Lawrence Berkeley National Laboratory

Abstract Model projections of permafrost thaw during the next century diverge widely. Here we used ecosys to examine how climate change will affect permafrost thaw in a polygonal tundra at Barrow AK. The model was tested against diurnal and seasonal variation in energy exchange, soil heat flux, soil temperature ( T s ), and active layer depth (ALD) measured during 2014 and 2015, and interannual variation in ALD measured from 1991 to 2015. During RCP 8.5 climate change from 2015 to 2085, increases in T a and precipitation ( P ) to 6.2 °C and 27% above current values, and in atmospheric CO 2 concentrations ( C a ) to763 μmol mol −1 , altered energy exchange by increasing leaf area index of dominant sedge relative to that of moss. Increased C a and sedge leaf area index imposed greater stomatal control of transpiration and reduced soil heat fluxes, slowing soil warming, limiting increases in evapotranspiration, and thereby causing gradual soil wetting. Consequently, increases in surface T s and ALD of 2.4–4.7 °C and 21–24 cm above current values were modeled after 70 years. ALD increase was slowed if model boundary conditions were altered to improve landscape drainage. These rates were smaller than those of earlier modeling studies, some of which did not account for changes in vegetation, but are closer to those derived from current studies of warming impacts in the region. Therefore, accounting for climate change effects on vegetation density and composition, and consequent effects on surface energy budgets, will cause slower increases in T s and ALD to be modeled during climate change simulations. Key Points Permafrost thaw will cause wetting of coastal tundra Permafrost thaw with climate change will be slowed by increasing vegetation density in coastal tundra Permafrost thaw with climate change will be slowed if drainage of coastal tundra increases