Global future heat stress projections: Regional variations of Humidex changes from high-resolution CMIP6 models

Abstract

This study investigates future projections of the extreme Humidex, a compound index integrating air temperature and relative humidity, as an indicator of heat stress under two Shared Socioeconomic Pathways (SSP1–2.6 and SSP5–8.5). Using multi-model ensemble simulations from 15 high-resolution CMIP6 models, we examine seasonal variations in the extreme Humidex at both global and regional scales across 21 land regions. The historical simulations from CMIP6 models were validated using ERA5 observational data, ensuring reliability and accuracy in model projections. Additionally, the climatology of Humidex values derived from MME simulations was compared with the climatology of the Heat Index obtained from MME simulations, revealing consistency between the two heat stress indicators. To quantify heat-related hazards, we calculate the number of seasonal Dangerous Humidex Days (DHDs). Furthermore, we assess the relative contributions of air temperature and relative humidity to projected changes in the extreme Humidex. The results reveal a marked intensification of heat stress under SSP5–8.5, with late-century (2080–2100) increases in the extreme Humidex reaching approximately 5 to 9 °C across most land regions, compared to around 1.5 to 3.5 °C under SSP1–2.6. The JJA season exhibits the greatest intensification, particularly over Eastern North America and Northern Asia. On average, DJF shows a 2.78-fold increase in Humidex anomalies, while JJA demonstrates an even larger amplification of approximately 2.90-fold when transitioning from a low to high-emission scenario. The number of seasonal Dangerous Humidex Days also increases significantly, with an additional 60 to 80 days during DJF across many tropical and subtropical regions of the Southern Hemisphere, and more than 80 additional days per JJA season in most tropical regions under the high-emission scenario. Sensitivity analyses indicate that air temperature is the dominant driver of future changes in the extreme Humidex, while relative humidity exerts a secondary but regionally important influence.