Humanity's Future Capacity for Physical Activity Limited by Climate Change and Population Aging

<title>Abstract</title> <p>Sustained physical activity is necessary for society, but will become harder to safely accomplish. Using a physiology-based biophysical model of human heat stress risk with bias-corrected climate model projections and demographic models, we show that humanity’s capacity for sustained physical activity while keeping core body temperature stable (“thermal compensability”) decreases with climate change and population aging over this century. At a 2°C global warming level, for adults in tropical regions, a physical activity intensity of household chores (3.3 METs) will be uncompensable 2.87% (2.25-3.98%) of the year; harvesting crops (4.8 METs) will be uncompensable for 37.69% (34.82-41.71%) of the year. Effects are worse for older adults and lessened at higher latitudes. Population aging drives significant global increases in heat risk; individual countries' primary heat risk drivers vary between climate and demographic changes. Our results underscore the urgent need for adaptation efforts that simultaneously address climate, physiology, and demographics.</p>