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