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 Arctic soil temperatures during winter and parameters such as snow effective thermal conductivity (K_eff) are not well constrained in land surface models which impacts our ability to accurately simulate wintertime soil carbon emissions. A point-model version of the Community Land Model (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 and using a K_eff parameterisation which is more representative of Arctic snowpack. Implementing this K_eff 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 K_eff parameterisation. The influence of snow K_eff 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. Furthermore, CLM5.0 simulations show that enhanced future air and soil temperatures increases the duration of the early winter (Sept–Oct) zero-curtain, a crucial period of soil carbon emissions, by up to a month and recent increases in both zero-curtain and winter CO₂ emissions appear set to continue to 2100. Modelled winter soil temperatures and carbon emissions demonstrate the importance of climate mitigation in preventing a significant increase in winter carbon emissions from the Arctic in the future.