Permafrost thaw and resulting soil moisture changes regulate projected high-latitude carbon dioxide and methane emissions

The fate of currently frozen permafrost carbon as high-latitude climate warms remains highly
uncertain and existing models give widely varying estimates of the permafrost carbon-climate
feedback. This uncertainty is due to many factors, including the role that permafrost thaw-induced
transitions in soil hydrologic conditions will have on organic matter decomposition rates and the
proportion of aerobic to anaerobic respiration. Large-scale permafrost thaw, as predicted by the
Community Land Model (CLM) under an unmitigated greenhouse gas emissions scenario, results in
significant soil drying due to increased drainage following permafrost thaw, even though permafrost
domain water inputs are projected to rise (net precipitation minus evaporation>0).CLM predicts
that drier soil conditions will accelerate organic matter decomposition, with concomitant increases in
carbon dioxide (CO₂) emissions. Soil drying, however, strongly suppresses growth in methane (CH₄)
emissions. Considering the global warming potential (GWP) of CO₂ andCH₄ emissions together, soil
drying weakens the CLM projected GWP associated with carbon fluxes from the permafrost zone by
more than 50% compared to a non-drying case. This high sensitivity to hydrologic change highlights
the need for better understanding and modeling of landscape-scale changes in soil moisture
conditions in response to permafrost thaw in order to more accurately assess the potential magnitude
of the permafrost carbon-climate feedback.