Steady but model-dependent polar amplification of the CO2-forced surface warming in CMIP6 projections

Using the CMIP6 historical and future emission scenarios, we examine sources of uncertainty in the projections of Polar amplification across all available fully coupled earth system models and in six large initial condition ensembles. In the multi-model mean, the annual mean Arctic amplification ratio is steady at 2.5, both in time and across scenarios, resulting in negligibly small scenario uncertainty. Deviations from the steady value can be found in the low and high emission scenarios due to different root causes, with the latter being mostly evident in the summer and autumn seasons. Best estimates of model uncertainty are at least an order of magnitude larger than scenario uncertainty. Finally, irreducible internal variability has a similar magnitude to inter-model uncertainty, except in the lowest emission scenario at the end of the 21st century when it could be twice as large. Antarctic amplification, on the other hand, is also steady in time and across scenarios, but has a magnitude of just greater than unity, comparable to amplification of warming over other landmasses. Additionally, inter-model uncertainty in Antarctic amplification is larger than uncertainty due to internal variability, suggesting more room for improvement in the understanding and modelling of Antarctic processes.