Greenland's firn responds more to warming than to cooling

Abstract. The porous layer of snow and firn on the Greenland Ice Sheet stores meltwater and limits sea level rise. This buffer is threatened in a warming climate. To better understand the nature and timescales of firn’s response to air temperature change, we use a physics-based model to assess the effects of atmospheric warming and cooling on firn air content. We identify an asymmetric response of Greenland’s firn to air temperature: firn loses more air content due to warming compared to the amount gained from commensurate cooling. In dry firn, this asymmetry is driven by the highly nonlinear relationship between temperature and firn compaction, as well as the dependency of thermal conductivity on the composition of the firn. The influence of liquid water accentuates this asymmetry. In wet firn areas, melt increases nonlinearly with atmospheric warming, thus enhancing firn refreezing and further warming the snowpack through increased latent heat release. Our results highlight the vulnerability of firn to temperature change and demonstrate that firn air content is more efficiently depleted than generated. This asymmetry in the temperature–firn relationship may contribute to the overall asymmetric mass change of the Greenland Ice Sheet in a changing climate across many timescales.