This Sunday, May 1, 2022, it was no less than 49.5 ° C in Nawabshah, a city in southeastern Pakistan. Exceptional, this maximum temperature level is not unprecedented. Because, as has been the case for several days, if not the past few weeks, a large part of the Indian subcontinent is facing extreme heat.
As Gaétan Heymes, forecaster at Météo France, indicates on Twitter, the mercury also rose to 49 ° C a little further north, in Jacobabad, or 47 ° C in Banda, in central India.
From “uninhabitable” to “uninhabitable”
How to qualify these temperatures? In our temperate latitudes, in casual discussions, they could easily be considered as “unbearable”, even “unlivable”. But, behind the misuse of language signifying the discomfort that one would have to undergo them, can one really say that these heats are “unlivable” ?
Scientists do not ask the question in these terms. But not far. They prefer to invoke the notion of zones “uninhabitable”.
A wet bulb story
But how, exactly, do they decree that an area is uninhabitable? The answer lies in a concept: that of wet bulb temperature (“temperature of a wet thermometer” in French,), which takes into account heat and humidity.
As Sarah Safieddine, researcher at the CNRS, reminds us, this item “was used by the US military to say, ‘Okay, we’re in Arizona, so training can’t last more than so many hours if the weather is X or Y'”.
X for heat, and Y for humidity, therefore. Because, in terms of resistance to a climate, this last fact is of decisive importance.
Indeed, some extreme heat is bearable if it is not accompanied by high humidity. On the other hand, in the event of very high humidity, sweating no longer makes it possible to regulate the temperature of the body which, in fact, overheats. And, in the event of prolonged exposure (6 hours) and in the absence of an artificial means of cooling the temperature (air conditioning), exceeding this threshold can have very serious consequences on human health.
Heat and humidity, a potentially deadly cocktail
“As the wet-bulb temperature approaches your body temperature, you lose the ability to cool yourself”summarizes NASA, in a blog post devoted to the subject. “It triggers changes in your body: you get dehydrated, your organs get ‘stressed’, especially your heart. Blood rushes to your skin in an attempt to release heat, starving your internal organs. The results can be deadly. »
This threshold of wet bulb temperature which makes life impossible in case of prolonged exposure has been set at 35° WT (WT, for “Wet-Bulb”).
This limit has not been exceeded in recent days in India. But the subcontinent came quite close to it. “With a temperature of more than 45°C and a humidity rate of 50%, we have a WT that approaches 35°”, confirms Sarah Safieddine. Not “uninhabitable” then, but almost.
Areas already deemed “uninhabitable”
On the other hand, some areas have already crossed the Rubicon. In July 2021, the Intergovernmental Panel on Climate Change (IPCC) thus indicated that, in the areas of Ras Al Khaimah (United Arab Emirates) and Jacobabad (Pakistan), this critical threshold was very regularly exceeded, which led the organization to qualify them “uninhabitable”.
The Telegraph specifies that in Jacobabad, this limit of what is humanly bearable has already been crossed on several occasions (1987, 2005, 2010 and 2012) and that it has been brushed against many other times.
The human toll of these threshold overruns is however difficult to achieve because, as the British daily reminds us, “the effects of entering the danger zone are likely to be mitigated, for example with cooler interiors that protect against the worst”.
Episodes of extreme humid heat will be more and more frequent
The worst, precisely, is it not to come? Perhaps, because in Jacobabad as in the other exposed regions, the 35° WT limit could be crossed more and more regularly, due to global warming.
Indeed, as indicated on Twitter Valérie Masson-Delmotte, research director at the CNRS and member of the IPCC, “the number of heat threshold days dangerous to human health increases in direct relation to each increment of additional warming”.
Other regions will soon switch
In the decades to come, the situation should therefore become critical in a large number of regions. “There are several studies that have shown that certain regions of the world are going to be uninhabitable”summarizes Sarah Safieddine.
In a report on Asia published in October 2021, the IPCC indicated for example that“In West Asia and the North China Plain in particular, the wet bulb temperature maximum should approach and perhaps exceed the physiological threshold of human adaptability”.
NASA indicates for its part that the list of regions where this threshold could be crossed regularly also includes “South Asia, the Persian Gulf and the Red Sea (by 2050) as well as eastern China, parts of Southeast Asia and Brazil (by 2070)”.
As the following map shows, these regions have already experienced the most intense episodes of humid heat in recent decades.
And NASA to specify that these extreme temperatures, at the limit of the bearable, could also be observed in the States of the American Midwest within fifty years. For the latter, technology and development can help counter these extreme heat waves. But for other, poorer regions, this may not be the case.
“Depending on the economy of the region or country where you are, either you have the capacity to adapt or you don’t”notes Robert Vautard, research director at the CNRS. “And it’s always on the same ones that it falls.” Maybe in the big cities of the Persian Gulf there is enough wealth that people can, on days when it’s not tenable, be in air-conditioned rooms, but I’m afraid that’s not the case for the tens of millions of people who live in northern India.
In these deprived areas, living conditions could therefore become very difficult. “For most people working outside, it will be extremely difficult conditions”predicts Robert Vautard.
A migration acceleration factor
In fact, these dantesque climatic conditions, bearable only with the artifices of technology, could push many people to take the path of exile.
Thus, in a report on climate migration published in 2021, the World Bank indeed indicated that “the number of days with a temperature exceeding the human tolerance threshold” was going to become increasingly important in certain regions, which would constitute “a migration push factor”.
The other factors? “Sea level rise, pollution…”, Sarah Safieddine list. The World Bank completes: lack of water, loss of soil productivity…
When it comes to global warming, mercury hides a galaxy of threats.