(19)

Reliable projections of permafrost change are crucial for estimating permafrost carbon loss. However, potential model biases in surface air temperature may yield unrealistic projections of future permafrost area. Here, by leveraging the emergent relationship between equilibrium climate sensitivity and projected changes in mean annual air temperature over High Mountain Asia (HMA), we mitigate the overestimated local warming rates and excessive thawing of permafrost associated with the hot model problem in models participating in the Coupled Model Intercomparison Project phase 6. After constraint, permafrost area over HMA will reduce by 37%, 64% and 99% in 2081-2100 relative to present-day under the SSP1-2.6, SSP2-4.5 and SSP5-8.5 scenarios, respectively. In contrast, the unconstrained projections tend to overestimate the loss of permafrost area by nearly 10% under the low and mid-emission scenarios due to the overestimation of local warming rates. These findings are crucial for policymaking and provide valuable insights into global permafrost projection. This study shows a comprehensive picture of permafrost changes over High Mountain Asia (HMA) in the coming future. A subset of the newest generation of climate models participating in Coupled Model Intercomparison Project phase 6 (CMIP6) have a hot model problem with high equilibrium climate sensitivity (ECS) exceeding the likely range of 2.5 degrees C-4 degrees C assessed by Intergovernmental Panel on Climate Change Sixth Assessment Report. This indicates that the surface temperature projections in response to changes in atmospheric carbon dioxide concentrations are higher than those expected based on other evidence. Here, taking HMA as a case study, we establish a relationship between ECSs and the future changes in mean annual air temperature over HMA to constrain the future projections of warming rates and permafrost degradation. The constrained projection is 0.2 degrees C, 0.4 degrees C and 0.5 degrees C lower than the unconstrained warming simulated by the CMIP6 ensemble during 2081-2100 under low, intermediate, and very high emission scenarios (SSP1-2.6, SSP2-4.5 and SSP5-8.5). Based on the more reliable projections of future warming, reductions of permafrost area by 37%, 64% and 99% in 2081-2100 under SSP1-2.6, SSP2-4.5 and SSP5-8.5 scenarios, respectively, are expected. The constraining solves the exaggerated projection of permafrost degradation caused by CMIP6 models. Models that overestimate warming rates tend to also overestimate permafrost degradation over High Mountain Asia An emergent relationship between equilibrium climate sensitivity and surface warming is used to constrain future permafrost projections The permafrost area will reduce by 37%, 64% and 99% in 2081-2100 relative to 2000-2016 under SSP1-2.6, SSP2-4.5 and SSP5-8.5 scenarios

来源平台：GEOPHYSICAL RESEARCH LETTERS