Increased methane emissions due to later soil freezing in Arctic tundra ecosystems

The concentration of atmospheric methane (CH4), a 28-times more powerful greenhouse gas than carbon dioxide (CO2), is increasing at the fastest rate in the last twenty years, making it critical to understand the controls on CH4 emissions from terrestrial ecosystems. We show that autumn terrestrial CH4 enhancements (atmospheric concentrations over background levels from in-situ tall tower measurements) in Northern Alaska are correlated with later soil freezing (R2 = 0.28, p-value < 0.001) using a 16-year data record. The later freezing is associated with the persistence of the "zero-curtain" (soil temperatures sustained near 0˚ C before freezing) which, we suggest allows sustained terrestrial CH4 emissions. Furthermore, site level ecosystem fluxes from five eddy co-variance tower sites in various Arctic tundra ecosystems in Alaska display significantly higher emission rates during zero-curtain conditions than after soil freezing at all five sites. These results suggest that CH4 may be produced in unfrozen soils during the autumn and readily released into the atmosphere, affecting the regional terrestrial CH4 enhancements. We also show that air temperature exhibits poor predictive ability (R2 = 0.13, AIC = 240) on CH4 fluxes relative to soil temperature (R2 = 0.30, AIC = 224) during autumn and suggest that the persistence of unfrozen soil layers should be considered to improve model simulations for the Arctic in the future.