Heat health during future summers in eastern Asia: The combined roles of circulation, temperature and humidity

Excessive relative humidity (RH) in combination with high temperature can lead to heat stress, often measured by the Wet Bulb Globe Temperature (WBGT). The Clausius-Clapeyron (CC) relationship implies that warming reduces RH if no extra moisture is added. Over coastal regions like eastern Asia, however, the predominant summer monsoon favours increased moisture transport from surrounding oceans as a result of enhanced evaporation driven by surface temperature increase. This would lessen the RH reduction by potentially two-thirds. Based on two ensembles of climate model simulations, this paper examines the competing effects of near-surface temperature, humidity and circulation patterns in this region and the consequential health risks. Under a high emissions scenario (RCP8.5/SSP5-8.5), surface temperature could increase by 4 - 7 degrees C with WBGT increases of several degrees by the end of the 21st century. Devastating extreme heat health events could therefore become a frequent occurrence as a result. Overall, our results show how humidity can be just as important as temperature when considering the risks to society of excessive heat.