Understanding the Intermodel Spread of Simulated Arctic September Sea-Ice Sensitivity

<p>We investigate the reasons for the intermodel spread of simulated Arctic September sea-ice sensitivity. Previous studies have found that Arctic September sea-ice area declines linearly with cumulative CO₂ emissions both in observations and climate-model simulations. However, the models&#8217; sensitivity differs substantially, with the models generally underestimating the sensitivity of sea-ice area to CO₂ emissions. We here examine the reasons for the large intermodel spread in order to be also able to understand the general underestimation.</p> <p>We identify a chain of processes contributing to the overall sea-ice sensitivity and investigate the simulation of each sub-process separately in each CMIP6 model. The process chain considers the global-mean temperature response to CO₂ increase, Arctic amplification, the increase in incoming longwave radiation, the total non-shortwave heat flux in the Arctic, and the resulting sea-ice loss. In addition, we separately examine the impact of the simulated incoming longwave radiation for the spread of sea-ice sensitivity. Doing so, we find that clouds play a minor role for the spread of simulated incoming longwave radiation but that temperature rise and water vapour content in the Arctic are relevant.</p> <p>Based on these analyses, we identify three processes whose different representation in climate models likely is the main cause for the intermodel spread of simulated sea-ice sensitivity, and which need to be improved to improve the modeled sensitivity of Arctic sea ice: firstly the global-mean temperature response to CO₂ increase, secondly the Arctic amplification and thirdly local sea-ice processes. The first two factors highly impact the evolution of temperature in the Arctic which affects the incoming longwave radiation and thus the evolution of sea ice.</p>