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Global warming has already made parts of the world hotter than the human body can withstand, decades earlier than climate models expected.

Measurements at Jacobabad in Pakistan and Ras al Khaimah in the United Arab Emirates have both repeatedly spent at least 1 or 2 hours over a deadly threshold, an analysis of weather station data has found.

Wet bulb temperature (TW) is a measure of heat and humidity, taken from a thermometer covered in a water-soaked cloth. Beyond a TW threshold of 35°C, the body is unable to cool itself by sweating. Lower levels can also be deadly, as was seen in the 2003 European heatwave, which killed thousands of people without passing a TW of 28°C.

Tom Matthews at Loughborough University, UK, and his colleagues analysed from around the world, and found that the frequency of wet bulb temperatures exceeding a series of temperature intervals between 27°C and 35°C had all doubled since 1979.

Most frequency increases were in the Gulf, India, Pakistan, the US and Mexico. But at Jacobabad and Ras al Khaimah, a TW of 35°C appears to have been passed, the first time the breach has been reported in scientific literature.

There is a degree of uncertainty, because there could be flaws with individual weather stations, such as how they are calibrated or where they are sited, but Matthews says the overall picture is clear.

“The crossings of all of these thresholds imply greater risk to human health – we can say we are universally creeping close to this magic threshold of 35°C,” he says. “It looks like, in some cases for a brief period of the day, we have exceeded this value.”

His team corroborated the breach by looking at a separate, widely used, , which also showed evidence for it occurring.

An analysis of that data set suggested several wider areas of the Gulf, not just a few hotspots, will see the possibility of a TW of 35°C happening once every 30 years at around 2.3°C of global warming. The world has already warmed about 1°C.

Such intense humid temperatures have so far largely affected affluent Gulf states, where air conditioning is
widely available to the rich.

But Matthews warns that with continued climate change, the extremes will affect more areas in Pakistan, as well as India, which may not have the capacity to adapt. Even if they could, it would require huge amounts of energy for cooling, possibly further exacerbating climate change.

Steven Sherwood at the University of New South Wales in Australia says the study makes a convincing case that the measurements are accurate, though it isn’t guaranteed. “The implications of this study are that such extreme conditions which push the tolerance of the human body are not as far off into the future as we thought, at least in a few locations on Earth,” he says.

Clare Heaviside at University College London says the work is  broadly in line with existing research, but cautioned against the focus on the TW threshold of 35°C.

“It is difficult to link a wet bulb temperature threshold to specific health outcomes, and for different population groups,” she says.

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