Flood in Morocco's Tafilalet Valley

Will floods in the southern Mediterranean, as pictured here in Morocco's Tafilalet Valley, increase with climate change?

© IRD/Thierry Ruf

Towards the intensification of extreme rainfall events in southern Europe


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For the first time, a team including an IRD researcher studied the impact of climate change on extreme rainfall patterns in the entire Mediterranean region. This research is crucial to assessing the risk of severe flooding in this region.

In the future, intense rainfall should intensify in southern Europe – and therefore France - warns a new study1. “This extreme rainfall could result in more severe floods, thereby causing significant human and economic damage”, cautions Yves Tramblay, hydrologist with the “HydroSciences Montpellier” laboratory, co-author of the study, and – as chance would have it – organiser of a conference on hydroclimatic risks in the Mediterranean, held in Montpellier from 9 to 11 October2.


Two climate scenarios

As a matter of fact, the basins of the Mediterranean region are already affected by intense rain events, which frequently exceed 100 millimetres in one day, i.e. one or even several months’ worth of rain depending on the region. Furthermore, several climate model projections?Computer programmes involving several mathematical equations, capable of modelling the climate in a given area, and simulating changes in this climate. showed that these extreme weather events could become more frequent and intense in the future, because of climate change linked to greenhouse gas (GHG) emissions in the atmosphere. However, most studies available focus on the European part of the Mediterranean Basin – i.e. its northern part - not the entire region – including the South.
To obtain projections for the entire Mediterranean region, Yves Tramblay and his colleague Samuel Somot, from the French Weather research centre, analysed rainfall data simulated by 11 regional climate models “with high spatial resolution of 12 kilometres”, explains the researcher.

The team looked at the 1950 to 2100 period, and two possible climate change scenarios. “The first one is consistent with the objective of the Paris Climate Conference (COP21), with a stagnation in GHG emissions after 2050. The second one, more pessimistic, is, according to recent studies3, the path we are following, with no reduction in these emissions”, points out the hydrologist.


Rainfall intensification in the northern Mediterranean region

Due to climate change, the increase in intense rainfall could exceed 20% in southern France, northern Italy, northern Greece and the Adriatic coast by 2100 (red).

© Y. Tramblay & S. Somot

Bloc de texte

Surprisingly, regardless of the scenario considered, simulations predict the intensification of extreme rain in the northern Mediterranean region. “In southern France - notably the Rhone basin - northern Italy, northern Greece and the Adriatic coast, the increase in intense rainfall could exceed 20% by 2100”, estimates Yves Tramblay. The results also indicate that this upward trend started early in the various simulations, in the early 2000s!

Conversely, in the south of the Mediterranean basin (North Africa), researchers have identified a downward trend in cumulative daily figures, “with considerable uncertainty, which does not support a robust conclusion”, stresses the scientist.

Researchers are now working to rigorously assess the risk of flooding associated with the intensification of extreme rainfall events. “To do this, we use hydrological models to simulate how the soil responds to heavy rains. Knowing that the response will vary depending on the surface cover: for example, the asphalt of cities is impervious to rainwater, and therefore increases the risk of flooding; conversely, dry soil covered in vegetation can absorb this water and therefore minimise the danger”, explains Yves Tramblay.

Crucial for the design and implementation of preventive measures, the initial results of this research are expected by the end of 2019.

Notes :

1. Y. Tramblay et S. Somot ; Climatic Change  ; 25 septembre 2018 ; Future evolution of extreme precipitation in the Mediterranean

2. 16th Plinius Conference on Mediterranean Risks

3. A. E. Raftery, A. Zimmer, D.M.W Frierson, R. Startz and P. Liu ; Nature Climate Change  ; 31 juillet 2017 ; Less Than 2 degrees C Warming by 2100 Unlikely

Contact :  yves.tramblay@ird.fr