How hot is too hot?

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The recent floods in Germany and other central European countries, as well as the meteorological conditions that favored the fires in Sardinia, are considered part of a very complex set of climatic phenomena which have long been the subject of study and analysis among scientists, and of increasingly frequent worried reflections in public opinion. There is a substantial convergence of views in the scientific community regarding the fact that climate change is somehow linked to a greater frequency of extreme weather events such as hurricanes, floods and droughts.

In addition to being a relevant factor involved in these phenomena, the increase in global average temperature – associated in turn with the increase in the presence of greenhouse gases in the atmosphere – often produces extreme conditions and serious consequences even during heat waves, with maximum temperatures over 40 degrees. At the end of June, at least 6,500 people died in India due to a heat wave with maximum temperatures higher than those of the past nine years. Over the same period and for the same reason, hundreds of people died in western Canada and the northwestern United States. In one of the cities most affected by the heat wave – Portland, Oregon – the people who died in Multnomah County were largely without air conditioning or fans.

– Read also: Floods in Europe have to do with climate change

Reflecting on the recent heatwaves, the site Slate he asked himself in the “Future Tense” section – in which he deals with public policies, emerging societies and technologies, in collaboration with the State University of Arizona – what are the temperatures above which the human habitability of certain currently populated areas it would be mortally compromised. First of all he refers to the difference between dry heat and humid heat, a distinction known and quite frequent even in common speeches (enough to have generated one of the most well-known clichés: “it’s not the heat, it’s the humidity”).

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An air-conditioned hospitality center set up in the Oregon Convention Center, Portland on June 27, 2021 (Nathan Howard / Getty Images)

Palm Springs is a popular Southern California resort town known for its luxury hotels, golf courses, and cultural events. Yet it is located in a desert area a few hours away by car from Death Valley, one of the most inhospitable places in the United States. With enough water – and in shaded spaces, to avoid sunstroke – humans can, for example, survive several hours in the heat of Palm Springs, with maximum temperatures of up to 50 degrees, being a hot and dry climate.

Things change completely if you move from a place like Palm Springs to one like Palm Beach, Florida, one of the wettest regions in the Western Hemisphere. A single day with 50 degrees would then cause thousands of deaths from hyperthermia, a condition characterized by the rapid increase in body temperature, common in environments with very high temperatures, high humidity and poor ventilation. Prolonged exposure to conditions of this type can cause the alteration of the body’s normal thermoregulation mechanisms by hindering the dispersion of heat that occurs through sweating.

– Read also: All about sweat

To better explain hyperthermia and understand a concept “fundamental for the survival and mitigation of the climate crisis”, according to Slate, it is useful to recall the notion of wet bulb temperature, a value different from the dry bulb temperature – that measured with a common thermometer – and generally measured through a thermometer wrapped in a piece of wet gauze. Due to the cooling effect of water evaporation, the temperature measured by the thermometer wrapped in the damp cloth in a certain environment will be lower than the dry bulb temperature in that same environment. The difference between the two temperatures corresponds to one of the humidity measurements: at 100 percent humidity, simplifying a lot, the dry and wet bulb temperatures will be the same.

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Sarajevo, Bosnia, 23 giugno 2021 (AP Photo/Kemal Softic)

It is believed that lethal alterations of normal human thermoregulation can occur in environments with wet bulb temperatures above 35 degrees (but also with values ​​lower than this, depending on other variables). When it’s hot outside and our bodies sweat, he explains Slate, the transformation of sweat from liquid on the skin to vapor in the air requires energy, and that energy comes from the heat of the body, which cools through this process and avoids an increase in internal temperature.

Dry heat is a more tolerable condition because evaporation occurs so quickly that we do not even notice the sweat on the skin. It is also the reason why dehydration is a more concrete risk in desert ecosystems, where dry air allows us to withstand the heat but in the meantime also involves a constant loss of fluids. In the presence of very high temperatures and air saturated with water vapor, however, the sweat remains on the skin and the body is unable to give up heat. The internal temperature then begins to rise and approach the external one until it reaches a dangerous point of equilibrium, a condition capable of causing death in a few hours, if the external temperature exceeds a certain threshold.

– Read also: What is the “perceived temperature” really

The concept of wet bulb temperature is considered a very significant parameter, useful for keeping track of the effects of climate change and for having more precise indications about when and where cities will become uninhabitable. In fact, it indicates conditions to which it is not possible to imagine that human beings get used to, given their fundamental physiological limits. Progressively increasing the amount of the world population with access to air conditioning, as well as having an enormous cost in environmental terms, is not considered a long-term solution to the problem, and the increase in demand would increase the likelihood of interruptions in the supply of air conditioning. electric energy.

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Outdoor showers in Baghdad, Iraq, Thursday July 1, 2021 (AP Photo / Khalid Mohammed)

The most shared climate models indicate that heat waves will be more frequent and more intense in large parts of the planet, with critical consequences as early as 2050. Although forecasting the trend of humidity is a more complex operation, it is in fact, it predicts that the increase in temperatures will affect both dry bulb and wet bulb temperatures, resulting in scenarios in which shade and hydration will not be sufficient for survival. The areas most exposed to the risk of frequent periods of uninhabitability in the United States, according to these forecasts, would not be the desert ones with the current highest temperatures but the areas that are already humid, such as most of the Midwest and the eastern United States. .

Extreme heat waves with wet bulb temperatures above 35 degrees, according to other climate models, will be regular and increasingly frequent also in the regions of the Persian Gulf, the Indian subcontinent and eastern China, some of the most densely populated areas in the world.

– Read also: The climate will trigger colossal migrations

Many projections point to the need to rapidly reduce greenhouse gas emissions to limit or slow global warming as much as possible and hopefully how often these extreme heat waves could occur in the future. They also underline the need to implement policies to safeguard the most vulnerable populations, which include the creation of air-conditioned centers for elderly residents and the systematic sending of preventive warnings in the event of imminent heat waves.

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