Arctic Ocean Becomes Ice Free Before 2050 in New, International Climate Simulations
We analyze the simulations of Arctic sea-ice area and volume from the latest generation of global climate models (CMIP6) and find that the observed evolution of Arctic sea-ice area lies within the spread of model simulations. In particular, the latest generation of models does a better job than earlier models at simulating the sea-ice loss for a given amount of CO2 emissions and for a given amount of global warming. In most simulations, the Arctic Ocean becomes practically sea-ice free in September at future anthropogenic CO2 emissions of less than 1000 Gt CO2, before the year 2050.
In recent decades, Arctic sea-ice area has decreased rapidly, and the signal of a forced sea-ice retreat has clearly emerged from the background noise of year-to-year variability. Because of this, the ability of climate models to plausibly simulate the observed changes in Arctic sea-ice coverage has become a central measure of model performance in climate model intercomparison projects. This study provides an initial overview of some large-scale metrics of model performance and the first analysis of future sea-ice evolution as indicated by CMIP6 simulations.
Most CMIP6 models have difficulties capturing the observed mean values of Arctic sea-ice area and volume, even accounting for observational uncertainty and internal variability. However, the CMIP6 ensemble mean provides a more realistic estimate of Arctic sea-ice area and its sensitivity in September to anthropogenic CO2 emissions and global warming, than in CMIP3 and CMIP5, although almost all CMIP6 models fail to simulate both metrics realistically at the same time. It is unclear to what degree these improvements are caused by a change in the forcing versus improvement of model physics. The relative role of internal variability increases, reducing scenario dependence until 2050. Based on the simulations that best capture the past evolution of Arctic sea ice, the Arctic Ocean in September becomes largely sea-ice free (with less than 1 million km2 of ice cover) before the year 2050 in each of the four Shared Socioeconomic Pathway (SSP) scenarios examined.