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In one manifestation of a warming planet billions of people could soon be exposed to such high levels of heat that spending longer periods outdoors during sweltering summer months could prove fatal.

Parts of the planet, such as the Sahara desert, are already largely unsuitable for people other than the hardiest souls. Yet other regions with currently temperate climates could also become too hot for much of the year. Once levels of heat stress rise to a certain threshold in these areas millions of people could suffer serious health effects, experts warn.

Across much of the planet, in other words, a warmer climate “will pose greater risk to human health,” says Tom Matthews, a climate scientist at Loughborough University in the United Kingdom. “[W]e can say we are universally creeping close to this magic threshold of 35°C,” he elucidates. “It looks like, in some cases for a brief period of the day, we have exceeded this value.”

Matthews has reached this conclusion after analyzing weather station data from around the world with his colleagues. What they have found is a marked increase in the recurrence of so-called wet bulb (WB) temperatures (which is a measurement of heat and humidity taken together) that exceed limits we can still safely handle.

Beyond a WB threshold of 35°C, our bodies can no longer cool themselves by sweating in humid weather. Just think of staying in a sauna for too long. As a result, we are likely to experience heat stroke and organ failure because the core temperature of out bodies remains too high. Yet since 1979 the frequency of dangerous heatwaves have doubled in countries such as India, Pakistan, parts of the United States and Mexico.

Even across much of Europe last summer thousands of people succumbed to extreme heat during long spells of unusual heat. Not only will such deadly heatwaves become more common but they will also continue to impact more and more people across an ever larger area, scientists say.

According to a new study, in just a couple of generations (in half a century) up to 3.5 billion people could find themselves living in areas that are too hot for humans throughout much of the year.

At present the planet seems to be on track for a warming of 3°C on average by the end of the century, which will make much of the planet uninhabitable for humans. Because land areas are warming faster than the oceans, temperatures in certain parts of the world could rise by as much as 7.5°C by 2070. The most-affected regions will include Sub-Saharan Africa, South America, Southeast Asia, Arabian Peninsula, the Indian subcontinent and Australia.

Prolonged droughts and other weather extremes could make things even worse for people living in these areas. “Absent climate mitigation or migration, a substantial part of humanity will be exposed to mean annual temperatures warmer than nearly anywhere today,” the scientists behind the study warn.