Climate warming over the past few decades has led to an increase in humidity, and work published in Nature Climate Change suggests that this has reduced people’s working capacity to 90% in peak months. The report predicts that this reduction in working capacity will be greater under future warming scenarios, with most tropical and mid-latitude regions expected to experience extreme heat stress, as well as humidity, by 2200.

An important aspect of greenhouse-gas-induced warming is a global increase in absolute humidity. This increase in humidity is thought to limit human activity in tropical and mid-latitudes during peak months of heat stress.

Most research related to moist temperature to date has focused on effects associated with heat waves, rather than sustained temperature increases. John Dunne and colleagues combine historical analysis of wet bulb temperature - which is largely determined by the amount of moisture in the air - and model projections with industrial and military guidelines for an individual’s capacity to safely perform sustained labour under environmental heat stress. They estimate that environmental heat stress has reduced labour capacity to 90% in peak months over the past few decades and project that there will be a further reduction to 80% in peak months by 2050. Under the highest CO2 scenario considered by the model, labour capacity would be reduced to less than 40% by 2200 in peak months. They report that this extreme scenario would expose most of the present tropical and mid-latitude populations to extreme heat stress in peak months, prohibit any safe labour in large areas and expose mid-latitude regions, such as the US east of the Rockies, to a level of environmental heat stress that is currently only experienced in the hottest regions of the world.

The authors emphasize, however, that these projections don’t include information about climate sensitivity, climate warming patterns, CO2 emissions, future population distributions and technological and societal change.