LETTERS Large tundra methane burst during onset of freezing Mikhail Mastepanov 1 , Charlotte Sigsgaard 2 , Edward J. Dlugokencky 3 , Sander Houweling 4,5 , Lena Stro ¨m 1 , Mikkel P. Tamstorf 6 & Torben R. Christensen 1 Terrestrial wetland emissions are the largest single source of the greenhouse gas methane 1 . Northern high-latitude wetlands con- tribute significantly to the overall methane emissions from wet- lands, but the relative source distribution between tropical and high-latitude wetlands remains uncertain 2,3 . As a result, not all the observed spatial and seasonal patterns of atmospheric methane concentrations can be satisfactorily explained, particularly for high northern latitudes. For example, a late-autumn shoulder is consistently observed in the seasonal cycles of atmospheric meth- ane at high-latitude sites 4 , but the sources responsible for these increased methane concentrations remain uncertain. Here we report a data set that extends hourly methane flux measurements from a high Arctic setting into the late autumn and early winter, during the onset of soil freezing. We find that emissions fall to a low steady level after the growing season but then increase signifi- cantly during the freeze-in period. The integral of emissions dur- ing the freeze-in period is approximately equal to the amount of methane emitted during the entire summer season. Three-dimen- sional atmospheric chemistry and transport model simulations of global atmospheric methane concentrations indicate that the observed early winter emission burst improves the agreement between the simulated seasonal cycle and atmospheric data from latitudes north of 606 N. Our findings suggest that permafrost-assoc- iated freeze-in bursts of methane emissions from tundra regions could be an important and so far unrecognized component of the seasonal distribution of methane emissions from high latitudes. Methane emissions from permafrost dominated tundra regions are well documented 5–7 and also recognized as considerable contri- butors to the dynamics of high-latitude atmospheric methane con- centrations 8,9 . The scale and dynamics of growing-season methane emissions from tundra settings have been documented mostly through flux measurements made with low time resolution using manual chambers 5,6,10 together with some at higher time resolution taken only during the growing season 7,11,12 . Here we report a data set that extends hourly CH 4 flux measurements from a high Arctic set- ting into the frozen season. The measurement site is located in Zackenberg Valley, northeast Greenland, 74.30u N 21.00u W. Six automated chambers provided flux measurements once per hour, in a typical fen area dominated by graminoids Eriophorum scheuch- zeri, Dupontia psilosantha and Arctagrostis latifolia. Methane concen- tration in the chambers was measured by a laser off-axis integrated- cavity output spectroscopy analyser (Fast Methane Analyser, Los Gatos Research). The instrument sensitivity is better than 10 p.p.b.; time resolution of the primary concentration data is 1 s. As part of the field season of the 2007 International Polar Year, the Zackenberg research station was kept open two months longer than normal. This gave us a chance to observe autumn and early-winter fluxes, which showed some surprisingly high emissions (Fig. 1; Supplementary Table 1). This very high and variable flux happened when the active layer was gradually freezing, so CH 4 that had accu- mulated in this layer was probably squeezed out through the frost action. This feature has not been observed in studies at lower lati- tudes, possibly because the permafrost bottom is necessary to prevent CH 4 from diffusing downwards. The autumn fluxes varied greatly over small distances (chambers were less than 1 m apart), probably because peat and vegetation structure provided pathways for emis- sion to the atmosphere. A late-autumn increase in methane emis- sions was observed in one of the early tundra flux studies 13 , but it lacked the time resolution needed to quantify the relative importance for the annual flux budget. The observed growing season emission dynamics are comparable to earlier work at the same 6,7 and at similar tundra sites 12 . Integrated summer season emissions, roughly 4.5 g CH 4 m –2 for the season, also match well with previous estimates for the same climatic and ecosys- tem setting 6,7 . Emissions decreased during September until they reached the pre- sumed low winter emission level (Fig. 1). However, at the onset of soil freeze-in, a substantial increase in emissions was observed and was sustained for several weeks, corresponding to the time required for a complete freeze-in of the entire soil and root zone profile. Freeze-in emissions were much more variable than summer emissions. Peak emissions during the freeze-in period in individual chambers reached levels of 112.5 mg CH 4 m 22 h 21 , which to our knowledge are the highest rates reported from tundra ecosystems (excluding hotspot emissions from thermokarst lakes 14 ), and they appear at a time when previous assumptions would put tundra emissions at a negligible level (see Supplementary Information for further discussion). Earlier studies have indicated the possibility of a spring burst from trapped methane during the winter 15,16 . We have early-season flux data from Zackenberg for 2006 (M. Mastepanov et al., manuscript in preparation) showing that spring emissions amounted to less than 2% of summer emissions (Fig. 1 insert; Supplementary Table 2), with summer emissions being very similar for 2006 and 2007 (Supplementary Tables 1 and 2). Emissions of methane during spring from this type of tundra environment are therefore not considered as a major contributor to annual methane emissions. To investigate the potential importance of the observed methane emissions during freezing of the permafrost surface layer at large scales, we carried out model simulations of atmospheric transport and compared them with observations. Model-simulated methane concentrations were sampled at the times and locations when mea- surements were taken at selected background monitoring sites of the NOAA Earth System Research Laboratory’s cooperative air sampling network 4 . Average seasonal cycles were constructed from air samples collected over the 4-year simulation period. Furthermore, back- ground sites were averaged into two latitudinal bands: 25–55u N 1 GeoBiosphere Science Centre, Physical Geography and Ecosystems Analysis, Lund University, So ¨lvegatan 12, 22362, Lund, Sweden. 2 Institute of Geography and Geology, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen, Denmark. 3 NOAA Earth System Research Laboratory, 325 Broadway, Boulder, Colorado 80305, USA. 4 SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands. 5 Institute for Marine and Atmospheric Research Utrecht (IMAU), Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands. 6 National Environmental Research Institute, University of Aarhus, Frederiksborgvej 399, 4000 Roskilde, Denmark. Vol 456 | 4 December 2008 | doi:10.1038/nature07464 628 ©2008 Macmillan Publishers Limited. All rights reserved