Future heat-stress regimes under CMIP6: a multi-index assessment of persistence and human-relevant thermal constraints

TIffanie Lescure, Dimitri Defrance

Human exposure to heat stress is increasing under climate change as rising temperatures interact with atmospheric moisture to constrain thermoregulation and outdoor activity. While numerous heat-stress indices are used in climate impact studies, their joint interpretation in terms of climatic regimes, persistence, and physiological relevance remains fragmented. Here, we provide a global, multi-model assessment of future human-relevant heat stress using a consistent set of biometeorological indices derived from CMIP6 climate projections.

We analyse daily bias-corrected projections from the NASA NEX-GDDP-CMIP6 dataset at 0.25$^\circ$ resolution, using five CMIP6 models under SSP1--2.6, SSP2--4.5, and SSP5--8.5 scenarios. Wet-bulb temperature (Tw), wet-bulb globe temperature (WBGT), Heat Index, and Humidex are computed consistently and evaluated against ERA5 for the historical period. Heat stress is characterised through mean conditions, peak daily stress, and chronic threshold exceedance, defined as at least 30 days per year above physiologically and operationally relevant thresholds.
All indices indicate a robust intensification of heat stress under future warming, reflecting a systematic elevation of baseline thermal environments rather than isolated extremes. Under SSP5--8.5, more than five billion people are projected to experience at least one month per year with WBGT $\geq$ 32$^\circ$C by late century, while over two billion may be exposed to chronic extreme humid heat (Tw $\geq$ 35$^\circ$C). These exposures are concentrated in densely populated tropical and subtropical regions, particularly in Asia and Africa, and are substantially reduced under lower-emission pathways.

WBGT-based exceedance highlights expanding constraints on sustained outdoor activity, whereas extreme wet-bulb temperature represents a fundamental atmospheric limit to evaporative cooling. By distinguishing between mean intensification, peak stress, and persistence-based exceedance across multiple indices, this study provides a climatologically grounded framework for assessing emerging heat-stress regimes relevant to applied climatology, climate services, and long-term adaptation planning.