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Water at the heart of global geopolitical issues

Water, a growing source of global tension

Hervé Douville, Researcher at Centre national de recherches météorologiques (CNRM) and Bertrand Decharme, CNRS Research Director at Centre national de recherches météorologiques (CNRM)
On January 24th, 2024 |
4 min reading time
Hervé Douville
Hervé Douville
Researcher at Centre national de recherches météorologiques (CNRM)
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.


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
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