A draft UN report to be released next year warns of dire consequences for billions of people if global warming continues unchecked.
Earlier climate models suggested that it would take nearly another century of unabated carbon pollution to spawn heat waves exceeding the absolute limit of human tolerance, but updated projections warn of unprecedented killer heatwaves on the near horizon.
A 4,000-page Intergovernmental Panel on Climate Change (IPCC) report, seen by reporters before its scheduled release in February next year, paints a grim picture.
Photo: AFP
If the world warms by 1.5°C above preindustrial levels — 0.4°C 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 0.5°C would add another 1.7 billion people, it says.
Worst hit would be burgeoning megacities in the developing world that generate additional heat of their own, including Karachi, Pakistan; Kinshasa; Manila; Mumbai, India; Lagos, Nigeria; and Manaus, Brazil.
It is 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 heat waves. “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.”
TW spikes above 27°C have already more than doubled since 1979, Raymond said.
His study predicts wet-bulb temperatures will “regularly exceed” 35°C at some locations in the next several decades if the planet warms 2.5°C above preindustrial levels.
The 2015 Paris Agreement calls for capping the increase at “well below” 2°C, and 1.5°C if possible.
Even if those targets are met, hundreds of millions of city dwellers in sub-Saharan Africa, as well as South and Southeast Asia, will likely be afflicted by at least 30 deadly heat days every year by 2080, the IPCC report says.
“In these regions, the population of cities is growing dramatically and the threat of deadly heat is looming,” said Steffen Lohrey, lead author of a study, still under peer review, cited in the report.
His calculations do not even take into account the so-called urban heat-island effect, which adds 1.5°C on average during heat waves compared with surrounding areas, Lohrey said.
Heat-absorbing tarmac and buildings, exhaust from air-conditioning and the sheer density of urban living all contribute to this increase in cities.
In central China and central Asia, “extreme wet-bulb temperatures are expected to approach and possibly exceed physiological thresholds for human adaptability,” the report says.
“Today’s children will witness more days with extreme heat when manual labor outside is physiologically impossible,” the report says.
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