Scientists predict that two billion people (22% of the projected population by the end of the century) will be exposed to dangerous heat conditions if we stay on track to reach 2.7°C of global warming.
New findings highlighting the human cost of climate inaction, led by researchers from the Global Systems Institute, University of Exeter and Nanjing University, show how the narrow subset of the Earth's habitable climate (the 'climate niche') is rapidly shrinking, putting millions at risk in the future - especially those living in the lowest emitting areas today.
The researchers reveal that countries across Africa will be particularly affected, and at 2.7°C of warming:
"Nigeria would have the second largest heat-exposed population at 2.7°C of warming, more than 300 million people, almost 100% of Burkina Faso and Mali will be dangerously hot for humans," they note, adding that if warming is successfully limited to 1.5°C, one-sixth of humanity would be saved from dangerous heat compared to 2.7°C. They therefore recall the urgent need for decisive action to rapidly reduce carbon emissions.
"Under current climate policies, more than a fifth of humanity will be exposed to dangerously high temperatures by 2100. Despite the Paris Agreement's commitment to keep global warming well below 2°C (compared to pre-industrial levels), current policies are projected to lead to 2.7°C of warming by the end of the century. "
The experts also revealed that around 60 million people are already exposed to dangerous heat (average temperature of 29°C or higher).
"Limiting warming to 1.5°C would leave 5% exposed - saving one-sixth of humanity from dangerous heat compared to 2.7°C of warming.
The lifetime emissions of 3.5 average global citizens today - or just 1.2 US citizens - expose one future person to dangerous heat," they say, explaining that "this highlights the injustice of the climate crisis, as these future heat-exposed people will live in places where emissions today are about half the global average".
According to Professor Chi Xu of Nanjing University, most of these people lived near the cooler 13°C peak of the niche and are now in the "middle ground" between the two peaks.
"While not dangerously hot, these conditions tend to be much drier and have not historically supported dense human populations."
Meanwhile, the vast majority of people who will be left outside this niche as a result of future warming will be exposed to dangerous heat.
"Such high temperatures have been linked to problems such as increased mortality, reduced labour productivity, reduced cognitive performance, impaired learning, adverse pregnancy outcomes, reduced crop yields, increased conflict and the spread of infectious diseases," he adds.
The scientists also found that exposure to dangerous heat begins to rise dramatically at 1.2°C (just above current global warming), increasing by about 140 million people for every 0.1°C of further warming.
"Assuming a future population of 9.5 billion people, India would have the largest exposed population at 2.7°C of global warming - more than 600 million. At 1.5°C, the number would be much lower, at about 90 million.
Nigeria would have the second largest heat-exposed population at 2.7°C global warming, with more than 300 million people. At 1.5°C, it would be less than 40 million," they note, adding that India and Nigeria are already "hotspots" with dangerous temperatures.
"At 2.7°C, almost 100% of some countries, including Burkina Faso and Mali, will be dangerously hot for humans. Brazil would have the largest area exposed to dangerous heat, although at 1.5°C almost no area would be exposed, while Australia and India would also see massive increases in exposed area."In the "worst-case scenarios" of 3.6°C or even 4.4°C global warming, half the world's population could be left outside the climate niche, posing what the researchers call an "existential risk".
Professor Tim Lenton, director of the Global Systems Institute at the University of Exeter, said the costs of global warming are often expressed in financial terms, but their study highlights the phenomenal human cost of failing to tackle the climate emergency.
"For every 0.1°C of warming above current levels, around 140 million more people will be exposed to dangerous heat.
This shows both the scale of the problem and the importance of decisive action to reduce carbon emissions," the professor said, adding that limiting global warming to 1.5°C instead of 2.7°C would mean five times fewer people exposed to dangerous heat in 2100.
Human population density has historically peaked in places with an average temperature of about 13°C, with a secondary peak at about 27°C (monsoon climates, especially in South Asia), the scientists said.
"Crop and livestock densities follow similar patterns, and wealth (as measured by GDP) peaks at about 13°C.
Mortality increases at both higher and lower temperatures, supporting the idea of a human "niche".
Although less than 1% of humanity currently lives in dangerously hot places, the study shows that climate change has already pushed 9% of the world's population (more than 600 million people) outside the niche.