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 uncertain2,3. 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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This work was carried out under the auspices of the GeoBasis programme and part of the Zackenberg Ecological Research Operations funded by the Danish Ministry of the Environment and the ISICaB project funded by the Commission for Scientific Research in Greenland (KVUG). ASIAQ–Greenland Survey provided climate data. The work was also supported by the Swedish Research Councils VR and FORMAS. We thank P. Bergamachi (JRC) and J.-F. Meirink (KNMI) for providing the TM5 model setup. T. Tagesson helped with the field work in Zackenberg. We are grateful for comments on earlier versions of this manuscript from A. Lindroth and B. Christensen.
Author Contributions T.R.C., M.P.T., M.M., C.S. and L.S. designed the field research; M.M. designed, constructed and set up the automatic measurement system in Zackenberg; C.S. operated the system and performed manual measurements; M.M. performed data analysis; E.D. and S.H. provided atmospheric CH4 data and designed and ran the atmospheric transport model experiments; T.R.C., M.M., S.H. and E.D. drafted the manuscript.
This file contains Supplementary Figures and Legends 1-3, Supplementary Tables 1-2, Supplementary Notes and Supplementary Discussion.