About 30% of the world’s population is currently exposed to potentially deadly heat for 20 days per year or more, and failure to reduce greenhouse gas emissions will increase the risk substantially, reports a paper published online this week in Nature Climate Change. The study suggests it is now almost inevitable that excess heat represents an increasing threat to human life, but that this threat will be greatly aggravated if greenhouse gas emissions are not considerably reduced.

Camilo Mora and colleagues conducted a global analysis of documented lethal heat events to identify the climatic conditions associated with human death, and then quantified the current and projected instances of such deadly climatic conditions worldwide. By reviewing papers published between 1980 and 2014, the authors identified 783 cases of excess human mortality associated with heat from 164 cities in 36 countries. They then calculated a global threshold beyond which daily mean surface air temperature and relative humidity become lethal. They predict that by 2100, up to 74% of the human population could be exposed to potentially deadly heat if greenhouse gas emissions continue to grow rapidly, or, up to 48% under a scenario with drastic emissions reductions.

Although lethal heatwaves are often mentioned as a key consequence of ongoing climate change, with reports typically referencing past major events such as Chicago in 1995, Paris in 2003, or Moscow in 2010, this study suggests that lethal heat events already occur frequently and in many cities worldwide. The authors report that, although it is understood that higher latitudes will undergo more warming than tropical regions, their results suggest that tropical humid areas will be disproportionately exposed to more days with deadly heat. They conclude that the consequences of exposure to deadly heat could be further exacerbated by an ageing population (who are highly vulnerable to heat) and increasing urbanization.