Thermal Transport by Rain into Thawing Permafrost Landscapes

Abstract:

Northern high latitudes are expected to get warmer and wetter. There is consensus that warming will intensify permafrost thaw and increase wetland methane emissions, facilitating a positive climate feedback. However, the effects of increased precipitation are uncertain. At two different thawing wetland complexes in Alaska, we found that rain rapidly altered soil temperature, both within the permafrost plateau and within the thaw wetland. To a first approximation, rain has the same temperature as air, and when air and soil temperatures are mismatched, rainwater inputs can rapidly change subsurface soil temperatures through thermal conduction. At one site, we found that when wetland soils were warmed by spring rainfall, methane emissions increased by ~30%. The warm, deep soils early in the growing season likely enhanced both microbial and plant processes that increased emissions. At the other site, data showed rapid thaw of frozen soil within the permafrost plateau during a large rain event. This result indicates, but does not prove, that thermal transport by rain could be an important mechanism for thawing permafrost. The collective datasets clearly demonstrate the ability of rain to advect thermal energy into soils, and indicate that through this mechanism, rain notably affects the radiative forcing of thawing permafrost landscapes.