In June of 2015, the death toll from a heatwave in Iran topped 3,500 people. In Pakistan, the same heatwave caused the deaths of over 1,200 people. Over 2,500 died during the same period in India.

This summer, hundreds have died already in record-breaking heat waves in Asia. Global warming threatens virtually every corner of the earth, but perhaps no area faces as bleak a forecast as South Asia, which is home to one-fifth of the world’s inhabitants, over a billion people. This massive population is now faced with a future imperiled by deadly heat and humidity as a result of global warming, according to a new study by Professor Elfatih Eltahir at Massachusetts Institute of Technology (MIT).

The worst-case scenario, where no emissions reductions occur, projects that the largest part of India, Pakistan, and Bangladesh will push the limits of human survivability by the year 2100. The report further estimates that the percentage of the population exposed to threatening humid heat waves could be as high as 30%.

Professor Eltahir emphasizes that the problems of global warming will hit hardest in an area that is already overpopulated and heavily dependent on agriculture, both factors that make it particularly vulnerable to unendurable heat and humidity.

The relationship between heat and humidity can create situations of greater danger than the same amount of heat without humidity.

Official weather stations generally measure temperature with two thermometers: the “dry bulb” instrument records the temperature of the air, and the “wet bulb” instrument measures heat taking into account the relative humidity in the air. Professor Eltahir’s study looked at the deadly impact on human survivability of extraordinarily high wet bulb temperatures.

In terms of human habitability, this wet bulb reading is the “survivability” rating.

The normal temperature of our skin is usually 35°C. If wet bulb temperatures reach 35°C – or higher – we lose our ability to cool ourselves by sweating, and even a young, healthy person would die in a matter of hours. But even a lower wet bulb temperature, for example of 31°C, is extremely dangerous for most people, and represents what used to be the upper limit of wet bulb readings in normal climatic conditions on the planet.

The researchers tested two different climate change scenarios: high emissions (no change in emissions from current levels), and containment to a 2°C increase, as per the Paris Agreement.

In the high emissions model, heatwaves would drive the wet bulb temperature to the 35°C maximum survivability threshold “over most of South Asia, including the Ganges river valley, north eastern India, Bangladesh, the eastern coast of China, northern Sri Lanka and the Indus valley of Pakistan,” according to the study, meaning that around 30% of the population would live in a climate in which the median maximum annual wet bulb temperature is 31°C or more.

If the Paris Agreement contains the rise in global temperature to just over two degrees, its primary goal, the fraction of the population living in such a climate drops to 2%.

This only highlights the importance of taking rapid action to address climate change. If no action is taken, deadly heat waves such as the one in 2015 will become much more common, changing from a once-in-two-decades event to a yearly occurrence. Even if the Paris goals are met, such heatwaves will still be more frequent.

Professor Eltahir made a last, critical point to the BBC. “This is something that is going to impact your most vulnerable population in ways that are potentially pretty lethal,” said Prof. Eltahir. “But it is avoidable, it is preventable.”

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