Large uncertainties in the budget of atmospheric methane, an important greenhouse gas, limit the accuracy of climate change projections. Thaw lakes in North Siberia are known to emit methane, but the magnitude of these emissions remains uncertain because most methane is released through ebullition (bubbling), which is spatially and temporally variable. Here we report a new method of measuring ebullition and use it to quantify methane emissions from two thaw lakes in North Siberia. We show that ebullition accounts for 95 per cent of methane emissions from these lakes, and that methane flux from thaw lakes in our study region may be five times higher than previously estimated. Extrapolation of these fluxes indicates that thaw lakes in North Siberia emit 3.8 teragrams of methane per year, which increases present estimates of methane emissions from northern wetlands (< 6-40 teragrams per year; refs 1, 2, 4-6) by between 10 and 63 per cent. We find that thawing permafrost along lake margins accounts for most of the methane released from the lakes, and estimate that an expansion of thaw lakes between 1974 and 2000, which was concurrent with regional warming, increased methane emissions in our study region by 58 per cent. Furthermore, the Pleistocene age (35,260-42,900 years) of methane emitted from hotspots along thawing lake margins indicates that this positive feedback to climate warming has led to the release of old carbon stocks previously stored in permafrost.

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 458 N using circumpolar databases of lake and permafrost distributions suggests that northern lakes are a globally significant source of atmospheric CH 4 , emitting approximately 24.2G10.5 Tg CH 4 yr K1. 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.

Polar ice-core records suggest that an arctic or boreal source was responsible for more than 30% of the large increase in global atmospheric methane (CH4) concentration during deglacial climate warming; however, specific sources of that CH4 are still debated. Here we present an estimate of past CH4 flux during deglaciation from bubbling from thermokarst (thaw) lakes. Based on high rates of CH4 bubbling from contemporary arctic thermokarst lakes, high CH4 production potentials of organic matter from Pleistocene-aged frozen sediments, and estimates of the changing extent of these deposits as thermokarst lakes developed during deglaciation, we find that CH4 bubbling from newly forming thermokarst lakes comprised 33 to 87% of the high-latitude increase in atmospheric methane concentration and, in turn, contributed to the climate warming at the Pleistocene-Holocene transition.

[1] This study reports an atmospheric methane (CH 4 ) source term previously uncharacterized regarding strength and isotopic composition. Methane emissions from 14 Siberian lakes and 9 Alaskan lakes were characterized using stable isotopes ( 13 C and D) and radiocarbon ( 14 C) analyses. We classified ebullition (bubbling) into three categories (background, point sources, and hot spots) on the basis of fluxes, major gas concentrations, and isotopic composition. Point sources and hot spots had a strong association with thermokarst (thaw) erosion because permafrost degradation along lake margins releases ancient organic matter into anaerobic lake bottoms, fueling methanogenesis. With increasing ebullition rate, we observed increasing CH 4 concentration of greater radiocarbon age, depletion of 13 C CH4 , and decreasing bubble N 2 content. Microbial oxidation of methane was observed in bubbles that became trapped below and later within winter lake ice; however, oxidation appeared insignificant in bubbles sampled immediately after release from sediments. Methanogenic pathways differed among the bubble sources: CO 2 reduction supported point source and hot spot ebullition to a large degree, while acetate fermentation appeared to contribute to background bubbling. To provide annual whole-lake and regional CH 4 isofluxes for the Siberian lakes, we combined maps of bubble source distributions with long-term, continuous flux measurements and isotopic composition. In contrast to typical values used in inverse models of atmospheric CH 4 for northern wetland sources (d 13 C CH4 = À58%, 14 C age modern), which have not included northern lake ebullition as a source, we show that this large, new source of high-latitude CH 4 from lakes is isotopically distinct (d 13 C CH4 = À70%, 14 C age 16,500 years, for North Siberian lakes).Citation: Walter, K. M., J. P. Chanton, F. S. Chapin III, E. A. G. Schuur, and S. A. Zimov (2008), Methane production and bubble emissions from arctic lakes: Isotopic implications for source pathways and ages,

Carbon dioxide, energy flux measurements and methane chamber measurements were carried out in an arctic wet tussock grassland located on a flood plane of the Kolyma river in NE Siberia over a summer period of 155 days in 2002 and early 2003. Respiration was also measured in April 2004. The study region is characterized by late thaw of the top soil (mid of June) and periodic spring floods. A stagnant water table below the grass canopy is fed by thawing of the active layer of permafrost and by flood water. The climate is continental with average daily temperature in the warmest months of 13 1C (maximum temperature at midday: 28 1C by the end of July), dry air (maximum vapour pressure deficit at midday: 28 hPa) and low rainfall of 50 mm during summer (JulySeptember). Summer evaporation (July-September: 103 mm) exceeded rainfall by a factor of 2. The daily average Bowen ratio (H/LE) was 0.62 during the growing season. Net ecosystem CO 2 uptake reached 10 lmol m À2 s À1 and was related to photon flux density (PFD) and vapour pressure deficit (VPD). The cumulative annual net carbon flux from the atmosphere to the terrestrial surface was estimated to be about À38 g C m À2 yr À1 (negative flux depicts net carbon sink). Winter respiration was extrapolated using the Lloyd and Taylor function. The net carbon balance is composed of a high rate of assimilation in a short summer and a fairly large but uncertain respiration mainly during autumn and spring. Methane flux (about 12 g C m À2 measured over 60 days) was 25% of C uptake during the same period of time (end of July to end of September). Assuming that CH 4 was emitted only in summer, and taking the greenhouse gas warming potential of CH 4 vs. CO 2 into account (factor 23), the study site was a greenhouse gas source (at least 200 g C equivalent m À2 yr À1 ). Comparing different studies in wetlands and tundra ecosystems as related to latitude, we expect that global warming would rather increase than decrease the CO 2 -C sink.

Arctic lakes are significant emitters of methane (CH 4 ), a potent greenhouse gas, to the atmosphere; yet no rigorous quantification of the magnitude and variability of pan-Arctic lake emissions exists. In this study, we demonstrate the potential for a new method using synthetic aperture radar (SAR) imagery to detect methane bubbles in lake ice to scale up whole-lake measurements of CH 4 ebullition (bubbling) to regional scales. We estimated ebullition from lakes, which is often the dominant mode of lake emissions, by mapping the distribution of bubble clusters frozen in early winter ice across surfaces of seven tundra lakes and one boreal forest lake in Alaska. Applying previously measured ebullition rates associated with four distinct classes of bubble clusters found in lake ice, we estimated whole-lake emissions from individual lakes. The percent surface area of lake ice covered with bubbles (R 2 = 0.68) and CH 4 ebullition rates from lakes (R 2 = 0.59) and were correlated with radar return values from RADARSAT-1 Standard Beam mode 3 for the tundra lakes, suggesting that with appropriate scaling and consideration for variability in lake-ice conditions, this technique has the potential to be used for estimating broader-scale regional and pan-Arctic lake methane emissions.(KEY TERMS: lakes; methane; climate change; remote sensing synthetic aperture radar; lake ice.)

The current concentration of atmospheric methane is 1774±1.8 parts per billion, and it accounts for 18% of total greenhouse gas radiative forcing [Forster et al., 2007]. Atmospheric methane is 22 times more effective, on a per‐unit‐mass basis, than carbon dioxide in absorbing long‐wave radiation on a 100‐year time horizon, and it plays an important role in atmospheric ozone chemistry (e.g., in the presence of nitrous oxides, tropospheric methane oxidation will lead to the formation of ozone). Wetlands are a large source of atmospheric methane, Arctic lakes have recently been recognized as a major source [e.g., Walter et al., 2006], and anthropogenic activities—such as rice agriculture—also make a considerable contribution.

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