weather report. But by the way, what are “unbearable” temperatures? Western France

Andalusian temperatures in Brittany. With France flooding from this Thursday 16 June 2022, hot air from Spain and the Maghreb will push temperatures to relatively new heights for the season.

Indeed, as Météo France hints on Twitter, the coming heatwave will cause the mercury column to rise to almost 40°C in certain regions, particularly in the south-west. Very high temperatures between 35 °C and 39 °C could also occur in the Great West.

From “uninhabitable” to “uninhabitable”

How can one qualify these temperatures? In casual discussions, they could easily be considered as such “unbearable”even “uninhabitable”. But behind the abuse of language that means the discomfort of having to undergo them, you can really tell these are runs “uninhabitable” ?

Scientists don’t ask the question in those terms. But not far. They prefer to refer to the notion of zones “uninhabitable”.

A wet bulb story

But how exactly do they determine that an area is uninhabitable? The answer lies in a concept: that of wet-bulb temperature (“temperature of a wet bulb” in French) that takes heat and humidity into account.

As Sarah Safieddine, researcher at CNRS, reminds us, this article “was used by the US military to say, ‘Okay, we’re in Arizona, so training can’t last more than that many hours if the weather is X or Y'”. So X stands for heat and Y for moisture. Because the latter is of crucial importance for climate resistance.

In fact, extreme heat is tolerable if not accompanied by high humidity. On the other hand, when the humidity is very high, sweating no longer allows regulation of body temperature, which in fact overheats. And with prolonged exposure (6 hours) and without artificial means of temperature cooling (air conditioning), exceeding this threshold can have very serious consequences for human health.

Heat and humidity, a potentially deadly cocktail

“As the wet-bulb temperature approaches your body temperature, you lose the ability to cool downsummarizes NASA in a blog post dedicated to the topic. This triggers changes in your body: you become dehydrated, your organs become “stressed, especially your heart. Blood rushes to your skin to release heat, starving your internal organs. The results can be deadly. »

This wet-bulb temperature threshold, which makes life impossible with prolonged exposure, has been set at 35° WT (WT, e.g “Wet Lightbulb”). That limit was not to be breached in France during the looming heatwave, but was approached in May in India or Pakistan, two countries experiencing a major heatwave.

Areas that are already considered “uninhabitable”.

Some areas have already crossed the Rubicon. In July 2021, the Intergovernmental Panel on Climate Change (IPCC) pointed out that this critical threshold is very regularly exceeded in the areas of Ras Al Khaimah (United Arab Emirates) and Jacobabad (Pakistan), which prompted the organization to qualify them “uninhabitable”.

The Telegraph specifies that this limit of what is humanly tolerable in Jacobabad has already been exceeded several times (1987, 2005, 2010 and 2012) and has been bumped into many times.

However, the human toll of these crossings is difficult to achieve because, as the British daily reminds us, “The effects of entering the danger zone should be mitigated, for example with cooler interiors protecting against the worst”.

Extreme humid heat episodes are becoming more common

The worst, exactly, shouldn’t it come? Perhaps because in Jacobabad, as in the other exposed regions, the 35° WT limit could be exceeded more and more frequently due to global warming.

As Valérie Masson-Delmotte, director of research at CNRS and member of the IPCC, stated on Twitter, “The number of unhealthy heat threshold days increases in direct proportion to each increase in additional warming”.

Other regions will switch soon

In the coming decades, the situation is therefore likely to become critical in a large number of regions. “There are several studies that have shown that certain regions of the world will be uninhabitable”summarizes Sarah Safieddine.

For example, the IPCC pointed this out in a report on Asia published in October 2021“Particularly in western Asia and the North China Plain, the maximum wet-bulb temperature is expected to approach and possibly exceed the physiological threshold of human adaptive capacity.”.

For its part, NASA points out that the list of regions where this threshold could be regularly exceeded is also included “South Asia, Persian Gulf and Red Sea (by 2050) and Eastern China, parts of Southeast Asia and Brazil (by 2070)”.

As the map below shows, these regions have already experienced the most intense episodes of humid heat in recent decades.

Map showing in dark brown the areas where the most intense episodes of humid heat have been recorded since 1979. | NOAA

And NASA specifies that these extreme temperatures, bordering on tolerable, could also be observed in the American Midwest within fifty years. For the latter, technology and development can help counteract these extreme heat waves. But for other, poorer regions, this may not be the case.

“Depending on the economy of the region or country you’re in, you either have the ability to adapt or you don’t.”notes Robert Vautard, research director at the CNRS. “And it’s always the same ones that it falls on.” Perhaps there is enough wealth in the great cities of the Persian Gulf that people can be in air-conditioned rooms on days when it is unsustainable, but I am afraid that is not the case for the tens of millions who live there north india.

In these disadvantaged areas, living conditions could therefore become very difficult. “It will be extremely difficult conditions for most people who work outside”predicts Robert Vautard.

A migration acceleration factor

In fact, these Dantesque climatic conditions, endurable only with the tricks of technology, could lead many people to take the road to exile.

Indeed, the World Bank pointed this out in a report on climate migration published in 2021 “Number of days with a temperature that exceeds the human tolerance threshold” would become increasingly important in certain regions “A migration push factor”.

The other factors? “Sea level rise, pollution…”, List Sarah Safieddine. The World Bank adds: water shortages, loss of soil productivity…

When it comes to global warming, mercury hides a galaxy of threats.

weather report. But by the way, what are “unbearable” temperatures?