Substantial Increase in the Joint Occurrence and Human Exposure of Heatwave and High-PM Hazards Over South Asia in the Mid-21st Century

Extreme heat occurrence worldwide has increased in the past decades. Greenhouse gas emissions, if not abated aggressively, will lead to large increases in frequency and intensity of heat extremes. At the same time, many cities are facing severe air pollution problems featuring high-PM episodes that last from days to weeks. Based on a high-resolution decadal-long model simulation using a state-of-the-science regional chemistry-climate model that is bias corrected against reanalysis, here we show that when daily average wet-bulb temperature of 25 degrees C is taken as the threshold for severe health impacts, heat extremes frequency averaged over South Asia increases from 45 5 days/year in 1997-2004 to 78 3 days/year in 2046-2054 under RCP8.5 scenario. With daily averaged PM2.5 surface concentration of 60 mu g/m(3) defined as the threshold for such "unhealthy" extremes, high-PM extremes would occur 132 8 days/year in the Decade 2050 under RCP8.5. Even more concerning, due to the potential health impacts of two stressors acting in tandem, is the joint occurrence of the heatwave and high-PM hazard (HHH), which would have substantial increases of 175% in frequency and 79% in duration. This is in contrast to the 73-76% increase for heatwave or high PM when assessed individually. The fraction of land exposed to prolonged HHH increases by more than tenfold in 2050. The alarming increases in just a few decades pose great challenges to adaptation and call for more aggressive mitigation. For example, under a lower emission pathway, the frequency of HHH will only increase by 58% with a lower frequency of high-PM extremes. Plain Language Summary Extreme heat occurrence worldwide has increased in the past decades. At the same time, many cities are facing severe air pollution problems featuring high-PM episodes (high concentration of particulate matter due to various sources) that last from days to weeks. We present an integrated assessment of human exposure to the joint occurrence of the heatwave and high-PM extremes, and possible future changes have been missing. In addition to the expected elevation in the heatwave and high-PM-related extremes, we also show that the rare jointed events would have quite large increases in the future with a 175% increase in frequency. The fraction of land exposed to prolonged HHH would increase by more than tenfold in 2050. The alarming rate of increases in just a few decades pose great challenges to adaptation. Key Points A regional-scale assessment for the present-day heatwave and high-PM occurrence and future changes is presented The rare heatwave and high-PM hazards (HHH) would have large increases, in contrast to smaller increases in heatwave or high PM individually The alarming increase rate in the next few decades poses great challenges to adaptation, calling for a holistic view of the health impacts