Boreal permafrost thaw amplified by fire disturbance and precipitation increases

Permafrost soils store huge amounts of organic carbon, which could be released if climate change promotes thaw. Currently, models predict that thaw in boreal regions is mainly sensitive to warming, rather than changes in precipitation or vegetation cover. Using a detailed process-based model, parameterised and validated on field measurements, we fundamentally challenge this assumption. We show that soil thermal regimes are controlled strongly by soil moisture and thus the balance between evapotranspiration and precipitation. Consequently, rainfall and warming strongly interacted to exacerbate thaw, with a 50% increase in rainfall more than doubling the thaw caused by 5ºC of warming under dense canopy cover. Furthermore, disturbance to vegetation promoted greater thaw through canopy cover loss and reduced evapotranspiration, which resulted in wetter, more thermally conductive soils. In such disturbed forests, increases in rainfall rivalled warming as a direct driver of thaw, with a 50% increase in precipitation at current temperatures causing as much thaw as 4ºC of warming. We find striking non-linear interactive effects on thaw between rising precipitation and loss of leaf area, which are of great concern given projections of greater precipitation and disturbance in boreal forests. Inclusion of robust vegetation-hydrological feedbacks in global models is therefore critical for accurately predicting permafrost dynamics; thaw cannot be considered to be controlled solely by rising temperatures.