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