Enhanced early 21st century Arctic sea ice loss due to CMIP6 biomass burning emissions

The use of the Global Fire Emissions Database (GFED) from 1997-2014 to create the CMIP6 historical biomass burning (BB) forcing allows for a more accurate representation of BB emissions in climate models, but also results in an unrealistic increase in their inter-annual variability compared to pre- and post-GFED years, especially in the Northern Hemisphere mid-latitudes. We find that this new BB forcing affects the simulated Arctic sea ice loss in several CMIP6 models, bringing them into better agreement with the observed sea ice decline by leading to enhanced sea ice loss in the early 21^st century. This suggests that BB emissions may have played a role in the acceleration of the observed early 21^st century Arctic sea ice loss.

Using the Community Earth System Model version 2 (CESM2), we conduct sensitivity experiments in which we use BB emissions with a fixed annual cycle over the GFED period, to remove the inter-annual variability between 40-70°N. These experiments show that the strong acceleration in sea ice decline since the late 1990s simulated by the CESM2 is caused by enhanced Arctic warming driven by the increased variability in BB emissions over the GFED period. We also find that about half of the increase in sea ice sensitivity to CO₂ and global mean surface temperature in the CESM2 compared to its CMIP5 counterpart, the CESM1, can be attributed to the change in BB emissions from CMIP5 to CMIP6, which suggests that the previously found improvement in sea ice sensitivity in CMIP6 models may in part be due to this new BB forcing and not only to changes in model physics. Overall, the results from this analysis highlight the influence of mid-latitude BB emissions on Arctic sea ice and provide new insights into the potential of a forced contribution to the observed accelerated early 21^st century Arctic sea ice loss. Furthermore, this work highlights the importance of avoiding temporal discontinuities in prescribed aerosol forcing datasets as well as the need to better understand inter-model contrasts within the CMIP6 archive related to sensitivity to BB emissions.