Due to global warming, these climatic and meteorological events are becoming more frequent and intense. But do you know what they mean exactly?
France experienced its first heat wave on Thursday, July 18. Almost 40 degrees in the South-East, 30 to 34 degrees inland… High temperatures were recorded throughout the country between Thursday and Saturday. But Météo-France did not use the term “heat wave”. For what reasons? When can we talk about a heat wave?
The return of these intense temperatures is in any case accompanied by other phenomena climatic and meteorological phenomena, amplified by global warming. Why is a zombie fire named after a living dead? How does the shrinking-swelling of clay, which cracks houses, form? What is a heat island? To help you see things more clearly, here is a glossary of these events.
Temperature under shelter
• What is this ? This is the air temperature that is reported in weather reports. It gets its name from the technique used to determine it. The temperature under cover is measured using a thermometer or probe, installed 1.5 meters above the ground, in an area that prevents rain and sunlight from distorting the readings. It should not be confused with the surface or ground temperature. Unlike the temperature under cover, the surface temperature is determined with infrared sensors on board satellites.
• How is it accentuated by the climate crisis? Monthly temperatures worldwide continue to break records due to global warming, and the summer months are no exception. According to data from Copernicus, a European observatory, we see that, since the 1990s, the mercury has been climbing higher and higher than the norms of the pre-industrial period.
In 2023, global average temperatures during the months of June, July and August were the highest ever measured, according to Copernicus, which noted “an average global temperature of 16.77°C”.
Heat wave
• What is this ? When temperatures are above seasonal norms for several consecutive days, it is called a heat wave. “The temperature levels and duration of the episode that allow [la] characterize vary according to the regions” of the world, specifies Météo-France. For the institute, two conditions must be met: the average temperature across the entire territory must be at least 25.3°C for one day, and at least 23.4°C for three days or more.
When the temperature falls below 23.4°C for two days in a row or below 22.4°C for one day, then we are out of the heat wave.
• How is it accentuated by the climate crisis? Heat waves are becoming more intense, more frequent and longer lasting as temperatures rise. “While France [hors territoires d’outre-mer] “It experienced an average of 1.7 days of heat waves per year before 1989, it has suffered 7.95 days per year since 2000 and 9.4 over the last decade”underlines Météo-France.
This dynamic is not about to change. “The frequency of events is expected to double by 2050warns Météo-France. By the end of the century, they could be not only much more frequent than today, but also much more severe and longer, with an extended period of occurrence from the end of May to the beginning of October. Controlling greenhouse gas emissions will be crucial for their stabilization in the second half of the 21st century.e century. “
Heat wave
• What is this ? We speak of a heat wave when the temperatures are “high” for at least three consecutive days, recalls Météo-France. Unlike heat waves, a heatwave does not necessarily affect the entire territory, because the temperature thresholds that define it depend on each department.
In Toulouse, for example, we are talking about a heatwave When “maximum temperatures [de jour] are above 36°C and the minimum temperatures [de nuit] above 21°C”explains Météo-France. In Lille, temperatures must be above 33°C during the day and above 18°C at night, the municipality reminds us on its website.
Heat wave “is linked either to a persistent anticyclone, which blocks cooler Atlantic disturbances, or to a persistent warm current coming from warmer regions”details the Ministry of Ecological Transition.
• How is it accentuated by the climate crisis? The heat waves that hit Europe and the United States in the summer of 2023 would have been “practically impossible” without global warming, according to the scientific network World Weather Attribution (WWA). Without it, the heatwaves of 2023 would have been 2.5°C cooler in Southern Europe, 2°C in North America and about 1°C in China, specifies the WWA. The latter, moreover, wants to be alarmist about the future: temperatures increasing on the globe, heatwaves will be even hotter, longer and more frequent if “The world is not going to stop burning fossil fuels anytime soon.”
Zombie Fire
• What is this ? It’s about “of a fire that we call ‘zombie’, because we don’t see it on the surface”summarizes Anthony Collin, teacher and member of the Feux research team at the University of Lorraine. Located up to one meter underground, this phenomenon feeds on soils that are full of carbon-rich material – peat, coal or lignite seams – and can burn for months.
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“When the humidity and temperature are high enough in the soil, the fire can start naturallyexplains Anthony Collin. But it also happens that a surface fire lights the fuse”as was the case in Landiras (Gironde) in 2022. When the weather conditions are right – drought, wind, heat – the zombie fire can resurface and burn outdoor vegetation, sometimes several months after it has been lit.
• How is it accentuated by the climate crisis? “As rapid climate warming leads to larger, longer and more intense wildfire seasons, hibernating fires are increasing”explain, in The Conversation, scientists who have worked on the subject in the boreal zones. And this, among other things, because the recurring drought and high temperatures linked to climate change weaken the vegetation, which becomes more flammable. A vicious circle then sets in, because “Reignition of hibernating fires triggers fire season earlier than in the past”note the researchers, while specifying that “further research” are needed on this topic.
Heat Dome
• What is this ? Imagine a very large bell jar under which temperatures would increase day after day. A heat dome forms with the appearance of an anticyclone: the high atmospheric pressures that accompany it “act like a lid (…) This compression heats the air in the dome and the heat reservoir thickens”explains Météo-France. Blocked by the pressure of the anticyclone, the strong heat cannot escape and the air compresses, releasing even more heat.
Clouds or rain, also driven away by the dome, cannot moderate the phenomenon. “In other words, a heat dome can be seen as a kind of greenhouse where the temperature is higher inside than outside.”illustrates climatologist Matthieu Sorel with Geo.
• How is it accentuated by the climate crisis? There is no consensus in the scientific community on the link between the frequency of this phenomenon and global warming, says Matthieu Sorel. But “What we can say with a relatively high degree of confidence is that heat domes tend to be more powerful because of climate change. With the base temperature being warmer, this phenomenon will push it even higher.”assures the climatologist.
Urban heat island
• What is this ? This term refers to the difference in air temperature between a city and the surrounding countryside. The urban heat island, expressed in degrees, is more pronounced at night, because “heat evacuation is blocked in very compact and dense cities”where the countryside cools more easily, notes Erwan Cordeau, researcher on climate, air and energy at the Paris Region Institute.
In addition to buildings that retain hot air, other factors can intensify this phenomenon: construction materials that store heat, lack of water and green spaces, air conditioning that rejects heat into the street, car traffic, etc. “As soon as there is a difference of +1°C at night [entre la ville et sa campagne environnante], we are talking about an urban heat islandcontinues the specialist. It can be +3°C, and rise to +10°C in large cities”.
• How is it accentuated by the climate crisis? With heat waves likely to be more frequent, “the development of heat islands in urban areas will therefore significantly increase urban overheating” the day, says Météo-France. Once the sun has set, “We will more regularly face tropical nights, characterized by high temperatures, which means that the city will have even more difficulty evacuating the heat”adds Erwan Cordeau.
Clay shrinkage-swelling
• What is this ? Soils are ecosystems composed of a multitude of minerals. Some are very concentrated in clay, we then speak of clayey soil. This type of soil “changes volume and consistency depending on its water content. When wet, it swells. When dry, it shrinks”explains the Ministry of Ecological Transition. This is what is called clay shrinkage-swelling (RGA).
These volume variations can cause cracks in buildings or roads located on these lands. According to a report (PDF) of the High Council for the Climate, in 2022, the damage caused by dry soils on buildings in France resulted in “8,000 requests from municipalities for the recognition of ‘natural disasters’ for the effects of the shrinkage-swelling of clays”.
• How is it accentuated by the climate crisis? The phenomenon “is intensifying with the accentuation of random cycles of intense droughts and rains, caused by global warming”notes the ministry. On the one hand, dry soils are favored “by the decrease in precipitation in spring and summer and by the increase in temperatures”climatologist Simon Mittelberger points out to franceinfo. On the other hand, “The warmer the air, the more water vapor it contains… which will potentially turn into intense downpours”points out the ministry.
Since the 19th century, the average temperature of the Earth has warmed by 1.1°C. Scientists have established with certainty that this increase is due to human activities, which consume fossil fuels (coal, oil and gas). This warmingunprecedented in its speed, threatens the future of our societies and biodiversity. But solutions – renewable energies, moderation, reduction of meat consumption – exist. Discover our answers to your questions about the climate crisis.