Extreme rainfall : can we anticipate the risk of flooding ?

Jan Pol­cher. Cli­mate change influences the water cycle. As a col­league des­cri­bed it best : with cli­mate change, dry regions become drier and wet regions become wet­ter¹. This is explai­ned by a well-known phy­si­cal pro­cess : the Clau­sius-Cla­pey­ron rela­tion­ship. An increase in the sur­face tem­pe­ra­ture of the atmos­phere leads to an increase in the amount of water vapour in the atmos­phere, of around 7% for each addi­tio­nal degree.

As a result, pre­ci­pi­ta­tion is more intense. Accor­ding to the latest report by the Inter­go­vern­men­tal Panel on Cli­mate Change (IPCC), there has been an increase in the fre­quen­cy and inten­si­ty of extreme pre­ci­pi­ta­tion events on a glo­bal scale across all conti­nents since the 1950s, par­ti­cu­lar­ly in Europe, North Ame­ri­ca and Asia². But it should be noted that our unders­tan­ding of this sub­ject has chan­ged lit­tle in recent decades, and we can­not say that we have a good unders­tan­ding of the impact of cli­mate change on extreme precipitation.

It has to do with the very nature of extreme pre­ci­pi­ta­tion : it is both high­ly inter­mit­tent and geo­gra­phi­cal­ly variable. It can rain hea­vi­ly in one val­ley for a few hours, and not at all in the next. Our rain­fall obser­va­tion sys­tem is not at all sui­ted to recor­ding this type of wea­ther event, which requires the ins­tal­la­tion of many ins­tru­ments such as rain gauges. While some regions, such as Europe and North Ame­ri­ca, are bet­ter equip­ped with ins­tru­ments, this is not the case in many areas, par­ti­cu­lar­ly tro­pi­cal zones.

This lack of data is com­poun­ded by the per­for­mance of the com­pu­ter models used to simu­late and bet­ter unders­tand the cli­mate. Conven­tio­nal models simu­late the pla­net’s cli­mate by divi­ding it into grids of around a hun­dred square kilo­metres each, a reso­lu­tion that is far too high to simu­late extreme rain­fall. We are wor­king to reduce this scale, but it is a colos­sal scien­ti­fic and tech­ni­cal challenge.

It is extre­me­ly dif­fi­cult. The phy­si­cal pro­cesses invol­ved – such as the Clau­sius-Cla­pey­ron rela­tion­ship – will always be the same, and we can rely on them for our fore­casts. But there are many other pro­cesses that make a rain­fall event a flood : they can be bio­lo­gi­cal (such as vege­ta­tion), che­mi­cal (such as the num­ber of aero­sols) or human (such as land use). 

All these para­me­ters are chan­ging at the same time as cli­mate change, so it is very dif­fi­cult to pre­dict hydro­lo­gi­cal trends, par­ti­cu­lar­ly extreme rain­fall. The only wide­ly accep­ted out­come is the glo­bal trend towards an increase in the fre­quen­cy and inten­si­ty of extreme rain­fall as the cli­mate warms. But this does not pro­vide any infor­ma­tion about local or sea­so­nal impacts.

No, eve­ryone is affec­ted. The impact of glo­bal war­ming on the water cycle is the most signi­fi­cant for human socie­ties. One of the main pro­blems is that man­kind has lear­ned to control hydro­lo­gi­cal resources since Anti­qui­ty, and this has been fun­da­men­tal to the deve­lop­ment of modern socie­ties. But hydrau­lic struc­tures – desi­gned to store water and control floods – are desi­gned for a cli­mate of the past. Now, with today’s cli­mate (and that of the future) being so dif­ferent, our infra­struc­tures are no lon­ger adap­ted, and we are losing this control.

Floo­ding and extreme rain­fall are two dif­ferent concepts. Extreme rain­fall does not always cause floo­ding, and vice ver­sa. The risk of floo­ding depends on the cli­mate, but also on the direct impact of human acti­vi­ties. This is an impor­tant fac­tor : in Europe, most of the hydro­lo­gi­cal varia­tions obser­ved can be explai­ned by the anthro­pi­sa­tion of the hydro­lo­gi­cal cycle³. This can be seen, for example, in soil sea­ling, which increases the risk of floo­ding, while cer­tain hydrau­lic struc­tures can reduce it. Many anthro­po­ge­nic fac­tors influence the conti­nen­tal water cycle – irri­ga­tion, urba­ni­sa­tion, mana­ge­ment of river navi­ga­bi­li­ty, hydroe­lec­tri­ci­ty, etc. It is very dif­fi­cult to sepa­rate the effects of these fac­tors. It is very dif­fi­cult to sepa­rate the impact of cli­mate change from that of human acti­vi­ties on the hydro­lo­gi­cal cycle, which makes future flood pro­jec­tions even more complex.

It’s almost impos­sible to anti­ci­pate it on a local level. All we know, as the IPCC points out, is that during per­iods of hea­vy pre­ci­pi­ta­tion, the inten­si­ty is increa­sed by the pre­sence of more water vapour in the atmos­phere⁴. But as floods are also affec­ted by human acti­vi­ties, as well as other cli­ma­tic phe­no­me­na (mel­ting gla­ciers, rising sea levels, drought), it is impos­sible to know whe­ther the risk of floo­ding will increase or even decrease in the future, for a given location.

Interview by Anaïs Marechal