A billion people will be affected by extreme heat stress if the climate crisis raises the global temperature by just 2C, according to research released by the UK Met Office at the Cop26 climate summit. The scientists said that would be a 15-fold increase on the numbers exposed today.

The key goal of Cop26 is to keep the chance of limiting global heating to 1.5C alive but delegates said there is much work to do to achieve this in the summit’s final week.

The Met Office assessed wet-bulb temperature, which combines both heat and humidity. Once this measure reaches 35C, the human body cannot cool itself by sweating and even healthy people sitting in the shade will die within six hours. The Met Office analysis used a wet-bulb temperature limit of 32C, at which workers must rest regularly to avoid heat exhaustion, for at least 10 days a year.

If efforts to end the climate emergency fail and temperatures rise by 4C, half of the world’s population will suffer from this extreme heat stress.

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Heat is the most obvious impact of global heating and extreme heat in cities across the world has tripled in recent decades, according to a recent study. In the summer of 2020, more than a quarter of the US population suffered from the effects of extreme heat, with symptoms including nausea and cramps.

At least 166,000 people died due to heatwaves around the globe in the two decades to 2017, according to the World Health Organization. The UK government has been repeatedly warned by its official climate advisers that the country is “woefully unprepared” for increased heat, particularly in vulnerable locations such as hospitals and schools.

The Met Office analysis is derived from research from the EU-funded Helix project, which also maps the rising risks of river flooding, wildfires, drought and food insecurity. Virtually the entire inhabited world is affected by at least one impact.

Andy Wiltshire, at the Met Office, said: “Any one of the climate impacts presents a scary vision of the future. But, of course, severe climate change will drive many impacts, and our maps show that some regions will be affected by multiple factors.”

Tropical countries including Brazil, Ethiopia and India are hardest hit by extreme heat stress, with some parts being pushed towards the limit of human liveability. But Prof Albert Klein Tank, director of the Met Office Hadley Centre, said: “These maps reveal areas of the world where the gravest impacts are projected to occur. However, all regions of the world – including the UK and Europe – are expected to suffer continued impacts from climate change.”

Scientists have been warning about deadly levels of heat and humidity for some years. A 2015 study showing the Gulf in the Middle East, the heartland of the global oil industry, set to suffer heatwaves beyond the limit of human survival if climate change is unchecked.

The deadliest place on the planet for extreme future heatwaves will be the north China plain, one of the most densely populated regions in the world and the most important food-producing area in the huge nation, according to 2018 research.