As much as one-third of the world's population will be exposed to Sahara Desert-like heat within half a century if greenhouse gas emissions continue to rise at the pace of recent years. Scientists from China, the US and Europe found that the narrow climate niche that has supported human society would shift more over the next 50 years than it had in the preceding 6000 years.

As many as 3.5 billion people will be exposed to near-unliveable temperatures averaging 29 degrees through the year by 2070. Less than 1 per cent of the Earth's surface now endures such heat.

That heat compares with the narrow 11- to 15-degree range that has supported civilisation over the past six millennia, according to research published Tuesday in the journal Proceedings of the National Academy of Sciences.

Absent climate mitigation or migration, a substantial part of humanity will be exposed to mean annual temperatures warmer than nearly anywhere today, the paper said.
Xu Chi, a researcher at China's Nanjing University and one of the paper's authors, said:
We were frankly blown away by our own initial results. As our findings were so striking, we took an extra year to carefully check all assumptions and computations.

Clearly we will need a global approach to safeguard our children against the potentially enormous social tensions the projected change could invoke.

Among the most exposed nations will be India - where many people live in already-hot places - with as many as 1.2 billion people likely to be forced to move if population and warming trends continue. For Nigeria, the number exposed could be 485 million, according to a media release distributed along with the paper.

The scenario used projected the total populations in India and Nigeria to reach 2.2 billion and 600 million, respectively, by 2070, Dr Xu told the Herald and The Age.

In Australia, areas of Western Australia and the Northern Territory home to about 200,000 people will be at risk.

The research extended current population and greenhouse gas emissions trends into the future, and excluded impacts from the coronavirus pandemic on both.

The researchers also considered possible rainfall changed. The global pattern of population distribution seems less constrained by precipitation - while there is also an optimum around 1000 mm [of rainfall a year ] - so we focused on temperature, Dr Xu said. Changes of precipitation regime would definitely have impacts, but such impacts together those of temperature change would be more complex to foresee.

Compared with pre-industrial-era conditions, temperatures globally will be about 3 degrees hotter by 2070. But as land warms faster than the oceans, the rise for people on average will be about 7.5 degrees, the paper found.

Should the world adopt strong emissions reductions - the so-called Representative Concentration Pathway 2.6 - it would substantially reduce the geographical shift in the niche of humans and would reduce the theoretically needed movement to about 1.5 billion people, the paper said.

Still, that number would account for about one in seven of the world's population.

Each degree of temperature rise above the current baseline roughly corresponds to 1 billion humans left outside the temperature niche, absent migration, it said.

The researchers added that upheavals among populations - and the ecosystems that support them - could happen well before 2070. Migration inevitably causes tension, even now, when a relatively modest number of about 250 million people live outside their countries of birth.

Tim Lenton, director of the Global Systems Institute at the University of Exeter and one of the paper's authors, said: The good news is that these impacts can be greatly reduced if humanity succeeds in curbing global warming. 

Marten Scheffer, a professor at Wageningen University and an author of the report, said the response to the coronavirus should give cause for some optimism that climate change's looming threats could also be tackled.

The COVID-19 response revealed that if a problem appears to be urgent and serious, humanity globally is able to act massively if needed, even if there are economic costs, he told the Herald and The Age. Perhaps this may serve to make it feel more doable to address global warming too. Our findings indicate that that would be worthwhile.