Life-threatening humid heatwaves will spread rapidly as climate warms, research suggests

Life-threatening periods of high heat and humidity will spread rapidly across the world with only a small increase in global temperatures, a new study has found, potentially causing a sharp acceleration in deaths resulting from the climate crisis.

The extremes, which can be fatal to healthy people in six hours, could affect hundreds of millions unused to such conditions. As a result, heat deaths could rise quickly without urgent and serious efforts to prepare populations, the researchers said.

The human body cools itself by producing sweat, which evaporates and takes heat away. But when humidity is high, evaporation is reduced.

The new study used a limit based on experiments on people showing that when combined heat and humidity, as measured by so-called wet bulb temperature, passes 31.5C, the body is no longer able to cool itself.

The researchers called this threshold "non-compensable heat stress", as sweating cannot compensate for the extreme conditions. Without cooling aids, such as cold water, fans or air conditioning, death is likely within hours.

The research analysed data from thousands of global weather stations to show 4% had already experienced at least one six-hour period of this extreme heat stress since 1970, with the frequency of such events doubling by 2020. However, these have been confined to date to hot places including the Gulf in the Middle East, Red Sea and north Indian plain, where people expect extreme heat.

The new analysis, which also used climate models, shows that extreme heat stress will spread rapidly to other regions with global heating of 2C. The climate crisis has already raised global temperature by about 1.2C. At 2C, more than 25% of the weather stations would suffer the extreme heat stress once a decade on average.