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

Model projections of future CO₂ and CH₄ exchange in Arctic tundra 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 net primary productivity (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 heterotrophic respiration (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. 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 net ecosystem productivity (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. More rapid nutrient cycling modelled with climate change favored dominance of deciduous sedge over evergreen moss which in turn affected the modelled responses of CO₂ and CH₄ exchange to climate change in the polygonal tundra. At a larger spatial scale, accelerated nutrient cycling modelled with climate change drove modelled shrub expansion across the North American Arctic tundra during the 21st Century. Faster-growing deciduous shrubs modeled with less efficient nutrient conservation dominated much of the low Arctic by 2100 where nutrient cycling became more rapid, while the slower-growing evergreen shrubs modeled with more efficient nutrient conservation dominated a wider latitudinal range that extended to the high Arctic where nutrient cycling remained slower.