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