James Paul 
Here we are; The 1.5 ° C threshold above the pre-industrial levels was crossed at the end of last year. What was only a gloomy forecast therefore became a reality and the prospects are even darker than what we had planned. A study published on February 4 in The review Nature Reviews Earth and Environment comes to drive the nail again: whole countries will disappearcrushed in heat.
If we arrive – and it’s a big ” if -to limit global warming to 2 ° C, as planned by the Paris Agreement, an area equivalent to the United States will become too hot to be inhabited, even for a healthy adult. Addition of only 0.5 ° C to our current temperature will multiply by three the areas of the planet where to live will simply be impossible; areas in which humans will no longer be able to adapt to environmental conditions.
When the thermometer climbs, humanity declines
The team identified two critical thresholds for characterize our ability to live in our own planet. “” HEAT ACCOMPENSABLE ” And ” Unsurvivable Heat ”, that we could respectively translate by ” Uncompensible heat “and” fatal warmth “.
To survive, the evolution has naturally equipped us with several internal biological mechanisms, including an absolutely essential: thermoregulation. It is thanks to it that our body evacuates excess heat to maintain our internal temperature around 37 ° C. Sweating, vasodilation and other physiological processes play this role of thermal regulators. But Like all systems, thermoregulation has its limits.
There uncompensible heat occurs when the environment saturates the thermal evacuation possibilities of the body. Even in the shade, even with a breeze, even perfectly hydrated, the body inexorably accumulates heat. The internal temperature then rises dangerously. A phenomenon normally circumscribed to inhospitable environments : specific places such as foundries or during extreme fires. However, today, we all face it during the oppressive heat waves that we live every summer.
Tom Matthews, researcher at King’s College in London and principal author of the study explains: ” The Persian Gulf, the Indo-Gangetic Plaine [NDLR : plaine qui s’étend des contreforts de l’Himalaya au nord jusqu’au golfe du Bengale au sud],, And even certain areas of the United States, Mexico and Australia have already crossed this threshold during extreme episodes, making the exterior physiologically intolerable, even for young adults in perfect health ».
Beyond the threshold said to mortal heat. At this stage, Body temperature reaches 42 ° C in less than six hours. Proteins are starting to distort, cell functions collapse and vital organs cease to operate normally.
In a world at +2 ° C above pre-industrial levels, this fatal border will be reached regularly in certain regions for people over 60, whose thermoregulation mechanisms are naturally weakened. At +4 ° C or +5 ° C, these areas of thermal lettality will extend, spanning no age class.
Lost territories
If our planet was to have 2 ° C warming, 35 % of the land emerged would rock beyond the habitability threshold for people over 60, against 21 % in our current world already strongly warmed. An area equivalent to that of the United States.
Ecuador becomes the epicenter of what one might call a inhabitability wave which will propagate towards the poles as the mercury climbs. At +4 ° C, The expansion of lethal zones will engulf 40 % of the landeven banish the most resistant organizations. The only remaining shelters will be the high boreal and southern latitudes, as well as certain areas of average altitude at temperatures modeled by elevation.
Current demographic distribution will only wake up the phenomenon, since tropical areas, foci of imminent extreme heat, Today shelters 40 % of humanity – or 3.2 billion people. These regions, already subject to water and food stress, will become real furnaces. Ironically, the populations who live there are often those who have contributed to climate change the least.
Matthews leaves no ambiguity as to this new reality : ” Unbearable heat thresholds, so far exceeded briefly for the elderly in the hottest regions, will even appear for young adults. Under these conditions, prolonged exposure outdoors – even in the shade, with a strong and well hydrated breeze – would probably cause a deadly heat stroke ».
Adaptation: a question of life or death
Faced with these scenarios, Two trajectories are emerging for humanity. The first,: radically decarbonizes our energy systems by accelerating the transition to renewable technologies and by sanctuarizing natural carbon wells – primary, peat bogs, mangroves – these biological infrastructures which sequest the atmospheric CO₂. The second: To prepare for a world where the exterior will become hostile from a thermal point of view.
“” As more regions will cross critical habitability thresholds, guarantee populations universal and uninterrupted access to air -conditioned environments will become a categorical public health imperative “, Underlines Matthews.
It’s here All our relationship to space that will be changed. Architects and urban planners will have to conceptualize (or adapt) cities incorporating natural ventilation corridors, thermally isolated underground infrastructure, and networks of air conditioned public spaces accessible quickly for each inhabitant. The very notion of habitat should also be redesigned, as a form of protective membrane against an exterior that has become lethal.
Our circadian rhythms, our professional practices, our school calendars or our agricultural practices; hasbsolument everything will have to be recalibrated to adapt to new temporal and spatial beaches of habitability.
Without immediate mobilization and international coordination, we therefore head straight Towards a gigantic crisis. The earth will continue its orbit, but the human species will be forced to concentrate in climaticly resilient areas and to abandon entire regions in which she will not even be able to live. This habitability is no longer a distant threat and each year that we lose to delay the rise in temperatures only increases the situation. Nature is well done and Darwin taught us; It gets rid of species that do not know how to adapt : The dice are thrown!
- The threshold of +1.5 ° C has been crossed, and an increase of +2 ° C will make immense territories uninhabitable, even for healthy adults.
- The tropical regions will become areas of fatal heat, where even perfect hydration and the shadow will no longer be enough to survive.
- Without immediate action, only a minority will be able to live in the rare areas still habitable.
