Reductions in labour capacity from heat stress under climate warming

A fundamental aspect of greenhouse-gas-induced warming is a global-scale increase in absolute humidity(1,2). Under continued warming, this response has been shown to pose increasingly severe limitations on human activity in tropical and mid-latitudes during peak months of heat stress(3). One heat-stress metric with broad occupational health applications(4-6) is wet-bulb globe temperature. We combine wet-bulb globe temperatures from global climate historical reanalysis(7) and Earth System Model (ESM2M) projections(8-10) with industrial(4) and military(5) guidelines for an acclimated individual's occupational capacity to safely perform sustained labour under environmental heat stress (labour capacity) here defined as a global population-weighted metric temporally fixed at the 2010 distribution. We estimate that environmental heat stress has reduced labour capacity to 90% in peak months over the past few decades. ESM2M projects labour capacity reduction to 80% in peak months by 2050. Under the highest scenario considered (Representative Concentration Pathway 8.5), ESM2M projects labour capacity reduction to less than 40% by 2200 in peak months, with most tropical and mid-latitudes experiencing extreme climatological heat stress. Uncertainties and caveats associated with these projections include climate sensitivity, climate warming patterns, CO2 emissions, future population distributions, and technological and societal change.