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