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