Why did it rain so much in 2024?

In France, 2024 was a year of heavy rain­fall. The spring was the 4^th wet­test on record since 1959, and cumu­lat­ive rain­fall exceeded 1,000 mm nation­wide since Novem­ber, which is more than all the rain accu­mu­lated in 2023. “As early as the end of Octo­ber, cumu­lat­ive rain­fall was in excess of the aver­age annu­al total for the peri­od 1991–2020,” com­ments Simon Mit­tel­ber­ger. “2024 was one of the wet­test years since weath­er records began in 1959. On the oth­er hand, the num­ber of rainy days is in line with the average.”

Is there a link between this par­tic­u­lar year and cli­mate change caused by human activ­ity? “2024 is mainly the res­ult of nat­ur­al cli­mate vari­ab­il­ity,” says Simon Mit­tel­ber­ger. Weath­er con­di­tions (pre­cip­it­a­tion, wind, tem­per­at­ure, etc.) are in fact mod­u­lated by nat­ur­al oscil­la­tions in the cli­mate as well as by the glob­al rise in tem­per­at­ures caused by human activ­it­ies. The World Met­eor­o­lo­gic­al Organ­isa­tion believes that it is neces­sary to con­sider a peri­od of 30 years to observe cli­mate change. “The short­er the time scales observed, the great­er the impact of nat­ur­al cli­mate vari­ab­il­ity,” explains Ber­trand Decharme. The annu­al scale is there­fore far too short to reveal the impact of cli­mate change on weath­er con­di­tions. “A series of favour­able weath­er con­di­tions explains the high rain­fall in 2024: numer­ous cold drops in spring, an atmo­spher­ic river in Septem­ber and high tem­per­at­ures in the Medi­ter­ranean,” points out Simon Mit­tel­ber­ger. It is there­fore not pos­sible to rely on the year 2024 to under­stand the impact of cli­mate change on rain­fall in France.

To do this, we need to look at the long-term trend in rain­fall. If we look at the his­tory of annu­al rain­fall in France, there is no dis­cern­ible trend. Cumu­lat­ive annu­al rain­fall in France has been around 935 mm since the 1980s, with nat­ur­al vari­ations from one year to the next. But when you change the scale, some sig­nals emerge. For example, annu­al rain­fall rose between 1961 and 2014 over a large north­ern half of France, while it fell in the south. “Since the 1960s, changes in rain­fall pat­terns have also been observed between sea­sons,” explains Simon Mit­tel­ber­ger. “We’re see­ing a marked increase in sea­son­al con­trasts, with more rain in winter, par­tic­u­larly in the north­ern half of the coun­try, and less rain in sum­mer, par­tic­u­larly in the south­ern half.”

By releas­ing green­house gases, human activ­it­ies increase the over­all tem­per­at­ure of the atmo­sphere. Tem­per­at­ure has a dir­ect influ­ence on the amount of water in the atmo­sphere: this well-known phys­ic­al rela­tion­ship is known as the Clausi­us-Clapeyron equa­tion. For each addi­tion­al degree, the humid­ity of the air at low alti­tude increases by 7%¹. As a res­ult, glob­al aver­age pre­cip­it­a­tion increases, by around 1 to 3% for each addi­tion­al degree.

In its latest syn­thes­is report, the Inter­gov­ern­ment­al Pan­el on Cli­mate Change (IPCC) sums up: “Glob­al aver­age pre­cip­it­a­tion and evap­or­a­tion increase less rap­idly than atmo­spher­ic humid­ity per 1°C of glob­al warm­ing, which lengthens the life­time of water vapour in the atmo­sphere and leads to changes in pre­cip­it­a­tion intens­ity, dur­a­tion and fre­quency, as well as an over­all intens­i­fic­a­tion, but not accel­er­a­tion, of the glob­al water cycle.” The regions affected in the future by an increase in mean annu­al pre­cip­it­a­tion are the Ethiopi­an High­lands, East, South and North Asia, south-east South Amer­ica, north­ern Europe, north and east North Amer­ica and the polar regions. Con­versely, aver­age rain­fall will decline in south­ern Africa, the West Afric­an coast, the Amazon, south-west­ern Aus­tralia, Cent­ral Amer­ica, south-west­ern South Amer­ica and the Mediterranean.

“France is in a trans­ition zone: in the north of Europe, rain­fall is set to increase as a res­ult of cli­mate change; con­versely, the Medi­ter­ranean basin is set to become drier,” explains Éric Sau­quet. “Is the trans­ition between these two sys­tems tak­ing place in the north of France? Or in Bel­gi­um? It’s still dif­fi­cult to get a clear answer from the cli­mate mod­els.” Cli­mate change means an increase – which can already be seen today – in the con­trast between sea­sons, but also between regions. “The high-res­ol­u­tion cli­mate mod­els we are work­ing on estab­lish the link between changes in pre­cip­it­a­tion in France and cli­mate change,” points out Simon Mittelberger.

The Explore 2 pro­ject², the res­ults of which were pub­lished in the sum­mer of 2024, is also explor­ing the pos­sible future of cli­mate and water in main­land France accord­ing to the IPCC’s cli­mate scen­ari­os, as Éric Sau­quet, the project’s sci­entif­ic co-lead­er, explained to us. “Between now and 2100, the pro­jec­tions don’t show any clear sig­nals about annu­al rain­fall,” points out Éric Sau­quet. “On the oth­er hand, rain­fall in the future will show great­er sea­son­al and region­al dis­par­it­ies: the trends are clear for a decrease in sum­mer rain­fall, and an increase in winter rain­fall under a scen­ario of high green­house gas emis­sions.” To put it plainly: the trends already observed today are set to continue.

Anaïs Marechal