A billion people could be living with lethal levels of heat stress if global warming were to reach 2°C above pre-industrial levels, research has found.
Heat stress, a combination of dangerous heat and humidity, affects 68 million people globally.
Modelling from a consortium of academics and the UK Meteorological Office showed that under a 2°C scenario, the number of people living in these conditions could rise 15-fold.
Heat stress is defined as a wet bulb globe temperature – a measurement taking into account temperature, humidity, wind speed and solar radiation – of above 32°C.
It can induce heat exhaustion, with symptoms including heavy sweating and rapid pulse, which can in turn put a strain on the heart and other organs.
The elderly, those with health conditions and people with physical, outdoor jobs are at the greatest risk.
Analysis shows the Earth was on course for 2.7°C of warming under countries’ carbon emissions reduction pledges before the Cop26 climate negotiations.
US climate envoy John Kerry revealed on Thursday that if the pledges made so far during the summit are kept, it would put humanity on course for 1.8°C, the International Energy Agency has said.
The Meteorological Office warned that in a future where runaway global warming reaches 4°C, half of the world’s population would be living with heat stress.
The heat stress maps are part of research projecting the future effects of climate change under 2°C and at 4°C, and also look at river flooding, wildfire risk, drought and food insecurity.
It was conducted by an international team of scientists in the EU-funded Helix project, and led by the University of Exeter
The Meteorological Office analysed where the most severe projected effects overlap.
“Currently, the [heat stress] metric is met in several locations, such as parts of India," said Dr Andy Hartley, climate effects leader at the Meteorological Office.
"But our analysis shows that with a rise of 4°C, extreme heat risk could affect people in large swathes of most of the world’s continents.”
Prof Richard Betts, of the University of Exeter and Meteorological Office, who led the Helix project, said: “This new combined analysis shows the urgency of limiting global warming to well below 2°C.
“The higher the level of warming, the more severe and widespread the risks to people’s lives, but it is still possible to avoid these higher risks if we act now.”
Dr Andy Wiltshire, head of earth system and mitigation science at the Meteorological Office, said: “Of course, severe climate change will drive many impacts, and our maps show that some regions will be affected by multiple factors.
“Perhaps unsurprisingly, parts of the tropics are most affected with countries like Brazil and Ethiopia potentially facing impacts from four of the hazards.
“Rapid emission reductions are required if we are to avoid worst consequences of unmitigated climate change.”