The heat in some parts of the world “could exceed the physiological tolerance of humans” by the end of this century, according to new research.

Even if global warming was restricted to 2 degrees Celsius, cities such as Karachi in Pakistan and Kolkata in India could experience deadly heatwaves like the one which killed more than 3,400 people in 2015 on an annual basis, scientists said in an academic paper.

‘Heat stress’ – the effect on the human body – rises at a faster rate than temperature, partly because warmer air can hold more moisture. Humid air absorbs more liquid at a slower rate so human’s main method of cooling down, sweating, is less effective.

The Paris Agreement on climate change committed the world to restricting global warming to 2C above pre-industrial levels, but it also urged countries to try to keep it to as close to 1.5C as possible.

However, in a paper in the journal Proceedings of the National Academy of Sciences, academics from the UK and Ireland wrote that the world would experience deadly heatwaves much more regularly even if these targets were met.

“During 2015, annual maxima for heat index were well above average across South Asia and around the Persian Gulf, with extreme values above 60C gaining widespread media attention,” they wrote.

“Some heat-prone megacity regions, such as Karachi and Kolkata, recorded their highest heat index values in at least 36 years.

“The extraordinary heat had deadly consequences, with over 3,400 fatalities reported across India and Pakistan alone.

“We show that, even in a climate held to 2C above pre-industrial, Karachi and Kolkata could expect conditions equivalent to their deadly 2015 heatwaves every year.”

Even the lower target would change the weather dramatically for the worse.

“With only 1.5C of global warming, twice as many megacities (such as Lagos, Nigeria, and Shanghai, China) could become heat stressed, exposing more than 350 million more people to deadly heat by 2050,” the paper said.

“The results underscore that, even if the Paris targets are realized, there could still be a significant adaptation imperative for vulnerable urban populations.”

Last year, planet Earth’s hottest on record, was 1.1C warmer than the average between 1850 and 1900, according to the UK Met Office, although it added the natural El Niño weather effect had played a role, contributing about 0.2C of warming.

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The researchers, from Liverpool John Moores University, Loughborough University, and Maynooth University in Kildare, said warming of 1.5C or 2C had been described in a way “modest enough for the urgency of the situation to be lost on non-experts”.

“Such interpretation may downplay the risk of climate change, which in turn, could make individuals less willing to take action to reduce climate change,” they wrote.

Some places currently inhabited by humans could become essentially uninhabitable within a lifetime.