‘Global warming is generating even more severe episodes of heavy precipitation’ – Technologist

On Thursday, October 17, six French departments south of Lyon faced “exceptional” rainfall of up to 650 millimeters. A torrent raged over the city of Annonay, with cars submerged and firefighters in diving suits. Aurélien Ribes, a researcher at Météo-France’s National Center for Meteorological Research, points out that this phenomenon is likely to recur with climate change.

Is this episode of heavy precipitation remarkable?

At this stage, the highest accumulations were recorded in Ardèch, in the Cévennes mountains. Locally, they have reached 600 to 700 mm in 48 hours, including around 500 mm in 24 hours, which is exceptional. Very few episodes of intense precipitation exceed 500 mm in a single day, even in the Cévennes region, which is the most frequently affected by such phenomena.

What’s also remarkable is the extent of the territories affected, with 33 departments placed under orange alert for rain and flooding and six under red alert on Thursday evening. In some departments in the northern half of France, between 30 and 50 mm of rain were expected, on soils already saturated with water, due to the passage of Storm Kirk last week.

What causes this phenomenon?

As is often the case with this type of phenomenon, a low-pressure system from the Atlantic brought up a mass of warm, humid air from the south. In the Cévennes and Alpes-Maritimes, this gave rise to what is known as a “Mediterranean episode.” As the air collided with the relief bordering the Mediterranean, it rose and cooled. It then began to condense, forming water droplets and triggering convection – in other words, very rapid upward movements up to an altitude of around 10,000 meters – which led to storms. The mountainous terrain makes it easier for storm cells to stay in one place, which can result in very high rainfall totals.

In the space of a few months, we experienced a sequence of several remarkable episodes of heavy precipitation. Before Kirk, there was also the exceptional flooding in the Aspe valley, in the Pyrénées region in September, and the flood that ravaged La Bérarde [near Grenoble] at the end of June. Parts of Central Europe were also hard hit in September. This series of events may be cause for concern, but it is consistent with the projection expected from climate change.

What are the effects of climate change on heavy rain events?

Generally speaking, climate change, by increasing temperatures, allows the air to contain more water vapor, leading to more frequent and intense episodes of heavy precipitation. The latest report by the Intergovernmental Panel on Climate Change (IPCC) makes this abundantly clear for the Mediterranean regions of France, as in many other parts of the world. On a global scale, we expect an average increase of 7% in the intensity of extreme precipitation events for each degree of warming. Put another way, any additional warming, any additional greenhouse gas emissions, will generate even more severe episodes of heavy precipitation.

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