Since the end of July, the entire Sahel region has been affected by unusually large floods, from the Atlantic coast to Ethiopia and Somalia. They have struck towns and the countryside indifferently, but it is naturally in the former that the victims are the most numerous, both because of the concentration of populations, the waterproofing of soils and shortcomings in urban planning. .
No region in the world is immune to the damage caused by exceptional floods, whether those which affected Central Europe in 2002 or the Mississippi in 2011, or more localized phenomena such as flash floods. which ravaged the Nice hinterland in the fall of 2020.
What happened this summer in certain regions of the Sahel and particularly in that of the middle course of the Niger River, however, presents all the characteristics of an entry into a new hydroclimatic era.
Today, climate intensification manifests itself mainly in the form of localized shocks, the effects of which are propagated to the entire sub-region, which acts as a shock absorber. Continued temperature increases will increase the intensity and spread of these shocks, generating large-scale socio-economic imbalances, a trend that this year’s flooding may be a harbinger of.
Global warming and devastating floods
The recurring question asked of scientists during any extreme event is whether it can be “attributed to climate change”. A legitimate question since global warming exceeded 1 ° C in 2015 (compared to a reference from the start of the industrial era) and has never fallen below this value over the past five years.
Given the impossibility of doing “clinical trials” with a second Earth spared by the anthropogenic increase in the greenhouse effect, this question can be addressed via climate simulations in the factual world of the increase in CO.2 atmospheric, which we compare with simulations for a counter-factual world as it would have been without an increase in CO emissions2. This made it possible to irrefutably attribute the tendency of temperature growth to anthropogenic emissions, in accordance with the theory of the greenhouse effect.
The theory and models also agree on the fact that a warmer atmosphere has a potential for more violent rains, in particular because it can contain more humidity: this is called hydroclimatic intensification, as evidenced by recently over West Africa, where the dry periods are more severe, but interrupted by more intense rainy periods.
The attribution of extraordinary floods to global warming alone is a more delicate subject. Precipitation is not the only cause, socio-demographic dynamics and changes in land use also playing a role. It is therefore a problem of global changes of which global warming is only one component. It increases the probability of occurrence of an exceptional rainfall sequence, such as that of August 2020 in the Sahel, but its hydrological effects can be damped or amplified depending on whether the water absorption capacity of the soil-vegetation complex is more or less important.
The case of Niger: local flood and regional flood
To fully appreciate what happened this summer, a few reminders on the hydrology of Niger are necessary. This river has its source in the mountains of Fouta-Djalon in Guinea, where rainfall is more abundant than in the semi-arid regions that the river crosses downstream. This precipitation is driven by the African monsoon and reaches its maximum during the summer (June to September), producing a flood which spreads by amplifying as it rains in the region, in a similar way to the famous flood. of the Nile.
After the end of the rainy season, this flood continues to spread downstream by spreading, but remains well marked: it reaches its maximum in the Niamey region, some 2,600 kilometers downstream, between mid-December and mid-January. The value of the flood peak can vary greatly from year to year, depending on whether the season was more or less abundant in Guinea and the Sahel, and depending on the temporality of these rains – more or less distributed or concentrated in the region. during the season.
Dikes were built to cope with this annual rise in water levels. In Niamey, they were sized on the basis of observations made since a hydrometric measuring station was put into service in 1929. Until 2012, the maximum flow had never exceeded 2,500 m3/ s. Then observed for the first time, it caused the displacement of more than 500,000 people.
On September 8, 2020, the level of the river exceeded the 7-meter mark, a new record, which corresponds to an estimated flow rate of 3,300 m3/ s (this value is an estimate which needs to be specified, given that we reach there never measured levels and for which the rating curves must therefore be extrapolated). The unsuitable dikes broke in places, flooding all areas of the right bank – including the Abdou-Moumouni University.
Just as much as its exceptional level, it is the date of occurrence of this flood that marks a break with past observations. A local flood between August 15 and September 15 has always existed, but, until the turn of the XXIe century, its level remained significantly lower than that associated with the regional flood (in a ratio of about two thirds).
However, since 2010, we have regularly observed, as this year, a local flood that is stronger than the regional flood. In 2012, where the peak of the first was 2,500 m3 / s, the maximum of the second was only 1,700 m3/s.
A new hydrological era in the Sahel
The rains observed in the region are clearly in excess this year, with particularly high anomalies in the Middle Niger region, where they locally reach nearly 200%. In the city of Niamey, however, they did not exceed the records observed in 1952 or 1998.
In the Sudanese climate band, between the Sahel and the Gulf of Guinea, rainfall is on the contrary in deficit. This dipole situation occurs when the intertropical convergence zone (ITCZ) is located at abnormally northern latitudes. There is nothing abnormal in itself and is one of the characteristic elements of natural climatic variability in this tropical region where the rains are driven by the African monsoon.
The floods of 2020, exceptional though they may be, are also part of an upsurge in these phenomena throughout West Africa over the past fifteen years, an upsurge that can be attributed to the ‘combined effect of increased rainfall and changes in land use: recent work carried out at the Institute of Environmental Geosciences shows that, on the tributaries of Niger, the ten-year flood of the 1960s now has a chance in two to occur every year, even though the annual rainfall remains lower than it was during this wet period.
Adapt to this new context
In addition to the intense rains that began on August 8 in the city of Niamey and more runny soils, there are – at least – three other phenomena that played an important role in the exceptional flood of 2020: on the one hand, strong inflows from tributaries on the right bank upstream of the city, caused by heavy rains over northeastern Burkina Faso from the end of July. On the other hand, the rise of underground water, linked to a lasting modification of regional hydrology. Finally, silting up of the Niger bed at Niamey, which results in a higher rating for the same flow.
The unprecedented floods that hit Niamey in September, and much of the region beyond, are therefore in reality the result of heavy regional trends and more circumstantial factors linked to the structure of the 2020 rainy season.
This new hydroclimatic era involves the implementation of adaptation policies, already widely identified: agroforestry and small works in rural areas to facilitate water infiltration, revision of sanitation schemes in cities and, moreover, general, updating of hydrological standards which serve as a basis for the sizing of infrastructures.
The fact that there are adaptation levers should not, moreover, make us forget the responsibility of the countries which emit high GHGs, which must take drastic and rapid mitigation measures to prevent the countries most vulnerable to the effects of global warming. climate are less and less habitable.
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