Methane Emissions From Lakes In Northeast Siberia And Alaska

Abstract

Large uncertainties in the budget of atmospheric methane (CH4), an important greenhouse gas whose relative greenhouse effect is 23 times stronger than that of carbon dioxide (CO2), limit the accuracy of climate-change projections. Concentrations of atmospheric CH 4 have been rising during recent decades, particularly at high northern latitudes. The causes of this increase are not well understood. Here I describe and quantify an important source of methane---bubbling from northern lakes---that has not been incorporated in previous regional or global methane budgets. I introduce a new method to accurately measure ebullition (bubbling), which accounted for 95% of CH4 emissions from North Siberian thaw lakes. Documenting the patchiness of ebullition increased previous estimates of CH4 flux from lakes 5-fold in Siberia and 2.5- to 14-fold in Alaska. Extrapolating estimates of measured fluxes, I show that North Siberian yedoma (Pleistocene-aged organic-rich loess) thaw lakes emit 3.8 Tg CH 4 yr-1. An independent mass-balance approach based on carbon lost from permafrost that thawed beneath lakes revealed that lakes emit 4-5 Tg CH4 yr-1. Adding these emissions significantly increases present estimates of northern wetland contributions (<6-40 Tg yr-1) to the atmospheric CH4 budget. Thermokarst (thaw) erosion was the primary driver of CH4 emissions in lakes. A 14.7% expansion of thaw lakes from 1974 to 2000 increased lake CH 4 emissions by 58% in Siberia, demonstrating a positive feedback to climate warming. The Pleistocene age of CH4 (14C age 35,570-42,800 years in Siberia and 14,760-26,020 years in Alaska) emitted from hotspots along active thermokarst margins of lakes demonstrated that recruitment of a previously sequestered carbon source contributes to this feedback. Finally, reconstruction of yedoma's distribution at the Last Glacial Maximum together with compilation of thaw lake basal ages that developed at the onset of Holocene warming, suggested that thaw lake development contributed up to 70% of the rapid increase in atmospheric CH4 during deglaciation. About 425 Gt C remain preserved in the yedoma ice complex in North Siberia. If this Siberian permafrost warms more rapidly in the future as projected, the positive feedback of ebullition from expanding thaw lakes could increase the rate of high-latitude warming.