India is likely to be among the G20 countries hit the worst by climate change, with its heat waves lengthening and economy ravaged, an international climate report released on Thursday says.

India’s heat waves are likely to “last 25 times longer by 2036-2065” given a scale of a close to 4°C global temperature rise by century end — the Intergovernmental Panel of Climate Change’s worst-case emission scenario — says the G20 Climate Impacts Atlas.

The heat waves will be “over five times longer (than the current average) if global temperature rise is constrained to about 2°C, and one-and-a-half times longer if emissions are very low and temperature rise only reaches 1.5°C”, adds the report, released ahead of the October 30-31 G20 summit in Rome.

Climate scientist Anjal Prakash, who has written the India-specific part of the report, told The Telegraph that India’s situation was “real red”.

He said that with about 54 per cent of India’s area being arid and “extremely prone to heat waves, India stands extremely vulnerable”.

Prakash added that India’s cities are “sitting on a time bomb in terms of climate change” and urged the government to take immediate action.

Subimal Ghosh, an IIT Mumbai professor and a lead author of the IPCC report, said “Indian cities like Calcutta, Delhi and others are extremely vulnerable in terms of both searing heat” and extreme rainfall events.

He said the density of concrete constructions relative to greenery in Indian cities “does not help the cause”. Ghosh and Prakash said most Indian cities were warming up faster than the global average.

The G20 report says global warming will increase the demand for water for agriculture while lowering its availability. It warns that “without any mitigation, climate change could severely undermine the development gains made in India in recent decades”.

“In a moderate climate-change scenario, India is projected to potentially lose between 0.8 (per cent) and 2 per cent of its GDP by mid-century,” it says.

“By the end of the century costs could double, reaching up to almost 10 per cent of the GDP (or 237 billion EUR) under a high emissions scenario.”

A senior official from the Union environment, forest and climate change ministry said: “The report is likely to push to the fore a global emissions cut agenda during the G20 summit, scheduled to be attended by several global leaders including Prime Minister Narendra Modi.”

The report, prepared by a team of more than 40 scientists at the Euro-Mediterranean Centre on Climate Change, says climate change will have devastating impacts on every G20 member.

“Climate impacts are already hitting the G20. Over the last 20 years, heat-related deaths have increased by at least 15 per cent in all G20 countries, while forest fires in the G20 have burnt an area one-and-a-half times the size of Canada. And if emissions keep rising, worse is to come,” the report warns.

“The impacts of climate change will spiral to tear through the world’s richest economies within 30 years, without urgent action to reduce greenhouse gas emissions... from droughts, heat waves and sea-level rise, to dwindling food supplies and threats to tourism… no country is immune.”

The G20 is a grouping that includes 19 countries and the European Union and works to address economic issues including those related to climate-change mitigation.