 4 , emitting approximately 24.2±10.5 Tg CH 4 yr −1 . Thermokarst lakes have particularly high emissions because they release CH 4 produced from organic matter previously sequestered in permafrost. A carbon mass balance calculation of CH 4 release from thermokarst lakes on the Siberian yedoma ice complex suggests that these lakes alone would emit as much as approximately 49 000 Tg CH 4 if this ice complex was to thaw completely. Using a space-for-time substitution based on the current lake distributions in permafrost-dominated and permafrost-free terrains, we estimate that lake emissions would be reduced by approximately 12% in a more probable transitional permafrost scenario and by approximately 53% in a ‘permafrost-free’ Northern Hemisphere. Long-term decline in CH 4 ebullition from lakes due to lake area loss and permafrost thaw would occur only after the large release of CH 4 associated thermokarst lake development in the zone of continuous permafrost.