Robust Critical Temperature Projections Reveal Diminishing Heat Safety Margins Under Global Warming

Uncompensable heat stress (UHS) occurs when human thermoregulation fails to maintain a stable core temperature, posing severe health risks. While previous studies emphasize humidity’s role in future heat stress, we introduce Critical Temperature Margin (CTM) - the buffer between ambient temperature and critical temperature causing UHS - to quantify heat safety under climate change. The CTM is based on human energy balance equations considering a variety of climate factors and key human physiological limits.Using ERA5 data (1980-2023) and CMIP6 projections, we quantify future trends in CTM under different radiation environments and classify heat stress by dominant mechanisms: dry heat (sensible heat and radiative components) and moist heat (evaporative component). Our analysis reveals that future UHS changes are primarily driven by increasing dry heat contribution associated with rising temperature and longwave radiation, while humidity effects of rising saturation vapor pressure and declining relative humidity during extreme events largely cancel out. Our key findings show: (1) CTM diminishes by -2.4°C per degree of global warming (-0.8°C locally) on average; (2) increasing prevalence of dry heat dominated regions over past 40 years; (3) outdoor conditions experience more dry heat stress than indoors; (4) sweat evaporation constraints on moist heat stress remain nearly constant, contradicting assumptions that humidity dominates future heat stress.We also find that human settlements historically avoided thermally challenging regions. However, diminishing CTM threatens millions of world populations, particularly outdoor workers in tropical and subtropical areas who will face similar evaporative cooling capacity but increasing radiative and dry heat loads. Typical moist-heat dominated regions such as India, Sahel, Southeast U.S., East China, Northern Australia will be subject to more than 50% dry heat in outdoor conditions.These findings clarify the ongoing humidity debate in heat-health research. Our results support targeted mitigation strategies: increased ventilation for moist heat, shade for radiation, and active cooling for dry heat. Our robust projection of the CTM provides critical insights for public health adaptation planning as thermal safety margins continue shrinking globally.