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Water, a growing source of global tension

Hervé Douville
Hervé Douville
Researcher at Centre national de recherches météorologiques (CNRM)
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Bertrand Decharme
CNRS Research Director at Centre national de recherches météorologiques (CNRM)
Key takeaways
  • Freshwater resources are unevenly distributed across the globe.
  • This lack of availability has a serious impact on the world's population, with 4 billion people experiencing severe shortages for at least one month of the year.
  • The IPCC states that climate change is increasing seasonal contrasts, such as the severity of droughts and wet events.
  • These shortages are due to reduced rainfall and increased evaporation.
  • By 2050, human activities (urbanisation, deforestation, pollution, etc.) could become the dominant cause of future global water shortages.
  • The scientific community cannot accurately predict the future of water resources, because future security will also depend on changes in socio-economic factors and governance.

835,000 km3 of fresh­wa­ter are avail­able to mankind world­wide. Most­ly stored in under­ground aquifers (630,000 km3), fresh­wa­ter is a large­ly renew­able resource that is suf­fi­cient to meet the needs of humans and ecosys­tems… in the­o­ry1. So, what’s the prob­lem? Water resources are uneven­ly dis­trib­uted in space and/or time. Four bil­lion peo­ple live at least one month a year in con­di­tions of seri­ous water short­age, because demand exceeds avail­abil­i­ty. All year round, 500 mil­lion peo­ple suf­fer from this sit­u­a­tion, which is get­ting worse.

Decreased water availability

Bertrand Decharme explains, “the most arid regions, includ­ing the Mediter­ranean basin, the east­ern Unit­ed States, south­ern Africa, south-east Asia, and India, are draw­ing heav­i­ly on water resources that are dimin­ish­ing over time.” On a glob­al scale, the avail­abil­i­ty of water on the con­ti­nents is decreas­ing. The bal­ance of arrivals (rain­fall) and depar­tures (evap­o­tran­spi­ra­tion) amounts to around ‑1 mm per year between 2001 and 20202, reflect­ing a deficit.”

How­ev­er, this aver­age cov­ers up major dis­par­i­ties. In par­tic­u­lar, the effect is large­ly vis­i­ble in the south­ern hemi­sphere (-3.5 mm/year). What’s more, while these vari­a­tions in annu­al aver­ages may appear small, they can mask an increase in sea­son­al con­trasts3. For exam­ple, accord­ing to the Inter­na­tion­al Pan­el on Cli­mate Change (IPCC)4, there has been an increase in the fre­quen­cy and sever­i­ty of droughts over recent decades in the Mediter­ranean, west­ern North Amer­i­ca and south-west­ern Aus­tralia. The cause: cli­mate change. “The con­se­quences of cli­mate change on ter­res­tri­al ecosys­tems and human soci­eties are main­ly man­i­fest­ed through changes in the water cycle”, writes the IPCC in its lat­est report.

Source5.

Before going into detail, let’s empha­sise one point: the direct impact (exclud­ing cli­mate change) of human activ­i­ties is by no means a sec­ondary con­sid­er­a­tion. Since the sec­ond half of the 20th Cen­tu­ry, the rivers feed­ing the Aral Sea have been divert­ed for irri­ga­tion, lead­ing to it almost entire­ly dis­ap­pear­ing. It has been clear­ly estab­lished that ground­wa­ter use for irri­ga­tion is now lead­ing to a sig­nif­i­cant reduc­tion in the resource; a decline that is being felt in the world’s most pro­duc­tive agri­cul­tur­al areas, such as Cal­i­for­nia, the great cen­tral plains of the Unit­ed States, the plains of north­ern Chi­na and the Ganges basin in India6. Ground­wa­ter use guar­an­tees food and health secu­ri­ty in these regions. How­ev­er, they can also be used for unsus­tain­able agri­cul­tur­al exports. Over­ex­ploita­tion of aquifers makes these pro­duc­tion meth­ods vul­ner­a­ble, and great­ly reduces the expect­ed social benefits.

On a glob­al scale, only the equiv­a­lent of 6% of the annu­al recharge of ground­wa­ter is extract­ed each year. But here again, there are major region­al dis­par­i­ties. “In a few aquifer basins in arid zones or in South-East Asia, with­drawals for irri­ga­tion are high­er than recharge, and ground­wa­ter lev­els are falling,” says Bertrand Decharme. “Even though these basins are few in num­ber, the effect is so strong that we can see it even when we look at glob­al water resources!” Hervé Dou­ville adds, “with cli­mate change, the dry sea­sons are get­ting dri­er and dri­er, and irri­ga­tion is on the increase. Unless we adapt our agri­cul­tur­al pro­duc­tion sys­tems, the impact of irri­ga­tion on the water cycle is like­ly to increase in the future.”

Changes in land use also affect water resources. Large-scale defor­esta­tion reduces evap­o­tran­spi­ra­tion (the evap­o­ra­tion of water from the soil) and gen­er­al­ly pre­cip­i­ta­tion. Con­verse­ly, urban­i­sa­tion favours local rain­fall and reduces ground­wa­ter recharge because of imper­me­able soils. These effects are of the same order of mag­ni­tude as the impact of irri­ga­tion. By 2050, water con­sump­tion could increase by 20–30%. As a result, human activ­i­ties could become the dom­i­nant cause of future glob­al water short­ages, all the more so if mit­i­ga­tion efforts are imple­ment­ed to lim­it glob­al warming.

More extreme rainfall

Cli­mate change is exac­er­bat­ing the impact of irri­ga­tion by pro­found­ly alter­ing the water cycle. The first major effects are on rain­fall. As the atmos­phere warms, its max­i­mum water con­tent increas­es by an aver­age of 7% for each degree of warm­ing. This encour­ages an increase in aver­age pre­cip­i­ta­tion of between 1% and 3% for each addi­tion­al degree. Above all, extreme pre­cip­i­ta­tion will be more intense, by around 7%. The IPCC points out that the sever­i­ty of extreme wet and dry events increas­es with glob­al warm­ing. “In sim­ple terms, water resources should increase where there is already an abun­dance of water, and decrease where it is need­ed, with a few excep­tions,” com­ments Bertrand Decharme. Arid­i­fi­ca­tion will par­tic­u­lar­ly affect the Mediter­ranean, south-west­ern Aus­tralia, south-west­ern South Amer­i­ca, South Africa, and west­ern North America.

The sever­i­ty of agri­cul­tur­al droughts may increase, and for­est fires may multiply.

The com­bined effect of changes in rain­fall and irri­ga­tion can already be seen on some water tables. Between 2001 and 2010, the decline exceed­ed 20 mm per year in some aquifers (Cal­i­for­nia, Mid­dle East, Sahara, Ganges, north­ern China).It is less marked (less than 10 mm per year) in the Ama­zon and Mekong basins.

The rise in glob­al tem­per­a­tures, caused by green­house gas (GHG) emis­sions, is giv­ing rise to anoth­er phe­nom­e­non: the increase in evap­o­tran­spi­ra­tion. This phe­nom­e­non refers to the water that evap­o­rates from the soil and the sur­face of rivers, lakes and oceans, and the trans­fer of water from the soil to the atmos­phere by plants. It is lim­it­ed by the water resources avail­able. “This is an impor­tant effect for under­stand­ing changes in water resources in soils and sur­face reser­voirs,” adds Hervé Dou­ville. “Even if water vapour increas­es in the atmos­phere, the dry­ing out of soils caused by glob­al warm­ing off­sets this effect in the low­er lay­ers of the atmos­phere above con­ti­nen­tal sur­faces7.” As a result, the sever­i­ty of agri­cul­tur­al droughts may increase and for­est fires may multiply.

At this stage, it is dif­fi­cult for the sci­en­tif­ic com­mu­ni­ty to accu­rate­ly pre­dict the future of water resources. The var­i­ous fac­tors involved – pre­cip­i­ta­tion, evap­o­tran­spi­ra­tion, irri­ga­tion – vary from region to region and accord­ing to inter­na­tion­al and region­al socio-eco­nom­ic choic­es. Evap­o­tran­spi­ra­tion is very like­ly to increase at con­ti­nen­tal lev­el, and annu­al pre­cip­i­ta­tion is like­ly to increase by 2% to 8% between now and 2100, depend­ing on the GHG emis­sion sce­nar­ios. “Cli­mate mod­els are get­ting bet­ter and bet­ter at pre­dict­ing pre­cip­i­ta­tion, but direct anthro­pogenic fac­tors, such as with­drawals, are not always tak­en into account or are poor­ly antic­i­pat­ed,” explains Bertrand Decharme. The researcher and his col­leagues have incor­po­rat­ed irri­ga­tion into the cli­mate pro­jec­tions tra­di­tion­al­ly used by the IPCC. They are study­ing the 218 largest aquifer basins in the world, over which 50% of the world’s pop­u­la­tion is expect­ed to live by 2100. By the end of the cen­tu­ry, almost 18% of the world’s pop­u­la­tion is like­ly to be direct­ly affect­ed by a drop in aquifer lev­els (com­pared with 9% if irri­ga­tion is not tak­en into account8). Ground­wa­ter qual­i­ty is also like­ly to be degrad­ed by increas­ing soil pol­lu­tion, high­er rain­fall inten­si­ty and extreme events that leach con­t­a­m­i­nants (pes­ti­cides, fer­tilis­ers, antibi­otics) into aquifers.

One thing is cer­tain, how­ev­er, accord­ing to the lat­est IPCC report: “The future secu­ri­ty of water resources will also depend on changes in socio-eco­nom­ic fac­tors and gov­er­nance”. By reduc­ing the activ­i­ties that are respon­si­ble for green­house gas emis­sions, and by lim­it­ing the way we use water, the pres­sure on water resources can be kept under control.

Anaïs Marechal
1Dou­ville, H., K. Ragha­van, J. Ren­wick, R.P. Allan, P.A. Arias, M. Bar­low, R. Cere­zo-Mota, A. Cher­chi, T.Y. Gan, J. Ger­gis, D. Jiang, A. Khan, W. Pokam Mba, D. Rosen­feld, J. Tier­ney, and O. Zoli­na, 2021: Water Cycle Changes. In Cli­mate Change 2021: The Phys­i­cal Sci­ence Basis. Con­tri­bu­tion of Work­ing Group I to the Sixth Assess­ment Report of the Inter­gov­ern­men­tal Pan­el on Cli­mate Change [Mas­son-Del­motte, V., P. Zhai, A. Pirani, S.L. Con­nors, C. Péan, S. Berg­er, N. Caud, Y. Chen, L. Gold­farb, M.I. Gomis, M. Huang, K. Leitzell, E. Lon­noy, J.B.R. Matthews, T.K. May­cock, T. Water­field, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cam­bridge Uni­ver­si­ty Press, Cam­bridge, Unit­ed King­dom and New York, NY, USA, pp. 1055–1210, doi: 10.1017/9781009157896.010.
2Yongqiang Zhang et al., South­ern Hemi­sphere dom­i­nates recent decline in glob­al water avail­abil­i­ty. Sci­ence, 382, 579–584(2023).DOI:10.1126/science.adh0716
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