The answer is far from obvious, but new knowledge now allows us to provide some elements.
Ahe department of Aude was placed on red alert on Monday, October 15, 2018 after very violent rains which caused, according to a provisional report, 12 deaths and several wounded. This summer, it is the monsoon storms of August which caused in the south-west of India the death of several hundred people and led to the displacement of thousands of others in the space of a few days.
On the Mediterranean rim, flash floods generally occur in autumn, when warm, humid air from the Mediterranean meets colder air on the continent. On October 3, 2015, 175 mm of rain was measured in two hours (equivalent to the average rainfall observed between September and October) over certain sectors of the Côte d'Azur, triggering a rapid rise in water levels that claimed 20 victims in the Cannes region.
More intense rainfall with global warming?
According to Clausius-Clapeyron's theorem, established in the first half of the 19th centurye century in the field of thermodynamics, the saturation steam pressure of air increases by about 7 % per degree of heating. In other words, the hotter the air is, the more water vapour it can hold.
In the context of global warming, scientists have hypothesized that extreme rainfall events should intensify by following Clausius-Clapeyron's law. which imposes a direct constraint on the amount of water available in the atmosphere. This hypothesis has since been confronted with numerous observations.
In 2013, a study showed that, overall, extreme daily precipitation have intensified during the XXᵉ centuryfrom 6 to 8 % per degree of global warming. But other work now indicates that intense short-duration, intense rainfall on the one-hour scale has intensified more rapidly than predicted by theory, with changes in theorder of 14 % per degree of heatingwhich is double that of the Clausius-Clapeyron relationship.
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This unexpected rate of intensification has also been detected recently. continental Australia as well as on the French Mediterranean rimfrom rainfall observation networks spanning more than five decades.
The origin of this phenomenon is still much debated in the scientific literature but the answer is most likely to be found in in cloud physics and in the release of latent heat at the time of condensation that "accelerates" convection (i.e. the upward movement in the atmosphere that causes many extreme rainfall events) and amplifies the intensity of the rains.
Other factors, such as the large-scale atmospheric circulation or the availability of moisture, can obviously regionally amplify or attenuate the rainfall response related to temperature, and explain the discrepancies between the observation and the Clausius-Clapeyron theory.
To simulate the future evolution of this intense rainfall, the scientific community uses climate models based on different climate change scenarios, models capable of simulating convection on fine meshes of the order of a kilometre. The results of these projections indicate an intensification of extreme rainfall events in different regions of the world including theAustralias, the United Statesand the United KingdomHowever, the rates of intensification vary across regions, seasons, models and greenhouse gas emission scenarios.
Theory, climate modelling and observations therefore seem to converge towards the same conclusion: rainfall has intensified in recent decades and will continue to intensify in the future as a result of global warming.
Will we see more flooding?
While rainfall intensity is essential, hydrologists have shown that other factors such as snowmelt, seasonal average flow, previous soil moisture conditions or land use also contribute to modulating the strength of flows.
The importance of each of these factors can vary by depending on season, region and watershed (i.e., the portion of the land drained by the watercourse and its tributaries).
A recent study journal entry ScienceThe study, to which Irstea researchers have contributed, indicates that global warming has already induced significant changes in the seasonality of floods across Europe.
Lower annual rainfall totals and drying of the soils in response to increased evaporation in these areas may have limited the rise in water levels during intense rainfall events.
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The size of the watersheds, and therefore their response time, is also a crucial element. Small watersheds are very sensitive to intense rainfall because it affects their entire surface area, whereas large watersheds are generally only partially affected and their response is more related to a slow accumulation of rainfall over several days or even weeks.
Resizing the infrastructure
We now have a high degree of certainty about the intensification of rainfall in recent decades and its future intensification. This trend could have a direct effect on flash floods in small watersheds. On the other hand projections are highly variable in space and more moderate for larger basins, due to the many additional factors influencing the rainfall-discharge relationship.
Our infrastructures (dams, bridges, etc.) have been designed and planned on the basis of periods of return of extreme rainfall and floods, assuming a stationary climate. However, this stationary hypothesis seems untenable in the current context. For example, Hurricane Harvey caused cumulative rainfall between August 25 and 28, 2017 in the Houston, Texas area, which corresponded to return periods greater than 1,000 years in the current climate.
To reduce our vulnerability to these hazards, we need to progressively revise the dimensioning of hydraulic works, water management and flood defence infrastructures as well as land use plans.
Renaud BarberoClimatology Researcher, Irstea ; Éric MartinRegional Director, Climate Change, Irstea ; Patrick ArnaudHydrology researcher, Irstea and Pierre JavelleHydrology researcher, Irstea
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