Over the past millennia, different human societies have been able to rely on a wide variety of climatic conditions to support their meteoric growth and progress. But today, the range of meteorological conditions that our species is gradually brought to meet is changing under the effect of global warming.
Conditions completely new could well appear in the coming decades. While technology can help us, this threat should not be taken lightly, because our first coping system is not mechanical, it is biological: it is our body - and it has its limits.
Our ability to regulate our internal temperature has played a key role in the colonization of the planet by humans. Walking on two legs (which calls for less energy than to move on four limbs), without fur and equipped with a cooling system based on sweat, we are well designed to beat the heat. However, the latter does influence our ability to move and stay healthy.
In fact, our physiology imposes limits to what we can take in terms of heat and humidity.
Dry temperature and wet temperature
The temperature usually indicated in the weather forecast corresponds to the temperature known as "Dry" (or "dry bulb"), measured in the absence of humidity. As soon as this temperature exceeds approximately 35 ° C, the body resorts to the evaporation of water (mainly through perspiration) to dissipate heat.
The so-called “wet” (or “wet bulb”) temperature is measured by taking into account the cooling effect due to evaporation on a thermometer: it is therefore normally lower than the dry bulb temperature.
The greater the difference between the “normal” temperature and the wet bulb temperature, the drier the air and water can evaporate. This affects how efficiently our sweat cooling system can operate.
But when the wet bulb temperature exceeds 35 ° C, the air is at the same time so hot and humid that the perspiration can no longer do its job: the sweat deposited on our skin no longer evaporates. Impossible therefore to lower our temperature. Staying above this threshold for too long can lead to death from overheating.
While this limit of 35 ° C may seem low, it is not.
When the UK suffered a record dry temperature of 38,7 ° C in July 2019, the humid temperature in Cambridge did not exceed 24 ° C. Even during the deadly heatwave in Karachi in 2015, the wet bulb temperature remained below 30 ° C.
In fact, outside of a hammam, few people have experienced a humid temperature close to 35 ° C. This threshold has so far remained largely outside of terrestrial climatic standards experienced over the past few millennia.
But our recent works show that things are changing: this limit of 35 ° C is approaching, leaving an increasingly reduced margin of safety in the hottest and wettest places on the planet.
Go beyond human limits
Our analysis wet bulb temperatures between 1979 and 2017 confirmed these caveats. Another important point: while previous studies focused on relatively large regions (at the scale of large metropolitan areas), we examined thousands of records from weather stations around the world. We have thus observed that on a more local scale, many sites are rapidly approaching the limit of 35 ° C.
The frequency of high temperatures in humid conditions (above 31 ° C for example) has more than doubled worldwide since 1979. In some of the hottest and wettest places, such as the coast of the United Arab Emirates, humid temperatures have already exceeded 35 ° C. We are entering here into situations that our physiology cannot handle.
So far, exceeding 35 ° C will have had only limited consequences, as the inhabitants of these regions are used to enduring extreme conditions - by suitable architecture or by sheltering in air-conditioned spaces.
But the massive use of artificial cooling to cope with increasing heat could have the effect of overload energy demand and leave a large population dangerously exposed to power outages. This solution would also abandon the most vulnerable members of society and not help those who have no choice but to get out.
The only way to avoid a growing confrontation with unfamiliar and hostile thermal territory is to reduce our greenhouse gas emissions.
The downturn in the economy during the Covid-19 pandemic did see a reduction in 4 to 7% of these emissions in 2020, bringing us closer to the level of 2010. But concentrations of greenhouse gases continue inexorably and rapidly to increase in the atmosphere.
We must therefore try to adapter, encouraging simple behavior changes (such as avoiding outdoor activities during the day) and strengthening emergency response plans when heat extremes are imminent. These measures will save time in the face of progression inexorable of these new climatic profiles.
Hopefully our research can help shed light on some of the challenges of climate change and rising temperatures. The emergence of unprecedented heat and humidity - beyond what our physiology can tolerate - is just a sample of it.
Let us also hope that the feeling of vulnerability caused by the outbreak of Covid-19 will strengthen global commitments in favor of carbon neutrality, by recognizing the interest of preserving the conditions that are familiar to us ... and still bearable.
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