Sensitivity of Methane Emissions to Later Soil Freezing in Arctic Tundra Ecosystems

The atmospheric methane (CH 4 ) concentration, a potent greenhouse gas, is on the rise once again, making it critical to understand the controls on CH 4 emissions. In Arctic tundra ecosystems, a substantial part of the CH 4 budget originates from the cold season, particularly during the “zero curtain” (ZC), when soil remains unfrozen around 0 °C. Due to the sparse data available at this time, the controls on cold season CH 4 emissions are poorly understood. This study investigates the relationship between the fall ZC and CH 4 emissions using long‐term soil temperature measurements and CH 4 fluxes from four eddy covariance (EC) towers in northern Alaska. To identify the large‐scale implication of the EC results, we investigated the temporal change of terrestrial CH 4 enhancements from the National Oceanic and Atmospheric Administration monitoring station in Utqiaġvik, AK, from 2001 to 2017 and their association with the ZC. We found that the ZC is extending later into winter (2.6 ± 0.5 days/year from 2001 to 2017) and that terrestrial fall CH 4 enhancements are correlated with later soil freezing (0.79 ± 0.18‐ppb CH 4 day −1 unfrozen soil). ZC conditions were associated with consistently higher CH 4 fluxes than after soil freezing across all EC towers during the measuring period (2013–2017). Unfrozen soil persisted after air temperature was well below 0 °C suggesting that air temperature has poor predictive power on CH 4 fluxes relative to soil temperature. These results imply that later soil freezing can increase CH 4 loss and that soil temperature should be used to model CH 4 emissions during the fall.