Authors : Binh pham-Duc, Florence Sylvestre, Fabrice Papa, Frédéric Frappart, Camille Bouchez and Jean-Francois Crétaux
Affiliated organisation : Nature Research
Site of publication : nature.com
Type of publication : Scientific Report
Date of publication : March 2020
Severe and recurrent droughts are the principal weather-related hazards for developing economies throughout sub-Saharan Africa, and the quality of long-term weather prediction is a bottleneck hampering drought mitigation and adaptation in the region. This is aggravated by uncertain impacts of the 21st century anthropogenic climate change on the continent’s rainfall and freshwater resources, due to at best fragmentary understanding on the effects of a warming atmosphere on the hydrological cycle at regional scales.
In this context, Lake Chad, located in the central Sahel sector, at the southern edge of the Sahara, rises up as a symbol of the current global climate change occurring in the region. After being ranked at the world’s sixth largest inland water body with an open water area of 25,000 km2 in the 1960s, it shrunk dramatically at the beginning of the 1970s and reduced to less than 2,000 km2 during the 1980s, decreasing by more than 90% its area.
The consequence of the 1970s and 1980s droughts was the subdivision of the lake into a northern pool and a southern pool, and the regular dryness of the northern pool alerted the international community of a possible lake’s disappearance.
Now, in the view of the challenges that have arisen in the Lake Chad area during the last decades, it is crucial to better characterize the Lake Chad hydrological cycle. Indeed, the Lake Chad region is currently facing multiple security risks, including livelihood and violent conflicts.
Even though the current conflict was triggered by violence linked to the armed groups known as Boko Haram, the crisis has deep roots in longstanding challenges. Widespread inequality and decades of political marginalization have instilled and entrenched sense of exclusion in the region. But several observations demonstrated that these challenges are further exacerbated by climate change.
Lake Chad – A complex hydrological system
The Lake Chad drainage basin covers ~2.5 × 106 km2, representing ~8% of the African continent. It is a hydrologically closed drainage system in the Central Sahel region of northern Africa, characterized by a south to north climatic gradient as a consequence of latitudinally decreasing rainfall.
Lake Chad is the terminal lake of this drainage system. Today, it is a shallow lake (<3 m), subdivided most of the time into three areas, the northern and the southern pools separated by an east-west vegetation-covered sand barrier named ‘The Great Barrier’. The third one located at the eastern part of the lake, named ‘The Archipelagos’, corresponds to an inland area formed by sand dunes which are inundated according to the seasonally water inflow into the lake.
Now, in the view of the challenges that have arisen in the Lake Chad area during the last decades, it is crucial to better characterize the Lake Chad hydrological cycle. Indeed, the Lake Chad region is currently facing multiple security risks, including livelihood and violent conflicts
Land Surface Water Extent during the last two decades
Times series and anomaly of the surface water extent of Lake Chad show that the southern pool area is quite stable over the last two decades, with a nearly flat linear trend, meaning that its water surface current has not been affected by drastic changes since the 2000s. The northern pool shows higher variability, with completely dry periods during dry seasons between 2005 and 2012, whereas it was partly inundated during dry seasons in other periods.
However, compared to the 1970s and 1980s when the northern pool was totally dry during both dry and wet seasons, until the beginning of the 2000s, water was still coming every year during the rainy season with an amount variable from year to year.
As a consequence, the total surface water extent of Lake Chad slightly reduced over the last 20 years, mostly due to the decreasing trend of the northern pool.
With the lowering of the lake during the 1970s and the 1980s combined with the increase of temperature, Lake Chad permanent vegetation cover increased by ~30% during the last two decades. Since the severe droughts during the 1980s, Lake Chad split into two pools, increasing favorable conditions for the vegetation growth, especially over the Great Barrier. For instance, it has been suggested by modeling experiments that without the split between the two pools during the 1980s, the northern pool should never dry out and should have recovered 82% of its 1963’s water area which was one of the highest extensions of the lake during the last 60 years.
Lake Chad in context of the climate change
For now, the ongoing climatic changes are expected to enhance the global hydrological cycle under the Tropics and contribute to increased precipitation.
Several modeling experiments demonstrated that despite the important uncertainties to reproduce properly the tropical convection over the Sahelian area, most of the global simulations are showing an increase in tropical rainfall, especially in the central Sahel.
As a consequence, the total surface water extent of Lake Chad slightly reduced over the last 20 years, mostly due to the decreasing trend of the northern pool.
It was evidenced that during global cold phases, the lake is shrinking, whereas during global warm phases, the lake is rising. However, some remote forcing factors could counterbalance the decadal trend. It has been evidenced that the sea surface conditions from North Atlantic ocean are closely linked with the rainfall in Sahel, and the differential gradient between tropics and extratropics acts as a driver of rainfall through an atmospheric pathway which triggers the atmospheric circulation over the Sahel and Sahara.
In summary, our findings show that Lake Chad’s surface water extent slightly reduced over the last two decades, mostly in the northern pool due to the increase of evaporation and vegetation cover, as well as the decrease of the Komadugu Yobe discharge.
However, the southern pool extent is stable and even, slightly increasing, as a consequence of stable local rainfall and the increase of the Chari-Logone river discharge. Despite the decrease in the northern pool, Lake Chad’s SWS shows an increasing trend for a 13-year period. The subsurface contributes to ~70% of Lake Chad’s TWS, meaning that most of its water is stored in soil moisture, and especially in groundwater with an estimated aquifer deep of ~70 m. As a consequence, despite the uncertainties in predicting the future climate, for now Lake Chad is not disappearing.
Several modeling experiments demonstrated that despite the important uncertainties to reproduce properly the tropical convection over the Sahelian area, most of the global simulations are showing an increase in tropical rainfall, especially in the central Sahel
When considering the amount of water stored in the groundwater reservoir accessible by pumping, with the recharge rate in the active basin, it represents today one of the best opportunities for buffering the huge inter-annual variability in rainfall that characterizes the current climate changes in Sahel.
This groundwater reservoir needs to be considered with attention for being integrated in appropriate development strategy and better managing the increasing pressure on resources in consequence of rapid population growth in Sahel.
Les Wathinotes sont soit des résumés de publications sélectionnées par WATHI, conformes aux résumés originaux, soit des versions modifiées des résumés originaux, soit des extraits choisis par WATHI compte tenu de leur pertinence par rapport au thème du Débat. Lorsque les publications et leurs résumés ne sont disponibles qu’en français ou en anglais, WATHI se charge de la traduction des extraits choisis dans l’autre langue. Toutes les Wathinotes renvoient aux publications originales et intégrales qui ne sont pas hébergées par le site de WATHI, et sont destinées à promouvoir la lecture de ces documents, fruit du travail de recherche d’universitaires et d’experts.
The Wathinotes are either original abstracts of publications selected by WATHI, modified original summaries or publication quotes selected for their relevance for the theme of the Debate. When publications and abstracts are only available either in French or in English, the translation is done by WATHI. All the Wathinotes link to the original and integral publications that are not hosted on the WATHI website. WATHI participates to the promotion of these documents that have been written by university professors and experts.