Constraining CMIP6 Projections of an Ice-Free Arctic Using a Weighting Scheme

Employing a model democracy in which each model is equally weighted may lead to a poor estimation of the true uncertainty in climate projections from phase 6 of the Coupled Model Intercomparison Project (CMIP6). The improvement and increase in number of CMIP6 models compared with previous phases of CMIP indicate that both model skill and independence need to be considered to provide convincing projections. In this study, we use a weighting scheme, which weights both the skill and independence of multi-model simulations, to efficiently constrain the large uncertainty in the projection of the timing of an ice-free Arctic. The uncertainty-constrained projections of CMIP6 show that the multi-model spread of the projected first year of an ice-free Arctic can be reduced by about 29 years under the Shared Socioeconomic Pathway (SSP)3–7.0 scenario (a regional rivalry scenario), indicating a faster tendency to an ice-free Arctic summer than projections based solely on model democracy. A fossil-fuel-based development scenario (i.e., SSP5-8.5) leads to an ice-free Arctic before the 2070s (ranging from 2038 to 2071), while an ice-free Arctic occurs slightly later (by ∼10 years) under the SSP2-4.5 scenario (i.e., intermediate scenario) and the SSP3-7.0 scenario, but is inevitable this century. The sustainable development scenario (i.e., SSP1-2.6) is likely to prevent the occurrence of an ice-free Arctic. Internal variability strongly affects the projection estimated by the equally weighted ensemble; however, it has a negligible impact on the results obtained by the weighting scheme, thereby indicating that the results of this study are robust and convincing.publishedVersio