Exceeding human heat tolerance in a warming, ageing world: a global projection modelling study

Background

In a warming world, understanding where, when, and to what extent human heat tolerance limits will be breached has become increasingly important. Previous projections applied heat limits for healthy young adults to all age groups, overlooking greater vulnerability in older ages. With global ageing, this oversight could seriously underestimate future heat risks. In this study, we aimed to examine the extent to which accounting for age-specific heat tolerance alters global projections of heat limit exceedances.

Methods

We integrated experimentally derived heat limits for young (18–39 years), middle-aged (40–59 years), and older adults (≥60 years) with climate projections and age-stratified population projections to assess where and to what extent these limits could be breached across age groups under 1–4°C of global warming above preindustrial levels. The climate projections include bias-corrected projections of dry-bulb temperature and wet-bulb temperature from 14 Coupled Model Intercomparison Project Phase 6 climate models under the shared socioeconomic pathway 5-8·5 scenario. We also highlighted countries with high exposure risk and low adaptive capacity.

Findings

More widespread and imminent breaches of heat limits were estimated among older adult populations than among other populations. Older adults faced more frequent and spatially extensive exceedances under the 1·5°C global warming scenario relative to the preindustrial period than young adults did under 4°C. Using age-specific thresholds more than doubled the global population having 180 h or more of exceedance annually as compared with previous estimates. Older adults in south Asia and the Persian Gulf could face sustained, day-and-night exposure for three consecutive months at 3°C or more warming. We also identified 13 countries under 3°C global warming above the preindustrial period where poverty rates exceed 50% and either at least 10 million older adults or at least 80% of the older adult population experiences 180 h or more of heat limit exceedance annually. India, Pakistan, Bangladesh, Myanmar, and Niger exceed both the absolute (10 million older adults) and proportional (80% of older adults) exposure thresholds.

Interpretation

Our results suggest accounting for age stratified vulnerability, intolerable heat will be experienced at much larger scales and for longer durations, affecting far more people than previously estimated. By identifying who and where people are exposed, our findings can inform targeted heat action plans and guide resource allocation and response planning.