The study has predicted increases in the frequency of deadly heat waves worldwide with six South Asian cities among the top ten facing the highest heat stress burden, a measure that combines the temperature, humidity and the population exposed to the heat.
The findings suggest that Calcutta, which experienced on average 16 days of extreme heat per year between 1979 and 2005, is likely to face 44 days of similar heat in the coming years or decades.
This doubling is projected to occur, when the average global temperature rises to 1.5°C above what it was in the late 19th century, or the pre-industrial era. Climate studies show that the average global temperature between 1986 and 2005 had already reached about 0.8°C above the pre-industrial level.
The research has also projected that over 350 million inhabitants of mega-cities across the world will experience intense heat waves in the coming decades, a four-fold increase compared to the global population exposed to such heat between the years 1979 and 2005.
"The higher the heat stress, the more the potential societal impacts," said Tom Matthews, a climatologist at Liverpool John Moore University in the UK, who led the study collaborating with researchers in Ireland and the UK.
"We think society will need to continue to learn to live with and adapt to episodes of extreme heat," Matthews told The Telegraph.
The study, published today in the US journal Proceedings of the National Academy of Sciences , has indicated that when the 1.5°C rise occurs, Calcutta will experience, almost every year, the conditions equivalent to the record heat wave that hit the city in 2015.
In the summer of 2015, India had experienced one of the worst heat waves in recent years with the combination of extreme high temperatures, reaching 44.5°C in Calcutta, and humidity contributing to deaths in several states. Climate scientists have documented 2016 as the hottest year since 1901.
Scientists are unsure when the Earth's average temperature will rise to 1.5°C above the pre-industrial level. "When that level of warming may be reached will depends largely on how humanity emits greenhouse gases in the coming years," Matthews said. "Under a plausible worst-case scenario, this could happen well before the 2050s."
When it does, Delhi is likely to experience on average 25 days of extreme heat per year compared to the eight days per year between 1979 and 2005.
Chennai will face a near-tripling of such days from 14 to 42 days, while Karachi in Pakistan will experience a five-fold rise in hot days from three to 15 days.
The international community had pledged in December 2015 through the Paris climate pact to try and limit global warming to below 2 C through actions such as expanding renewable energy, cutting down on dependence on fossil fuels, and adding forests that can serve as sinks for greenhouse gases.
Matthews and his collegues who used climate models to forecast heat stress burden have also found that Beijing, New York, and Tokyo are among several cities that under the 1.5°C temperature rise will face heat stress levels beyond what they have historically experienced.
The scientists say their findings highlight the importance of mega-cities preparing for future heat waves, citing a "heat-health action plan" announced by the city of Ahmedabad three years ago as a model for other cities.
The Ahmedabad Municipal Corporation, collaborating with specialists at the Natural Resources Defence Council in the US and academic institutions, had outlined a plan relying on early warnings through seven-day forecasts and public outreach activities to minimise health impacts of dangerous heat waves.