Last Saturday the thermometer approached 46 ° C in the Indian capital New Delhi, while the day before the cities of Jacobabad and Sibi in neighboring Pakistan recorded around 47 ° C (29/4/2022). But is it a heat wave, or a heat wave in the true sense of the term?
“In general, we consider that a heat wave is simply a period of high temperature and that a heat wave is a heat wave that lasts. replies Thibault Laconde, president of the Callendar startup specialized in climate risk assessment, contacted by GEO.fr. France defines a heat wave as “a very high level of heat day and night for at least three consecutive days“(Government.fr)
If he”there is no universal definition of a heat wave“whatever the meaning given to this term, what we currently observe in India corresponds to it”no doubt“says the engineer.”Technically, the Indian Meteorological Department (equivalent to Météo France in India, ed) heat wave defines a period in which the temperature exceeds 40 ° C in the plains and 30 ° C on the hills. The 40 ° C threshold was exceeded in April in almost the whole country, with the exception of the southern tip and some coastal regions“, He specifies.
⋙ Climate: when it comes to heatwave?
Consider the wet bulb temperature
Are these temperature records harmful, even fatal, to humans? To answer this question, we must first understand the difference between the air temperature, the one we usually talk about, and another equally important parameter, called “wet bulb temperature” (humid temperature in English, abbreviated to TW) or “wet bulb temperature”. Its calculation takes into account the relative humidity of the environment, which affects the ability of the human body to lose heat by evaporation of transpiring water at the level of the skin.
“It is generally believed that the human body cannot withstand a wet bulb temperature above 35 ° C“explains Thibault Laconde.”Beyond that, the body’s temperature regulation mechanisms no longer work. Your body temperature will rise and lead to hyperthermia, or “heat stroke”. You will literally die of heat within hours, even if you are healthy, at rest, hydrated and in the shade …“
While air temperatures in India certainly exceeded 40 ° C in the shade, those measured with a wet bulb would not have crossed the fateful threshold that leads to death. “Based on preliminary data from the European Earth Observation Program, Callendar estimates that the maximum wet bulb temperature reached in April in India is around 32 ° C. It must be said that it is still very rare that the threshold of 35 ° C of humid temperature is exceeded, these are not conditions that are normally encountered on the planet.“Says the engineer.
Human health, affected by the heat wave
However, this does not exclude the mortal risk for the inhabitants. “This threshold (35 ° C wet bulb temperature) can be much lower, for example, if you practice physical activity or if you are exposed to the sun”, Thibault Laconde shadow. “Especially since hyperthermia is far from the only risk. In fact, when we look at the statistics, we see that nearly all causes of death increase during heatwaves – even if they apparently have nothing to do with heat: more people die from cardiovascular or respiratory disease. .but also infectious or endocrine diseases.“
“It is for this reason that, even if human physiological limits are very rarely exceeded, heat is the climatic danger that currently claims the greatest number of victims.“, underlines the president of Callendar. Between 1998 and 2017, more than 166,000 people died from heat waves or heat waves (World Health Organization). In India, these phenomena have killed more than 6,500 people since 2010.
More heatwaves with climate change
“Before human activities increased global temperatures, heat like the one that hit India earlier this month would only be seen once every 50 years. We can now expect such high temperatures about once every four years.“, Explained Dr Mariam Zachariah, researcher at Imperial College London, quoted by AFP (29/4/2022).
It is in fact likely that fatal episodes of heat waves “worsen in frequency and intensity“with climate change”.According to several recent studies, it is possible for heat waves with humid temperatures above 35 ° C to occur in the Persian Gulf, South Asia and China, especially on coasts where the air is more humid.“Warns Thibault Laconde. Heat waves that would be directly fatal to exposed humans.
“But the thermometer (even wet, ed) does not say everything: for example, the same temperature will have a much stronger impact in France than in Spain or Algeria, simply because historically warmer regions are more suitable. There is an urgent need to adapt to a warmer climate and, of course, to reduce our emissions by greenhouse gases“, Concludes the engineer.
Per Chandni Singh, member of the Intergovernmental Panel on Climate Change (IPCC) and researcher at the Indian Institute of Human Settlements (IIHS), quoted by CNN, “you can only adapt to a certain extent. This heatwave is testing the limits of human survivalIn addition to the fires that have raged in landfills near Delhi, India is also facing severe agricultural losses, as well as power outages due to the increased energy demand for building air conditioning.
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