1. GeoBiosphere Science Centre, Physical Geography and Ecosystems Analysis, Lund University, Sölvegatan 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

Correspondence to: Torben R. Christensen¹ Correspondence and requests for materials should be addressed to T.R.C. (Email: Torben.Christensen@nateko.lu.se).

Abstract

Terrestrial wetland emissions are the largest single source of the greenhouse gas methane¹. Northern high-latitude wetlands contribute significantly to the overall methane emissions from wetlands, 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 methane at high-latitude sites⁴, 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 significantly during the freeze-in period. The integral of emissions during the freeze-in period is approximately equal to the amount of methane emitted during the entire summer season. Three-dimensional 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 60° N. Our findings suggest that permafrost-associated 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.

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