Droughts exacerbated by climate change

In Janu­ary 2025, viol­ent wild­fires hit the Los Angeles region. Ini­tial ana­lyses¹ show that the arid­ity and heat of the pre­vi­ous months are among the factors behind the intens­ity of these fires²: they con­trib­uted to dry­ing out the veget­a­tion – which was par­tic­u­larly dense because of the pre­vi­ous year’s rain­fall – and increas­ing the amount of fuel avail­able. “In recent years, the dry­ing out of the envir­on­ment has glob­ally increased the length of the fire sea­son across much of the world, con­trib­ut­ing to forest fires of unpre­ced­en­ted sever­ity,” points out the IPCC in its 6^th assess­ment report³. Drought is a major nat­ur­al haz­ard. Between 1970 and 2019, only 7% of nat­ur­al dis­asters were drought related. Yet they con­trib­uted dis­pro­por­tion­ately to 34% of dis­aster-related deaths, mainly in Africa, as sum­mar­ised by the IPCC⁴. In the United States, droughts cost $250bn and killed nearly 3,000 people between 1980 and 2020⁵.

Attri­bu­tion stud­ies, which estab­lish the impact of cli­mate change on extreme events, have already shown on numer­ous occa­sions the con­tri­bu­tion of cli­mate change to the fre­quency or sever­ity of droughts. Of course, not all droughts can be explained by cli­mate change, and droughts occurred even before the cli­mate was mod­i­fied by human activ­it­ies. But of the 103 droughts stud­ied to date (sum­mar­ised by Car­bon Brief⁶), 71 have been made more severe or likely by cli­mate change. The group behind these attri­bu­tion stud­ies, the World Weath­er Attri­bu­tion, sum­mar­ises⁷: “We can attrib­ute an increase in the sever­ity and like­li­hood of droughts in the Medi­ter­ranean, South Africa, Cent­ral and East Asia, south­ern Aus­tralia and west­ern North Amer­ica to glob­al warm­ing with a high degree of confidence.”

To bet­ter under­stand how cli­mate change is influ­en­cing droughts, let’s take a step back. What exactly are we talk­ing about? “There is no uni­ver­sal cri­terion for what con­sti­tutes a drought,” notes research­er Toby R. Ault in an art­icle Ault⁸. In the broad­est sense, droughts are defined by a lack of water, or drier-than-nor­mal con­di­tions in a giv­en loc­a­tion, which can vary in dur­a­tion. “Met­eor­o­lo­gic­al droughts are marked by a defi­cit in rain­fall (of vary­ing dur­a­tion); they can also be hydro­lo­gic­al, with reduced levels in rivers or water tables (some­times for sev­er­al years), and agri­cul­tur­al, with soil drought hav­ing a poten­tial impact on crops and nat­ur­al veget­a­tion,” explains Her­vé Douville. Most droughts begin with a lack of rain­fall (a met­eor­o­lo­gic­al drought) and can devel­op into agri­cul­tur­al and hydro­lo­gic­al droughts⁹. Veget­a­tion and human activ­it­ies, such as irrig­a­tion and soil arti­fi­cial­isa­tion, can increase or decrease the sever­ity of drought and its socio-eco­nom­ic impact.

Human activ­it­ies also have an indir­ect impact on drought. Cli­mate change is one such factor. Cli­mate change is intensi­fy­ing the water cycle, as we have already explained in pre­vi­ous art­icles. As the atmo­sphere warms, its max­im­um water con­tent increases by an aver­age of 7% for each degree of warming.

This has sev­er­al implic­a­tions when it comes to droughts. Firstly, pre­cip­it­a­tion is chan­ging: its sea­son­al­ity and intens­ity are chan­ging. “In Europe, for example, cli­mate change is lead­ing to a reduc­tion in the num­ber of rainy days and an increase in extreme rain­fall,” explains Her­vé Douville. This less fre­quent but more intense rain causes more sur­face run-off and is gen­er­ally less effect­ive at rechar­ging the water tables. Rising glob­al tem­per­at­ures are also redu­cing snow stocks in some regions, affect­ing the flow of rivers fed by spring snow­melt. Finally, in the 6^th Syn­thes­is Report, the IPCC states that sub­trop­ic­al regions will also exper­i­ence a sig­ni­fic­ant drop in annu­al pre­cip­it­a­tion – this impacts the Medi­ter­ranean, South Africa, south-west Aus­tralia and South Amer­ica, Cent­ral Amer­ica, West Africa and the Amazon basin.

“In addi­tion to rain­fall, cli­mate change is also increas­ing sur­face evapo­tran­spir­a­tion, which is mak­ing a major con­tri­bu­tion to agri­cul­tur­al drought,” points out Her­vé Douville. On the con­tin­ents, evapo­tran­spir­a­tion refers to the flow of water that evap­or­ates from the soil and the sur­face of rivers or lakes, but also to the trans­fer of water from the soil to the atmo­sphere via plant tran­spir­a­tion. How­ever, the warm­ing of sur­face tem­per­at­ures is great­er over the con­tin­ents than over the oceans. As a res­ult, more energy is avail­able for water to evap­or­ate, and the sever­ity of droughts increases. This effect is partly off­set by the effects of CO₂ on plants, which can increase their effi­ciency in using water from the soil. But this is not enough: evapo­tran­spir­a­tion has increased since the 1980s.

The effects of these phe­nom­ena linked to glob­al warm­ing vary from region to region. While met­eor­o­lo­gic­al droughts have so far been rel­at­ively unaf­fected by cli­mate change, there has been an increase in agri­cul­tur­al droughts through­out the world – only north­ern Aus­tralia has been spared¹⁰. This under­lines the major impact of increased evapo­tran­spir­a­tion on droughts, which the IPCC points out is very likely to be due to anthro­po­gen­ic green­house gas emis­sions. A clear link has even been estab­lished between warm­ing linked to human activ­it­ies and an increase in the fre­quency and sever­ity of droughts over recent dec­ades in the Medi­ter­ranean, west­ern North Amer­ica and south-west Australia.

In the future, these effects will increase as aver­age glob­al tem­per­at­ures con­tin­ue to rise. Depend­ing on green­house gas emis­sion scen­ari­os, the dur­a­tion of droughts could be 0.5–1 months to 2 months longer than today in Cent­ral Amer­ica, the Medi­ter­ranean, the Amazon basin, south-west­ern South Amer­ica, west­ern North Africa, south­ern Africa and south-west­ern Aus­tralia. “All these regions will become drier as a res­ult of reduced pre­cip­it­a­tion and increased evapo­tran­spir­a­tion […] even for low green­house gas emis­sion scen­ari­os,” sum­mar­ises the IPCC. Without a drastic reduc­tion in green­house gas emis­sions, around a third of the world’s land area is likely to suf­fer at best mod­er­ate drought by 2100. Only a few regions and sea­sons should see their risk of drought decrease: high lat­it­ude areas in North Amer­ica and Asia, and the mon­soon sea­son in South Asia.

This will have major socio-eco­nom­ic con­sequences. In a study pub­lished in 2021¹¹, a team estim­ated that, in the absence of cli­mate action, drought-related dam­age could rise from $9–65bn each year in the European Uni­on and the United King­dom. “The impact of floods and droughts is expec­ted to increase in all eco­nom­ic sec­tors, from agri­cul­ture to energy pro­duc­tion, with neg­at­ive con­sequences for glob­al pro­duc­tion of goods and ser­vices, indus­tri­al pro­duc­tion, employ­ment, trade and house­hold con­sump­tion,” sum­mar­ises the IPCC. By redu­cing our green­house gas emis­sions and imple­ment­ing adapt­a­tion meas­ures, we can sig­ni­fic­antly lim­it the impact on our communities.

Anaïs Marechal