The Times reported that scientists from the University of Exeter in the UK, as well as teams from Europe, China and the US, found that one-fifth of people could live in regions above the upper limit for safe temperatures by 2100.

Countries including Indonesia and Pakistan could face periods of mass emigration as a result of climbing temperatures, the scientists said.

Using an annual average temperature of 29 degrees Celsius as the upper limit of tolerable conditions, which 0.9 percent of people contend with today, the teams found that the figure could reach 23 percent of the world’s population by 2100.

Tim Lenton from the University of Exeter said: “It’s not just the mean annual temperature. It’s what are going to be those hottest days of the year that might reach a physiologically dangerous level for human life?”

He added: “The threshold for which people and animals keel over and die is about 35 degrees Celsius. Above 28 degrees Celsius it starts to become physiologically damaging.”

Marten Scheffer of the Netherlands’ Wageningen University said: “It will be a very natural adaptation to … consider migration, and not just migration of just tens of millions of people, but a billion or so.

“It’s a human cost. It will be a cost for everyone because it has to be accommodated in some way.”

But if the Paris Agreement on climate change — which sets a 1.5 degrees Celsius limit on warming — could be met, it will result in five times fewer people living with extreme heat by 2100 than the projected model shows, Lenton said.

Since 1980, the percentage of the global population living with annual average temperatures above 29 degrees Celsius has grown from 0.3 percent to 0.9 percent.