How high will the mercury rise in the coming days in southern Europe? No doubt very high as another week of heat waves is expected in several regions of the northern hemisphere. If with the exception of the Alpes-Maritimes, France should be relatively spared, up to 48 ° C are expected in Sardinia. In Rome, we are preparing to reach 42 ° C on Tuesday, an absolute record for the capital. We are also suffocating in Greece and Spain, particularly in Andalusia, where we could exceed 44°C.
To explain this new heat wave, the excuse ” El Nino – of which a new episode has just started – does not hold. This climatic phenomenon, which is characterized by abnormally high water temperatures in the eastern Pacific, tends to increase global temperatures. “We are only at the very beginning of this phenomenon. [qui dure habituellement un an] and Europe is in any case very little affected in summer, if at all”, explains Olivier Boucher, researcher at the Pierre-Simon Laplace Institute (CNRS).
A rate twice as fast in Europe
The climatologist places this new heat wave much more in line with the global temperature increases observed in recent years in connection with climate change. Over the decade 2012-2023, the average temperature on the scale of the planet has increased by 1.15°C compared to the period 1850-1900, taken as a reference by the Intergovernmental Panel on Climate Change (IPCC) and corresponding to the pre-industrial era.
But in Europe, this warming is even greater. Since the 1980s, the region has warmed at a rate of 0.5 degrees per decade, twice the global average, calculate the World Meteorological Organization (WMO) And Copernicusthe European Earth monitoring programme, in a report published on June 19. In 2022, the average European temperature was about 2.3°C higher than the pre-industrial average (1850-1900). And 2023 should not bring down the thermometer.
Oceans that bring down the global average
How can this faster warming be explained? Olivier Boucher begins by recalling that this average of +1.15°C at the planetary level would be much higher if we did not take into account the seas and oceans. “They have the ability to absorb heat and bury it deep within. Since they cover 70% of the surface of the globe, they bring down the world average,” he explains. “Always over this decade 2012-2023 compared to the period 1850-1900, the oceans have “only” warmed up by 0.93°C, confirms Christophe Cassou, climatologist, co-author of the IPCC and director of research at the European center for research and advanced training in scientific computing (Cerfacs). If we only take the continents, we are at + 1.65°C. »
With +2.3°C, Europe remains above average. Christophe Cassou evokes a first reason which may seem paradoxical. “The level of air pollution by particles (aerosols) – emitted by our industries, our cars – has decreased significantly in Europe and North America over the last three decades when it has often increased elsewhere, particularly in South Asia. East. This is a good thing, the health issues linked to this pollution being very strong. Nevertheless, this concentration of aerosol has a cooling power that once masked global warming linked to the increase in greenhouse gases. This is much less the case today. »
A drop in soil humidity around the Mediterranean
Christophe Cassou mentions another factor which particularly affects Mediterranean Europe, where the warming is more accentuated than on the rest of the continent. “It is the drying of the soil, points out the climatologist. A warmer atmosphere results in more efficient evaporation of water from soils and plants. Again, this evaporation has a cooling effect. But as it sucks up all the moisture in the soil, it dwindles. “It is surely the dominant process which explains the strong heat waves around the Mediterranean in recent years and at the moment, continues Christophe Cassou. This decrease in evaporation, which normally occurs mainly at night, prevents nighttime temperatures from falling. So that the day dawns with an already very large heat capital. And all this accumulates day by day, until you have mega heat waves. »
The Cerfacs climatologist adds a third factor: atmospheric circulation, which “promotes the transport of hot air from the Maghreb and/or the Mediterranean rim to northern Europe”. “The phenomenon is quite complex to understand,” he says. Are these circulations another facet of climate change, or are they part of the natural variability of the climate which means that we are in a period where flows are more from South to North? We don’t really know how to answer. »
A warming twice as fast… not only in Europe
In any case, taken together, these factors mean that Europe is warming twice as fast as the average. It is not the only region. It is also said of the Middle East, California, Russia or the Canada that they warm up at a similar rate. “Not necessarily for the same reasons,” says Christophe Cassou.
While soil drying is also one of the key factors of warming in California or the Middle East, another phenomenon comes into play for Canada and Russia: the loss of snow cover. “It’s a matter ofalbedo, specifies the climatologist. The very white surface of the snow has a strong reflective power. It reflects heat back into the atmosphere. As it melts, it exposes darker surfaces which will capture the heat and keep it on the surface. It’s the same phenomenon as in the Arctic with the melting of glaciers and sea ice. But at an even faster rate: the Arctic is warming four times faster than the global average.