Changes in Precipitation and Air Temperature Contribute Comparably to Permafrost Degradation in a Warmer Climate
A study at the North Slope of Alaska shows that increased precipitation accelerates permafrost degradation beyond the degradation caused by recent and 21st-century climate surface air warming. The study examines (1) how changes in precipitation affect Active Layer Depths under recent and future climate and (2) the relative importance of changes in surface air temperature and precipitation on permafrost degradation in the continuous permafrost zone (>90% of the area underlain by permafrost).
Projected permafrost degradation may result in several ecological and climatic feedbacks that affect the carbon cycle. Permafrost regions store a huge amount of carbon, which may be available for microbial decomposition. Uncertainties in projected 21st-century precipitation trends strongly affect simulated permafrost degradation. Earth system models, which do not account for changes in soil thermal regime driven by precipitation heat transfer, likely underestimate predicted increases in thaw depth and therefore their effects on high-latitude carbon interactions with the atmosphere.
Surface energy budgets of high-latitude permafrost systems are poorly represented in Earth System Models (ESMs), yet permafrost is rapidly degrading and these dynamics are critical to future carbon-climate feedback predictions. A potentially important factor in permafrost degradation neglected so far by ESMs is heat transfer from precipitation, although increases in soil temperature and thaw depth have been observed following increases in precipitation. Modeled active layer depth (ALD) in simulations that allow precipitation heat transfer agreed very well with observations from 28 Circumpolar Active Layer Monitoring (CALM) sites (R2=0.63; RMSE = 10 cm). Simulations that ignored precipitation heat transfer resulted in lower spatially-averaged soil temperatures and a 39 cm shallower ALD by 2100 across the NSA. Using a mechanistic ecosystem model, ecosys, the results from our sensitivity analysis show that projected increases in 21st-century precipitation deepen the active layer by enhancing precipitation heat transfer and ground thermal conductivity, suggesting that precipitation is as important an environmental control on permafrost degradation as surface air temperature. We conclude that ESMs that do not account for precipitation heat transfer likely underestimate ALD rates of change, and thus likely predict biased ecosystem responses.