The importance of aerosol scenarios in projections of future heat extremes

Global climate models project a large increase in the frequency and intensity of heat extremes (HEs) during the 21st century under the Representative Pathway Concentration (RCP8.5) scenario. To assess the relative sensitivity of future HEs to the level of greenhouse gas (GHG) increases and aerosol emission decreases, we contrast Community Earth System Model (CESM)'s Large Ensemble projection under RCP8.5 with two additional ensembles: one keeping aerosol emissions at 2005 levels (but allowing all other forcings to progress as in RCP8.5) and the other using the RCP4.5 with lower GHG levels. By the late 21st century (2060-2080), the 3 A degrees C warmer-than-present-day climate simulated under RCP8.5 could be 0.6 A degrees C cooler (0.9 A degrees C over land) if the aerosol emissions in RCP8.5 were not reduced, compared with a 1.2 A degrees C cooling due to GHG mitigation (switching from RCP8.5 to RCP4.5). Aerosol induced cooling and associated HE reductions are relatively stronger in the Northern Hemisphere (NH), as opposed to GHG mitigation induced cooling. When normalized by the global mean temperature change in these two cases, aerosols have a greater effect than GHGs on all HE statistics over NH extra-tropical land areas. Aerosols are more capable of changing HE duration than GHGs in the tropics, explained by stronger dynamical changes in atmospheric circulation, despite weaker thermodynamic changes. Our results highlight the importance of aerosol scenario assumptions in projecting future HEs at regional scales.