Snow thermal conductivity controls future winter carbon emissions in shrub–tundra

Abstract. The Arctic winter is disproportionately vulnerable to climate warming and approximately 1700 Gt of carbon stored in high-latitude permafrost ecosystems is at risk of degradation in the future due to enhanced microbial activity. Few studies have been directed at high-latitude cold season land–atmosphere processes and it is suggested that the contribution of winter season greenhouse gas (GHG) fluxes to the annual carbon budget may have been underestimated. Snow, acting as a thermal blanket, influences the Arctic soil temperatures during winter and parameters such as snow effective thermal conductivity (Keff) are not well constrained in land surface models, which impacts our ability to accurately simulate wintertime soil carbon emissions. To address this, we investigated the impacts of implementing a Keff parameterisation more suitable to Arctic snowpacks into the Community Land Model (CLM5.0). A point-model version of CLM5.0 forced by an ensemble of NA-CORDEX (North American Coordinated Regional Downscaling Experiment) future climate realisations (RCP 4.5 and 8.5) indicates that median winter CO2 emissions will have more than tripled by the end of the century (2066–2096) under RCP 8.5. Implementing the refined Keff parameterisation increases simulated winter CO2 in the latter half of the century (2066–2096) by 130 % and CH4 flux by 50 % under RCP 8.5 compared to the widely used default Keff parameterisation. The influence of snow Keff parameterisation within CLM5.0 on future simulated CO2 and CH4 is at least as significant, if not more so, than climate variability from a range of NA-CORDEX projections to 2100. The average difference in refined Keff compared with the default Keff raises minimum winter soil temperatures by 4–7 °C by the end of the century under RCP 4.5 and 8.5. Furthermore, CLM5.0 simulations using the refined Keff show an extension of the early winter (September–October) zero-curtain period, by nearly a month under RCP 8.5. Consequently, recent increases in both zero-curtain duration and winter CO2 emissions appear set to continue to 2100. Modelled winter soil temperatures and carbon emissions further highlight the importance of climate mitigation in preventing a significant increase in winter carbon emissions from the Arctic in the future.