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What is the capacity of human beings to resist the new climatic conditions?

Heat and humidity are liable to intensify with climatic deregulation. Although human beings have shown their capacity to confront and even live in extreme climatic conditions, from desert to arctic zones, might there not be a risk of reaching the limit of this capacity of adaptation? This is the question raised by American researchers in a study published on 8 May 2020 in the review Science Advances, based on an original indicator: wet bulb temperature.


Faster and more intense occurrence

35°C TW, that is to say 35°C wet bulb temperature, appears to be the physiological threshold above which the human body is incapable of surviving. It corresponds to a level of temperature combined with a level of humidity that would prevent human beings from efficiently evacuating heat by perspiration, leading to fatal bodily overheating, even in the case of hydration. This phenomenon has never occurred on Earth, since this variable has almost never exceeded 30°C.

However, climate warming, which increases temperatures, could also increase the degree of atmospheric humidity. Climate models have forecast the first occurrences of 35°C TW for the middle of the 21st century. The challenge risks becoming even more complex to overcome if this phenomenon of wet heat is more intense and occurs more rapidly than foreseen. That is the warning sounded by these researchers, who base their results on the study of data from nearly 8,000 meteorological stations located around the world between 1979 and 2017.


According to them, a wet bulb temperature of 30°C has already been reached more than a thousand times during the period covered in highly localized zones, and that of 33°C, up to recently thought unreachable, was recorded 80 times. A wet bulb temperature of 35°C has even been recorded in several subtropical areas. Another indicator, the frequency of extreme humid heat (higher than 27°C TW) has more than doubled globally since 1979. Recent overshoots of 35°C of the maximum temperature of the sea’s surface on the worldwide scale confirm these dangerously high TW values. Deadly heatwaves could arrive 30 years earlier than predicted.


A major societal challenge

In mid-June 2019, extreme heat in Bihar, north-eastern India, killed 78 people in two days

Whereas, from time to time, this critical value is exceeded for one or two hours in Pakistan and the United Arab Emirates, in the future this risk will affect larger areas more regularly. However, they include some of the most highly populated regions of the world: South Asia, the Middle East and the southeast of North America. These regions are all located in subtropical, coastal regions, so they all lie close to the extremely hot surface waters of the oceans and are exposed to continental heatwaves. The authors of the study estimate that, together, these conditions lead to the occurrence of extreme wet bulb temperatures, and they also emphasize that the critical threshold could be exceeded regularly, with an increase of less than 2.5°C in comparison to the pre-industrial era, the least worst-case scenario.

To measure the threat, let us compare it with the heatwave of 2003 that caused the death of 70,000 people in Europe. During this period, the wet bulb temperature did not exceed 28°C, since it was an essentially a dry heatwave.

Experiences of lethal heat already encountered over the last few decades point to a continuously rising trend towards extreme wet bulb temperatures, and our results underline that their different significant and increasing impacts constitute a major societal challenge for the coming decades, the authors conclude

They add that the economic and health risks linked to these temperatures are currently considerably underestimated.

This study is a new scientific warning of the possible impacts of major climatic change and is not alone. Four days before its publication, another study, which appeared in the American journal Proceedings of the National Academy of Sciences announced that in fifty years’ time, a third of humanity (i.e. 3.5 billion people) could live in regions as hot as the Sahara now. This raises the issue of Earth’s habitability.

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