Areas of the planet that are currently home to one-third of humans will become as hot as the hottest parts of the Sahara within 50 years unless greenhouse gas emissions fall.

The predictions come from research by scientists from China, US and Europe published in the journal Proceedings of the National Academy of Sciences. The rapid heating would mean that 3.5 billion people would live outside the climate ‘niche’ in which humans have thrived for 6,000 years.

Human populations are largely concentrated in narrow climate bands, with most people living in places where the average annual temperature is about 11-15C and a smaller number of people living where the average temperature is about 20-25C. The researchers found that people, despite all innovations and migrations, have mostly lived in these climate conditions for several thousand years. “This strikingly constant climate niche likely represents fundamental constraints on what humans need to survive and thrive,” says Professor Marten Scheffer of Wageningen University, who coordinated the research with his Chinese colleague Xu Chi, of Nanjing University.

But, as temperatures are projected to increase rapidly as a result of human greenhouse gas emissions, the temperature experienced by the average person will rise by 7.5C by 2070 unless dramatic action is taken. That is more than the expected global average temperature rise of slightly over 3C because land will warm much faster than the ocean and also because population growth is biased towards already hot places.

This rapid temperature rise, combined with projected global population changes, mean about 30 per cent of the world’s projected population (3.5 billion people) will live in places with an average temperature above 29C within 50 years, if greenhouse gas emissions continue to increase. These climate conditions are currently experienced by just 0.8 per cent of the global land surface, mostly in the hottest parts of the Sahara desert, but by 2070 the conditions could spread to 19 per cent of the planet’s land area.

“The good news is that these impacts can be greatly reduced if humanity succeeds in curbing global warming,” says study co-author Tim Lenton, climate specialist and director of the Global Systems Institute at the University of Exeter. “Our computations show that each degree warming above present levels corresponds to roughly one billion people falling outside of the climate niche. It is important that we can now express the benefits of curbing greenhouse gas emissions in something more human than just monetary terms.”

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