Global Increases in Lethal Compound Heat Stress: Hydrological Drought Hazards Under Climate Change

Previous studies seldom consider humidity when examining heat-related extremes, and none have explored the effects of humidity on concurrent extremes of high heat stress and low river streamflow. Here, we present the first global picture of projected changes in compound lethal heat stress (T-h)-drought hazards (CHD) across 11,637 catchments. Our observational datasets show that atmospheric conditions (e.g., energy and vapor flux) play an important role in constraining the heat extremes, and that T-h (32% +/- 11%) yields a higher coincidence rate of global CHD than wet-bulb temperature (28% +/- 11%), driven by lower relative humidity (RH) and thus air dryness in T-h extremes. Our large model ensemble projects a 10-fold intensification of bivariate CHD risks by 2071-2100, mainly driven by increases in heat extremes. Future declines in RH, wind, snow, and precipitation in many regions are likely to exacerbate such water and weather-related hazards (e.g., drought and CHD).

Automated summary

Previous studies have not considered the role of humidity in heat-related extremes. This study presents the first global assessment of the projected changes in lethal heat stress-drought hazards, showing that atmospheric conditions and lower relative humidity play a crucial role in these compound hazards. The findings indicate a significant increase in the risks of compound lethal heat stress-drought hazards in the future due to increases in heat extremes.