Will the intertropical zone be uninhabitable within a few decades?

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If global warming were to exceed 1.5 ° C, a significant fraction of the intertropical zone would be subject to heat exceeding the threshold of human adaptability. In any case, this is what a new study indicates. Significant results which remind us that a tenth of a degree overall can make a big difference at the level of the territories. The results were published in the journal Geoscience of nature this March 8.

The concept of climate injustice is that the countries most affected by global warming are those that have participated the least in greenhouse gas emissions (GHG). In other words, despite limited historical liability, they are among the most affected. Conversely, the countries which issued the majority of GHG over the past century are relatively unaffected. Note that we are not considering the temperature increase as such here, but its socio-economic, environmental and health implications.

This inequality between the developed countries of the north and those of the intertropical strip – known as from South – is structural. Indeed, it is due to the configuration of the climate system which makes the tropical zone a more violent world: hurricanes, episodes of lethal heat, major droughts, intense rains, etc. Incidentally, it is not fortuitous that the countries located in this band of latitudes are globally less developed and, in fact, more vulnerable. The 1.5 ° C objective mentioned in the Paris Agreements is precisely linked to this type of consideration.

Heat at the limits of human tolerance in the intertropical zone

A new study illustrates just how much of a difference keeping warming at 1.5 ° C or not can make all the difference. In particular, in tropical regions already subject to a high level of heat. The authors were interested in what is called the temperature of the wet thermometer (noted Tw). This is a parameter that takes into account both the temperature and the humidity of the air. Widely used in bioclimatology, it allows for example to know when a heat is approaching the tolerance threshold for humans.

Evolution of global temperature for 12,000 years (top) and 2,000 years (bottom). Note the recent spike, showing the brutality of the change underway. Credits: PAGES 2k network.

“The general idea is that the body does not react only to temperature, it responds to humidity”, notes Kristina Dahl, climatologist who did not participate in the study. “The body cools down mainly through perspiration and the evaporation of sweat from the skin. At a certain level of heat-humidity, it becomes “thermodynamically difficult” for this to happen ”.

This limit defines the biological tolerance threshold from which heat conditions become lethal. For humans it is around 34 ° C to 35 ° C of Tw. From this level, the body is no longer able to regulate its temperature and anyone exposed to the outside air is life threatening. And for good reason, the inability to cool the skin by evaporation very quickly leads to a state of hyperthermia. While the sweat beads in large drops, no drop in temperature manages to occur. Thus, without urgent intervention, the outcome is quickly fatal.

Global warming: every tenth of a degree counts

Using a set of climate simulations, the researchers found that global warming limited to 1.5 ° C would allow most countries in the intertropical zone to avoid unbearable temperatures. The study area extends from 20 ° N to 20 ° S and includes the Amazon, a large part of Africa, India as well as Southeast Asia. Otherwise, the lethal heat episodes would generalize and go as far as question the habitability of certain geographic areas.

intertropical zone
The average tropical warming in a world at 1.5 ° C is indicated by a black line. The fraction (vertical axis) of the tropical surface affected by a given warming (horizontal axis) is shown in blue (for the Tw maximum) and red (for the maximum T). Credits: Yi Zhang et al. 2021.

However, lead author Yi Zhang recalls that heat below the level of human adaptability also has strong impacts on human health. A warming of 1.5 ° C is therefore already a difficult change for most intertropical countries. One way among many to show that every tenth of a degree counts. “The results translate a political objective into a potential impact on the real world”, points out Kristina Dahl. Finally, such prospects obviously raise the question of the potential mass migrations which would result from it.

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