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This story was originally published by The Guardian and appears here as part of the Climate Desk collaboration.

Global heating will drive billions of people out of the “climate niche” in which humanity has flourished for millennia, a study has estimated, exposing them to unprecedented temperatures and extreme weather.

The world is on track for 2.7 C of heating with current action plans and this would mean two billion people experiencing average annual temperatures above 29 C by 2030, a level at which very few communities have lived in the past.

Up to one billion people could choose to migrate to cooler places, the scientists said, although those areas remaining within the climate niche would still experience more frequent heat waves and droughts.

However, urgent action to lower carbon emissions and keep global temperature rise to 1.5 C would cut the number of people pushed outside the climate niche by 80 per cent, to 400 million.

The analysis is the first of its kind and is able to treat every citizen equally, unlike previous economic assessments of the damage of the climate crisis, which have been skewed toward the rich.

In countries with large populations and already warm climates, most people will be pushed outside the human climate niche, with India and Nigeria facing the worst changes. India is already suffering from extreme heat waves and a recent study found that more than a third of heat-related deaths in summer from 1991 to 2018 occurred as a direct result of human-caused global heating.

Prof. Tim Lenton at the University of Exeter, U.K., who led the new research, said: “The costs of global warming are often expressed in financial terms but our study highlights the phenomenal human cost of failing to tackle the climate emergency.

“Economic estimates almost always value the rich more than the poor because they have more assets to lose, and they tend to value those alive now over those living in the future. We’re considering all people as equal in this study.”

Global heating will push billions outside ‘human climate niche’. #ClimateCrisis #ExtremeWeather #ClimateEmergency #GlobalHeating

Prof. Chi Xu at Nanjing University in China, and also part of the research team, said: “Such high temperatures [outside the niche] have been linked to issues including increased mortality, decreased labour productivity, decreased cognitive performance, impaired learning, adverse pregnancy outcomes, decreased crop yield, increased conflict and infectious disease spread.”

Prof. Marten Scheffer at Wageningen University, the Netherlands, and a senior author of the study, said those pushed outside the climate niche might consider migrating to cooler places: “Not just migration of tens of millions of people but it might be a billion or so.”

The idea of climate niches for wild animals and plants is well established but the new study, published in the journal Nature Sustainability, identified the climate conditions in which human societies have thrived.

It found most people lived in places with mean annual temperatures spread around 13 C or 25 C. Conditions outside those are too hot, too cold or too dry and associated with higher death rates, lower food production and lower economic growth.

“The climate niche describes where people flourish and have flourished for centuries, if not millennia, in the past,” Lenton said. “When people are outside [the niche], they don’t flourish.”

Scheffer said: “We were surprised how sharply limited humans have remained when it comes to their distribution relative to climate. This is a fundamental thing we’ve put our finger on.”

The scientists then used climate and population models to examine likely future changes in the number of people outside the climate niche, which they defined as above an annual average temperature of 29 C.

There are 60 million people living outside the niche and exposed to dangerous heat, the researchers said. But with each rise of 0.1 C in global temperature above the 1.2 C of human-caused global heating already seen today, an extra 140 million people are driven outside the niche.

If global temperature continues to rise towards 2.7 C, the heating combined with a growing global population will mean two billion people living outside the niche by 2030 and 3.7 billion by 2090.

In a worst-case scenario, with the climate more sensitive to greenhouse gas rises than expected, a global temperature would rise of 3.6 C and leave almost half of the world’s population outside the climate niche.

Rapid and deep cuts to emissions to keep heating to 1.5 C would hugely reduce the number of people outside the niche, the researchers found. For example, 90 million people in India would live with an average temperature above 29 C, compared with 600 million if the global temperature rose to 2.7 C. Other countries that would be severely affected by high temperatures include Indonesia, the Philippines and Pakistan.

Lenton said the study emphasized the “huge inequality” of the climate emergency with those people with low emissions suffering the greatest changes in extreme heat exposure.

He said the most practical and immediate option to adapt to high temperatures was to increase green spaces in cities: “This can shave 5 C off extreme temperatures and provide shade — that’s huge.”

Dr. Richard Klein at the Stockholm Environment Institute in Sweden, and not part of the team, said: “What this study shows very well is the direct human suffering that climate change could cause — living outside the niche means suffering due to an unbearably hot and possibly humid climate.”

Dr. Laurence Wainwright at the University of Oxford, U.K., said: “Humans have got used to living in particular areas at certain temperatures. When things change, serious problems arise, whether in terms of physical health, mental health, crime and social unrest.”

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