Heat waves are the most lethal weather phenomenon in the world. For example, the 2003 heat wave across Europe is estimated to have killed more than 70,000 people; significantly more than even the most major floods, hurricanes and tornadoes. As our planet continues to warm, heat waves are expected to occur more frequently and to last for longer.

In India the increase in number and duration of heat waves is already evident, with a clear rise in dangerous heat waves in recent years. The five most severe of the top-ten heat waves since 1951 occurred after 1990.

Previous work has investigated how heat-wave frequency and duration might increase in the future across India, but until now little had been done to assess the impact on population exposure. It isn't always the hottest heat waves that pack the biggest punch. In the 2015 event an estimated 2500 people died from heat-related causes, making it India's second most deadly heat wave. However, this heat wave failed to be classified in the top ten because it was localized over the east of the country.

Vimal Mishra from the Indian Institute of Technology (IIT) Gandhinagar and his colleagues combined climate models with population scenarios for India to investigate how many people might be exposed to heat waves across India in the future. Climate models project that the frequency of severe heat waves in India will increase 30-fold by the end of the century under a 2°C warming scenario. Under a business-as-usual scenario (RCP 8.5) heat-wave frequency increases 75-fold.

"Heat waves like 1998 [where more than 2000 people died] are projected to occur every year in the late 21st century under a business-as-usual scenario," said Mishra.

By looking at the number of days that heat waves were expected to last, and the estimates of population increase, the researchers estimated the number of people likely to be exposed to heat waves each year, and totted up the number of days that they were likely to be exposed.

The results show that population exposure to heat waves is expected to increase by around 18-fold by the end of the century under a 1.5°C warming scenario, 92-fold under a 2°C warming scenario, and a massive 200-fold under business-as-usual warming.

When it comes to mitigating for heat-wave exposure, the researchers note that strategies to reduce population growth in India during the 21st century may not reduce exposure to heat waves as much as hoped.

"The heat-wave exposure is dominated by the increase in length of heat waves, rather than population increase," they said. Instead, the largest mitigation effects are seen by reducing greenhouse-gas emissions. "We show that low-warming scenarios can provide substantial benefits in reducing the frequency of severe heat waves in India. Limiting global temperatures to 1.5°C would reduce exposure by half by the mid-21st century, compared to business-as-usual," Mishra added.

In the meantime, whichever climate scenario unfolds, India is going to have to look at serious adaptation measures such as increased provision of shelter, more cooling systems, modification of daily behaviour patterns and developing emergency public services to cope with heat-wave associated problems. But reducing global temperature will be key.

"Slowing the rate of global warming would provide vital time for further development of measures to reduce actual exposure," writes the team in Environmental Research Letters (ERL) .

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