What impact does climate change have on the seasons ?

It’s a fact : cli­mate change is increa­sing tem­pe­ra­tures, drought fre­quen­cy and extreme wea­ther events. But what effect is it having on the seasons ? 

Obser­ving the sea­sons and how they change is cru­cial. Win­ter, spring, sum­mer, and autumn illus­trate the rhythm of vege­ta­tion : bud­ding, the appea­rance of the first leaves, fol­lo­wed by flo­we­ring and then leaf fall. They control eve­ry­thing and work in sym­bio­sis with all the ele­ments of our ecosystem.

For the last thir­teen years, Jadu Dash has been stu­dying the evo­lu­tion of vege­ta­tion over time around the world, using satel­lite obser­va­tions. The pro­fes­sor of remote sen­sing in geo­gra­phy and envi­ron­men­tal sciences at the Uni­ver­si­ty of Sou­thamp­ton has access to some fif­ty years’ worth of satel­lite data, enabling him to deter­mine varia­tions in vege­ta­tion growth over time. “We use a tech­nique that involves exa­mi­ning the degree of green­ness of the vege­ta­tion in a spe­ci­fic area, in order to iden­ti­fy the start and end of the gro­wing sea­son,” explains the researcher.

Accor­ding to his stu­dies, glo­bal war­ming has chan­ged dura­tion of the sea­sons. Spring arrives a fort­night ear­lier on ave­rage, and autumn two weeks later. In other words, the vege­ta­tion gro­wing sea­son has been leng­the­ned by an ave­rage of one month over the last five decades. These effects are not the same eve­ryw­here on the pla­net. “In the nor­thern hemis­phere, i.e. Europe and North Ame­ri­ca, we are seeing a much more pro­noun­ced change in sea­so­na­li­ty,” says Jadu Dash.

This change in sea­sons is main­ly due to rising tem­pe­ra­tures. In France, for example, 2022 was 2.7°C war­mer than 1961–1990. In spring, high tem­pe­ra­tures send a signal to plants that trig­gers buds to open and leaves to unfurl. Then, in autumn, the drop in tem­pe­ra­ture brings vege­ta­tion to a halt. So, accor­ding to the researcher’s work, rising tem­pe­ra­tures have a major impact on the gro­wing sea­son for plants. 

The leng­the­ning of the plant gro­wing sea­son has mul­tiple conse­quences for our eco­sys­tem. “Vege­ta­tion stays lon­ger, so it is more expo­sed to spring frosts, para­sites and diseases, but also to droughts during the sum­mer”, explains Jadu Dash. His obser­va­tions have also shown that there are dis­cre­pan­cies bet­ween cer­tain bio­lo­gi­cal events. Pol­li­na­ting insects, for example, depend on flo­we­ring. When these occur ear­lier, by the time these ani­mals arrive, there may no lon­ger be enough flo­wers for them to move around and trans­port the pol­len. In the same way, migra­to­ry birds depend on vege­ta­tion and know where to feed. “If they expect to find a cer­tain type of vege­ta­tion in a given place, but it’s not there because it’s fini­shed or late, this could have an impact on their abi­li­ty to sur­vive,” adds the professor.

The impact of this change in sea­so­na­li­ty varies accor­ding to geo­gra­phi­cal area, but also accor­ding to the type of vege­ta­tion. The pro­fes­sor of remote sen­sing has noted that the large forests of Rus­sia are more res­pon­sive to changes in tem­pe­ra­ture than the coni­fe­rous forests of Europe. Fur­ther­more, grass­lands, made up of shal­low-roo­ted grasses, are very sen­si­tive to changes. “Spring comes ear­lier, they turn green, lose a lot of water and are the­re­fore more sen­si­tive to drought during the sum­mer than forest trees, which are more dee­ply roo­ted”, explains Jadu Dash.

In its inven­to­ry publi­shed in Octo­ber 2023, the Ins­ti­tut natio­nal de l’in­for­ma­tion géo­gra­phique fores­tière noted a major increase in tree mor­ta­li­ty, of around 80% bet­ween 2013 and 2021. Is this the result of the chan­ging sea­sons ? For the resear­cher, it could be a com­bi­na­tion of fac­tors. “Droughts have become more frequent. If they occur towards the peak of the gro­wing sea­son, severe water stress can lead to tree mor­ta­li­ty. We are also seeing new diseases affec­ting forests, poten­tial­ly lin­ked to changes in sea­so­na­li­ty, all of which makes them more vulnerable.”

Should we expect the sea­sons to conti­nue to evolve in the future to become com­ple­te­ly dif­ferent from those we know today ? “It’s not enti­re­ly clear. Cli­mate fore­casts for the future are very uncer­tain,” replies Jadu Dash, “but if we pass a tip­ping point, we could still see the vege­ta­tion per­iod leng­then. In the long-term, the entire plant com­po­si­tion around us is like­ly to change.” The scien­tist is alrea­dy obser­ving tree spe­cies migra­ting to higher lati­tudes that suit them bet­ter. In the Uni­ted King­dom, wine­gro­wing is expan­ding consi­de­ra­bly, whe­reas before it was non-existent. “Far­ming prac­tices are bound to change over time, due to the chan­ging sui­ta­bi­li­ty of the land for dif­ferent crops,” anti­ci­pates Jadu Dash. For the rest, the future evo­lu­tion of the sea­sons depends lar­ge­ly on our actions to reduce our impact on the environment.

Sirine Azouaoui

Réfé­rences : https://www.ign.fr/files/default/2023–10/memento_ign_2023_2.pdf