Torrential rains: the other side of global warming

After Kirk, the flood. A week after the passage of the storm, which caused significant damage in its wake, a very intense Cévennes episode hit French territory on Thursday October 17.

In Ardèche, the town center of Annonay is under water. According to Météo France, the department experienced its rainy episode “ the most intense ever recorded over two days », with 627 millimeters falling in 48 hours. Or twice as much as what fell cumulatively during the three weeks of flooding in Pas-de-Calais in November 2023.

The mercury is rising, the water cycle is disrupted

“These are totally exceptional volumes of rain, but will be much more common in the future,” explains Françoise Vimeux, climatologist at the Institute of Research for Development. We will have to wait for scientific work to quantify very precisely the part of global warming in the scale of the phenomenon, but it is already certain that the rise in temperatures increases the capacity of our atmosphere to gorge itself with humidity and intensifies the water cycle.

The mechanism is simple: the warmer the ocean, the more surface water evaporates. At the same time, the atmosphere is also warming. And the warmer the air, the more humidity it is able to contain: this explains, for example, that laundry dries more quickly in summer. With each additional degree of warming, the atmosphere can hold 7% more humidity.

With climate change, air masses are therefore more loaded with water, leading to greater precipitation when they cool: “ The potential volume of water that can fall on us is much greater than in the past”explains Françoise Vimeux.

Torrential rains: the new climate norm

Torrential rains, floods and floods… As the climate changes, these exceptional events increase. And risk becoming the new norm, as a report from the World Meteorological Organization said very recently.

« Events described as exceptional a few decades ago will likely be normal events in the near futurepoints out the climatologist. As for the exceptional events of tomorrow, they will be even more devastating than today. Hence the need to adapt to survive. »

According to a recent report from the NGO Oxfam on adaptation, 62% of the French population is exposed to a high or very high degree to climate risks. The association points to a delay and a lack of anticipation of policies related to climate change, and calls for the establishment of an ambitious and coherent national adaptation plan.

« In particular, we need to improve local alert systems, with mobile phone alerts extended to all affected areas so that people can get to safety and stay at home,” completes Françoise Vimeux.

Adapt to avoid the worst

Beyond crisis management, measures can be put in place to limit the risk of flooding, which depends on a certain number of factors: amount of precipitation, topography and soil permeability. On flat land, heavy rain infiltrates into the soil if it is permeable enough. However, land covered in concrete is completely impermeable, which makes cities more vulnerable to flooding in the event of intense rain.

“Urbanization increases the risk of flooding”underlines the research director at IRD. Hence the idea of ​​increasing the surface area of ​​non-artificialized soil to prevent water from accumulating on the surface. And if water cannot infiltrate into the ground, then it must be able to evacuate it: “Rainwater evacuation systems must be sized to take into account the risks of tomorrow, explains Françoise Vimeux. Most systems are designed for precipitation from several decades ago. This is nonsense. »

The government plans to announce its long-awaited national climate change adaptation plan (PNACC) on October 25. But for now, the 2025 finance bill does not plan to allocate any specific envelope to adaptation.