Warm-season net CO2 uptake outweighs cold-season emissions over Alaskan North Slope tundra under current and RCP8.5 climate

Abstract

Arctic warming has increased vegetation growth and soil respiration during recent decades. The rate of Arctic warming will likely amplify over the 21st century. Previous studies have revealed that the most severe Arctic warming occurred during the cold season (September to May). The cold-season warming has posited significant CO _2 emissions to the atmosphere via respiration, possibly offsetting warm-season (June to August) net CO _2 uptake. However, prevailing Earth system land models poorly represent cold-season CO _2 emissions, making estimates of Arctic tundra annual CO _2 budgets highly uncertain. Here, we demonstrate that an improved version of the energy exascale Earth system model (E3SM) land model (ELMv1-ECA) captures the large amount of cold-season CO _2 emissions over Alaskan Arctic tundra as reported by two independent, observationally-constrained datasets. We found that the recent seven-decades warming trend of cold-season soil temperature is three times that of the warm-season. The climate sensitivity of warm-season net CO _2 uptake, however, is threefold higher than for the cold-season net CO _2 loss, mainly due to stronger plant resilience than microbial resilience to hydroclimatic extremes. Consequently, the modeled warm-season net CO _2 uptake has a larger positive trend (0.74 ± 0.14 gC m ^−2 yr ^−1 ) than that of cold-season CO _2 emissions (0.64 ± 0.11 gC m ^−2 yr ^−1 ) from 1950 to 2017, supported by enhanced plant nutrient uptake and increased light- and water-use efficiency. With continued warming and elevated CO _2 concentrations under the representative concentration pathway (RCP) 8.5 scenario, the increasing rate of warm-season net CO _2 uptake is more than twice the rate of cold-season emissions (1.33 ± 0.32 gC m ^−2 yr ^−1 vs 0.50 ± 0.12 gC m ^−2 yr ^−1 ), making the modeled Alaskan Arctic tundra ecosystem a net CO _2 sink by 2100. However, other geomorphological and ecological disturbances (e.g. abrupt permafrost thaw, thermokarst development, landscape-scale hydrological changes, wildfire, and insects) that are not considered here might alter our conclusion.

Keywords

• soil respiration
• carbon budget
• Alaskan Arctic tundra
• tundra plant resilience
• hydroclimatic extremes
• E3SM land model (ELM)