Modelling climate change impacts on an Arctic polygonal tundra: Changes in CO2 and CH4 exchange depend on rates of permafrost thaw as affected by changes in vegetation and drainage

Model projections of CO₂ and CH₄ exchange in Arctic tundra during the next century diverge widely. Here we used ecosys to examine how climate change will affect CO₂ and CH₄ exchange in troughs, rims and centers of a coastal polygonal tundra landscape at Barrow AK. The model was shown to simulate diurnal and seasonal variation in CO₂ and CH₄ fluxes associated with those in air and soil temperatures (T_a and T_s) and soil water contents (q) under current climate in 2014 and 2015. During RCP 8.5 climate change from 2015 to 2085, rising T_a, atmospheric CO₂ concentrations (C_a) and precipitation (P) increased NPP from 50 - 150 g C m^-2 y^-1, consistent with current biometric estimates, to 200 - 250 g C m^-2 y^-1. Concurrent increases in R_h were slightly smaller, so that net CO₂ exchange rose from values of -25 (net emission) to +50 (net uptake) g C m^-2 y^-1 to ones of -10 to +65 g C m^-2 y^-1. Large increases in R_h with thawing permafrost were not modelled. Increases in net CO₂ uptake were largely offset by increases in CH₄ emissions from 0 - 6 g C m^-2 y^-1 to 1 - 20 g C m^-2 y^-1, reducing gains in NEP. These increases in net CO₂ uptake and CH₄ emissions were modelled with hydrological boundary conditions that were assumed not to change with climate. Both these increases were smaller if boundary conditions were gradually altered to increase landscape drainage during model runs with climate change.