Varved lake sediments provide opportunities for high-resolution paleolimnological investigations that may extend monitoring surveys in order to target priority management actions under climate warming. This paper provides the synthesis of an international research program relying on >150 years-long, varved records for three managed perialpine lakes in Europe (Lakes Geneva, Annecy and Bourget). The dynamics of the dominant, local human pressures, as well as the ecological responses in the pelagic, benthic and littoral habitats were reconstructed using classical and newly developed paleo-proxies. Statistical modelling achieved the hierarchization of the drivers of their ecological trajectories. All three lakes underwent different levels of eutrophication in the first half of the XXth century, followed by re-oligotrophication. Climate warming came along with a 2°C increase in air temperature over the last century, to which lakes were unequally thermally vulnerable. Unsurprisingly, phosphorous concentration has been the dominant ecological driver over the last century. Yet, other human-influenced, local environmental drivers (fisheries management practices, river regulations) have also significantly inflected ecological trajectories. Climate change has been impacting all habitats at rates that, in some cases, exceeded those of local factors. The amplitude and ecological responses to similar climate change varied between lakes, but, at least for pelagic habitats, rather depended on the intensity of local human pressures than on the thermal effect of climate change. Deep habitats yet showed higher sensitivity to climate change but substantial influence of river flows. As a consequence, adapted local management strategies, fully integrating nutrient inputs, fisheries management and hydrological regulations, may enable mitigating the deleterious consequences of ongoing climate change on these ecosystems.
The Chari-Logone Basin (CLB), encompassing 610,000 km2 as the hydrologically active segment of the Lake Chad basin, is the focal point of this study aiming to characterize the evolution of vegetation cover in response to rainfall. The primary focus relies on spatialized precipitation and NDVI data. Analysis of these spatial rainfall data reveals trends that predominantly align with a south-north gradient typical of the Sudano-Sahelian region, with the highest values observed in the southwestern part of the basin, coinciding with the direction of the monsoon flow into the area. The NDVI results depict spatial and seasonal changes, mirroring the seasonal variation in climate, particularly rainfall. In the Chari-Logone basin, the vegetation undergoes a gradual transition from Sudanian to Sahelian vegetation, with grasses predominantly forming at the end of the dry season and regenerating only after the onset of the first rains. Key words: Chari-Logone, Lake Chad, rainfall, vegetation, spatio-temporel, NDVI.
L’assechement du lac dans le cadre de la desertification de la zone sahelienne est une idee courante au Fitri, tant chez les gestionnaires que parmi les populations residentes. Qu’en est-il exactement d’apres l’analyse diachronique d’images satellitaires et des donnees climatologiques ? Les resultats d’une etude en teledetection basee sur les images multispectrales Landsat montrent la variabilite intra-annuelle de la crue pour une annee (2015), et inter-annuelle entre 1972 (episode d’assechement du lac) et 2015. Si l’extension des eaux libres est plus importante aujourd’hui que dans les annees 1980, celle des terres de decrue exploitees pour l’agriculture et l’elevage est plus difficile a evaluer. Neanmoins, les images montrent clairement un deplacement des secteurs exploites vers l’exterieur du lac, egalement mentionne par les residents du lac.
The Tamga mountain range system is characterized by Holocene diatom-rich deposits, located along the nvers Oued El Abid and Tidjikja, dated between 9 630 14C B P and 7 550 I4C BP. The diatom studies provide the reconstruction of the palaeoenvironments of the wadi flood plain, composed of small lakes separated by narrow still water reaches. The results evidence a humid phase at 9 000/8 000 14C B P , followed by drier conditions after 8 000 I4C BP.
<p>In a near future, the Sahara and Sahelian regions could experience more rainfall than today as a result of climate change. Wetter conditions in the hottest and driest place of the planet today raise the question of whether the near future might hold in store environmental transformations, particularly in view of the growing human-induced climate, land-use and land-cover changes. Reflecting an enhancement of the global hydrological cycle under warmer conditions, some experiments provide support for the notion of a strengthening of the monsoon in the future and more rainfall in central Sahel and Sahara. However, some remote forcing could counterbalance the decadal trend. Modeling experiments suggest that the freshwater discharge coming from Greenland melting could significantly impact the sea surface temperature of North Atlantic and induce a decrease in Sahel rainfall for the next decades, remaining left open the question how Sahara will be in a warmer climate?</p><p>By chance, Lake Chad, located at the southern edge of the Sahara, is recognized for being the best site in Africa for deciphering hydrological and climate change. After being ranked at the world&#8217;s sixth largest inland water body with an open water area of 25,000 km<sup>2</sup> in the 1960s, it shrunk dramatically at the beginning of the 1970s and reached less than 2000 km<sup>2</sup> during the 1980s, decreasing by more 90% in area. Because it provides food and water to 50 millions of people, it becomes crucial to observe precisely its hydrological cycle during the last 20 years.</p><p>Here by using a new multi-satellite approach combined with ground-based observations, we show that Lake Chad extent has remained stable during the last two decades, slightly increasing at 14,000 km<sup>2</sup>. We extend further this reconstruction by adding new data from the hydrological year 2019-2020, which is considered at an extreme in precipitation recorded over the Sahel. Moreover, since the 2000s, groundwater which contributes to 70% of Lake Chad&#8217;s annual water storage, is increasing due to water supply provide by its two main tributaries draining a catchment area 610,000 km<sup>2</sup> wide. Because the current climate change seems to be characterize by a higher interannual variability affecting from year to year the amount of precipitation during the rainy season and increasing the vulnerability of the economy of the region mainly based of agropastoral activities, we investigate the yearly cycle and see how it is impacted the hydrological cycle of Lake Chad and changed over time.</p>
