Abstract
Heat stress is one of the leading natural causes of mortality in India. Aerosols can potentially impact heat stress by modulating the meteorological conditions via radiative feedback. However, a quantitative understanding of such impact is lacking. Here using a chemical transport model WRF-Chem, we showed that high aerosol loading in India was able to mask the heat stress (quantified by the Wet Bulb Globe Temperature, WBGT) by 0.3-1.5C in 2010 with a regional heterogeneity across the major climate zones in India. However, the cooling effect of aerosol direct radiative forcing is partially compensated by an increase in humidity. To understand the potential impact of air quality improvement (i.e., reducing aerosol load) on heat stress in the future, WBGT was projected for 2030 under two contrasting aerosol emission pathways. We found that the heat stress would increase by >0.75C in all the climate zones in India except in the montane zone under the RCP4.5 scenario with a bigger margin of increase in the mitigation emission pathway relative to the baseline emission pathway. On the contrary, under the RCP8.5 scenario, the heat stress is projected to increase in limited regions such as the tropical wet and dry, north-eastern part of the humid subtropical, tropical wet, and semi-arid climate zone in peninsular India. Our results demonstrate that aerosols modulate heat stress, and therefore, the heat stress projections in India and anywhere else with high aerosol loading should consider aerosol radiative feedback.