PARIS: Searing, unrelenting heat scorches large swathes of the Earth, killing millions who have no means to escape. Shade is useless, and shallow bodies of water are warmer than the blood coursing through people’s veins.

This is a scene from a new sci-fi novel, but the suffocating horror it describes may be closer to science than fiction, according to a draft United Nations report that warns of dire consequences for billions if global warming continues unchecked.

Earlier climate models suggested it would take nearly another century of unabated carbon pollution to spawn heatwaves exceeding the absolute limit of human tolerance.

But updated projections warn of unprecedented killer heat waves on the near horizon, according to a 4,000-page Intergovernmental Panel on Climate Change (IPCC) report, seen exclusively by Agence France-Presse before its scheduled release in February 2022.

The chilling report by the UN’s climate science advisory panel paints a grim — and deadly — picture for a warming planet.

If the world warms by 1.5 degrees Celsius (C) — 0.4 degrees above today’s level — 14 percent of the population will be exposed to severe heat waves at least once every five years, “a significant

increase in heatwave magnitude,” the report says.

Going up half a degree would add another 1.7 billion people.

Worst hit will be burgeoning megacities in the developing world that generate additional heat of their own, from Karachi to Kinshasa, Manila to Mumbai, Lagos to Manaus.

It’s not just thermometer readings that make a difference — heat becomes more deadly when combined with high humidity.

It is easier, in other words, to survive a high temperature day if the air is bone-dry than it is to survive a lower temperature day with very high humidity.

That steam-bath mix has its own yardstick, known as wet-bulb temperature.

Experts say that healthy human adults cannot survive if wet-bulb temperatures (TW) exceed 35 C, even in the shade with an unlimited supply of drinking water.

“When wet-bulb temperatures are extremely high, there is so much moisture in the air that sweating becomes ineffective at removing the body’s excess heat,” said Colin Raymond, lead author of a recent study on heatwaves in the Gulf.

“At some point, perhaps after six or more hours, this will lead to organ failure and death in the absence of access to artificial cooling.”