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Rapid Pleistocene desiccation and the future of Africa's Lake Victoria
Abstract
The effects of precipitation changes on tropical East African ecosystems and human populations is poorly understood due to the complex interplay between global and regional processes and missing data from key regions and time periods. We generate a water-budget model for Lake Victoria, the largest tropical lake in the world, the source of the White Nile, and a region that supports some of the densest human populations in Africa, that assesses the impact of changing climate on lake levels and the rate of lake level change. Model results demonstrate that significant changes in the size and volume of Lake Victoria are possible in response to changes in temperature, precipitation, and orbital forcing. This modeling indicates that Lake Victoria can transition back and forth between modern lake levels and complete desiccation in centuries to a few millennia, which is rapid enough to allow for two previously observed desiccation events between 14-18 ka, during which time the lake drained and refilled twice. Combined observations from modeling and estimates of paleoprecipitation indicate that Lake Victoria was likely desiccated between 94-36 ka. This dry interval partially overlaps the megadrought (140-70 ka) identified in Lakes Malawi and Tanganyika further south, and the cooler, drier conditions identified in the Gulf of Aden between 75-50 ka. This prolonged desiccation was probably driven by eccentricity-enhanced precession and high-latitude forcing that affected the Congo Air Boundary convergence. Using future climate projections, our model also predicts that at current rates of temperature change and previous rates of lake level fall, Lake Victoria could have no outlet to the White Nile within 10 years, and Kenya could lose access to the lake in <400 years, which would significantly affect the economic resources supplied by Lake Victoria to the East African Community.
... Lake Victoria is also home to a large array of haplochromine cichlid fish, whose diversity, origins, and endemism are the focus of continued significant research (e.g., Seehausen, 1996;Meier et al., 2017;Muschick et al., 2018;Verheyen et al., 2003). Lake Victoria is an extremely sensitive archive of past hydroclimate variability (e.g., Vanderkelen et al., 2018;Beverly et al., 2020;Ogondo et al., 2022). It completely desiccated at least once in the Late Pleistocene (e.g., Johnson et al., 1996) and has likely desiccated and refilled multiple times since it formed (e.g., Scholz et al., 1998). ...
... Today, rainfall in the Lake Victoria basin is distributed bimodally, with boreal spring Figure 1. Map of equatorial eastern Africa with the modern extent of Lake Victoria (modified after Beverly et al., 2020, andTryon et al., 2015). Yellow squares denote previously collected short lake sediment core locations, and purple diamonds mark archaeological sites. ...
... A significant portion of the water input into Lake Victoria (50 %-80 % of total input) comes from precipitation falling directly over the lake rather than from drainage running into the lake from the catchment area (Flohn and Burkhardt, 1985;Yin and Nichol-son, 2002;Tate et al., 2004). Evaporation, while important for the lake's water balance, has low variability in modern Lake Victoria, and hence variations in lake levels are primarily controlled by changes in rainfall (Yin and Nicholson, 2002;Smith and Semazzi, 2014;Beverly et al., 2020). ...
Lake Victoria, which is bordered by Uganda, Tanzania, Kenya, and has a catchment that extends to Rwanda and Burundi, is home to the largest human population surrounding any lake in the world and provides critical resources across eastern Africa. Lake Victoria is also the world's largest tropical lake by surface area, but it is relatively shallow and without a major inlet, making it very sensitive to changes in climate, and especially hydroclimate. Furthermore, its size creates abundant habitats for aquatic fauna, including the iconic hyper-diverse cichlids, and serves as a major geographic barrier to terrestrial fauna across equatorial Africa. Given Lake Victoria's importance to the eastern African region, its sensitivity to climate, and its influences on terrestrial and aquatic faunal evolution and dispersal, it is vital to understand the connection between the lake and regional climate and how the lake size, shape, and depth have changed through its depositional history. This information can only be ascertained by collecting a complete archive of Lake Victoria's sedimentary record. To evaluate the Lake Victoria basin as a potential drilling target, ∼ 50 scientists from 10 countries met in Dar es Salaam, Tanzania, in July 2022 for the International Continental Scientific Drilling Program (ICDP)-sponsored Lake Victoria Drilling Project (LVDP) workshop. Discussions of the main scientific objectives for a future drilling project included (1) recovering the Pleistocene and Holocene sedimentary records of Lake Victoria that document the dynamic nature of the lake, including multiple lacustrine and paleosol sequences; (2) establishing the chronology of recovered sediments, including using extensive tephra fingerprinting and other techniques from deposits in the region; (3) reconstructing past climate, environment, lacustrine conditions, and aquatic fauna, using an integrated multi-proxy approach, combined with climate and hydrologic modeling; and (4) connecting new records with existing sedimentary snapshots and fossils exposed in deposits around the lake, tying archaeological, paleontological, sedimentological, tectonic, and volcanic findings to new drilling results. The LVDP provides an innovative way to address critical geological, paleontological, climatological, and evolutionary biological questions about Quaternary to modern landscapes and ecosystems in eastern Africa. Importantly, this project affords an excellent opportunity to help develop conservation and management strategies for regional responses to current and future changes in climate, land use, fisheries, and resiliency of at-risk communities in equatorial Africa.
... Precipitation occurs in two rainy seasons (March to May and October to December) as a result of the changing position of mesoscale convection (Nicholson, 2018) and is, on long time-scales, influenced by the seasonal migration of the Afrotropical rain belt, the position of the Congo Air Boundary and the strength of the Indian and Atlantic monsoons predominantly controlled by orbital, greenhouse gas and North Atlantic forcing (Beverly et al., 2020;Castañeda et al., 2016;Nicholson, 2018;Stager and Johnson, 2000;Verschuren et al., 2009). ...
... Today, the water budget of the lake is mainly controlled by direct precipitation and evaporation (Yin and Nicholson, 1998). A climate-water-budget model for Lake Victoria (Beverly et al., 2020) suggests that, depending on paleoclimate scenarios including temperature, precipitation, and orbital forcing, Lake Victoria can completely dry out and re-fill to modern levels within centuries to a few millennia. Importantly, in the flat lake basin, small changes in water depth translate into large changes in lake surface area (Olaka et al., 2010). ...
... higher cloudiness which reduces evaporation (Yin and Nicholson, 1998) and, (iii) the size of the lake controls precipitation in the catchment; today ca. 80 % of the inflow in Lake Victoria is recycled moisture from the large lake itself (Beverly et al., 2020;Yin and Nicholson, 2002). Accordingly, Lake Victoria would dry out within centuries if precipitation rates fell below 75 % of modern values (Beverly et al., 2020). ...
Reconstructing hydrological variability is critical for understanding Lake Victoria's ecosystem history, the evolution of its diverse endemic fish community, the dynamics of vegetation in the catchment, and the dispersal of aquatic and terrestrial fauna in the East African Rift system during Latest Pleistocene and Holocene times. Whereas consensus exists on widespread desiccation of Lake Victoria ~18-17 ka, the refilling history (16-13 ka) has remained highly controversial. Here, we present data from four new sediment cores along a depth transect. We use lithostratigraphic core correlation, sediment facies, XRF data, wetland vegetation analysis (Typha pollen), and 14 C chronologies of unprecedented precision to document Latest Pleistocene lake-level variability. At our coring site in the central basin, local Typha wetlands existed >16.7 ka, alternating with periods of desiccation. Moisture increased slightly between ca. 16.7-14.5 ka and wetlands with permanent, shallow ponds established simultaneously in the center and the marginal, more elevated parts of the flat lake basin. After ca. 14.0 ka, lake levels increased; wetlands in the central basin were submerged and replaced by lacustrine environments and a >50 m deep lake established ca. 13.5 ka, likely with intermittent overflow most of the time. The lake reached modern or even above-modern levels around 10.8 ka. This lake-level history is consistent with regional terrestrial paleoenvironmental reconstructions, notably the expansion of Afromontane and rainforest. Our data suggest a complex picture of paleoclimatic conditions in Eastern Africa and tele-connections to the North-Atlantic and Indian Ocean domains.
... The paleontology, archaeology, and geology of both localities have been the subject of ongoing research by the Lake Victoria Prehistory Project since 2009. Rusinga is an island in Lake Victoria connected to mainland Kenya during much of the LGP (Tryon et al., 2016;Beverly et al., 2020), with the Pleistocene Wasiriya Beds yielding abundant large mammal fauna collected across multiple stratigraphic intervals spanning ∼100-36 ka (Tryon et al., 2010. Rusinga's fossil record is dominated by R. atopocranion, with this single species accounting for 34% of the recovered mammalian fauna (Tryon et al., 2016). ...
... Additionally, three out of four individuals analyzed have notably high amplitude fluctuations, indicating movement during the period of tooth formation. Lake Victoria was desiccated during the time when these animals lived, allowing for movement to and from islands like Rusinga (Beverly et al., 2020), but the high 87 Sr/ 86 Sr values of Rusinga individuals (>0.708) do not suggest that these individuals roamed over the dry bed of Lake Victoria during tooth development, as Quaternary lake sediments would likely have low 87 Sr/ 86 Sr values like those of Rusinga (∼0.705). However, more information is needed regarding the true geologic nature of the sediments and bedrock underlying Lake Victoria. ...
... Migrating long distances would be possible in the eastern Lake Victoria Basin so long as grassy ecosystems persisted and there were no obstacles along the migration path. From 100-36 ka, this would have likely been the case, as all lines of evidence suggest reduced lake levels (Beverly et al., 2020) and extensive grasslands (Marean, 1992;Steele, 2007;Faith et al., 2012Faith et al., , 2015Tryon et al., 2012Tryon et al., , 2016Beverly et al., 2015). However, the previously desiccated Lake Victoria filled after 36 ka (Beverly et al., 2020). ...
For many animals, migration is an important strategy for navigating seasonal
bottlenecks in resource availability. In the savannas of eastern Africa, herds of
grazing animals, including blue wildebeest (Connochaetes taurinus), Thomson’s
gazelle (Eudorcas thomsonii), and plains zebra (Equus quagga), travel hundreds
of kilometers annually tracking suitable forage and water. However, we know
nearly nothing about migration among the extinct species that often dominated
Late Pleistocene communities. Using serially sampled 87Sr/86Sr and δ13C, we
characterize the prehistoric movement and diet of the enigmatic wildebeest
Rusingoryx atopocranion from two localities (Karungu and Rusinga Island) in
the Lake Victoria Basin of western Kenya. We find clear evidence for migration
in all four individuals studied, with three 87Sr/86Sr series demonstrating highamplitude fluctuations and all falling outside the modeled isoscape 87Sr/86Sr
ranges of the fossil localities from which they were recovered. This suggests
that R. atopocranion exhibited migratory behavior comparable to that of its
closest living relatives in the genus Connochaetes. Additionally, individuals show
seasonally-variable δ13C, with a higher browse intake than modern and fossil
eastern African alcelaphins indicating behavioral dierences among extinct taxa
otherwise unrecognized by comparison with extant related species. That this
species was highly migratory aligns with its morphology matching that of an
open grassland migrant: it had open-adapted postcranial morphology along with
a unique cranial structure convergent with lambeosaurine dinosaurs for calling
long distances. We further hypothesize that its migratory behavior may be linked
to its extinction, as R. atopocranion disappears from the Lake Victoria Basin fossil
sequence coincident with the refilling of Lake Victoria sometime after 36 ka,
potentially impeding its past migratory routes. This study characterizes migration
in an extinct eastern African species for the first time and shapes our ecological
understanding of this unique bovid and the ecosystems in which Middle Stone Age
humans lived.
... Lake Victoria is the second-largest freshwater lake in the world and the largest lake in Africa. The lake supports more than 30 million people in the three countries (Uganda, Kenya, and Tanzania) around it and modulates the regional climate [1][2][3]. It plays an important role in regional sustainable development and in the protection of the ecological environment [4,5]. ...
... Lake Victoria (0 • 31 N-3 • 05 S, 31 • 35 E-34 • 54 E) is the world's largest tropical lake, with a surface area of~69,000 km 2 [3]. It is in the western part of the African Great Rift Valley [1,2] (Figure 1), mainly located in north-western Tanzania and southern Uganda and bordering on eastern Kenya. Lake Victoria is a shallow lake with a mean depth of 85 m and a mean water storage of~2760 km 3 [7]. ...
... Lake Victoria (0°31′N-3°05′S, 31°35′E-34°54′E) is the world's largest tropical lake, with a surface area of ~69,000 km 2 [3]. It is in the western part of the African Great Rift Valley [1,2] (Figure 1), mainly located in north-western Tanzania and southern Uganda and bordering on eastern Kenya. Lake Victoria is a shallow lake with a mean depth of ~85 m and a mean water storage of ~2760 km 3 [7]. ...
To effectively monitor the spatio–temporal dynamics of the surface water extent (SWE) in Lake Victoria, this study introduced a novel methodology for generating a seamless SWE time series with fine resolution by integrating daily a Moderate-resolution Imaging Spectroradiometer (MODIS) and Landsat imagery. In the proposed methodology, daily normalized difference vegetation index (NDVI) time series data with 30 m resolution were first generated based on the constructed pixel-by-pixel downscaling models between the simultaneously acquired MODIS-NDVI and Landsat-NDVI data. In the compositing process, a Minimum Value Composite (MinVC) algorithm was used to generate monthly minimum NDVI time series, which were then segmented into a seamless SWE time series of the years 2000–2020 with 30 m resolution from the cloud background. A comparison with the existing Landsat-derived JRC (European Joint Research Centre) monthly surface water products and altimetry-derived water level series revealed that the proposed methodology effectively provides reliable descriptions of spatio–temporal SWE dynamics. Over Lake Victoria, the average percentage of valid observations made using the JRC’s products was only about 70% due to persistent cloud cover or linear strips, and the correlation with the water level series was poor (R2 = 0.13). In contrast, our derived results strongly correlated with the water level series (R2 = 0.54) and efficiently outperformed the JRC’s surface water products in terms of both space and time. Using the derived SWE data, the long-term and seasonal characteristics of lake area dynamics were studied. During the past 20 years, a significant changing pattern of an initial decline followed by an increase was found for the annual mean SWE, with the lowest area of 66,386.57 km2 in 2006. A general seasonal variation in the monthly mean lake area was also observed, with the largest SWE obtained during June–August and the smallest SWE observed during September–November. Particularly in the spring of 2006 and the autumn of 2020, Lake Victoria experienced intense episodes of drought and flooding, respectively. These results demonstrate that our proposed methodology is more robust with respect to capturing spatially and temporally continuous SWE data in cloudy conditions, which could also be further extended to other regions for the optimal management of water resources.
... Importantly, fossils of R. atopocranion co-occur with hominin fossils attributed to H. sapiens (Grine, 2016;Pearson et al., 2020) and Middle Stone Age (MSA) artifacts (Tryon et al., 2010(Tryon et al., , 2012Faith et al., 2015;Blegen et al., 2017;Jenkins et al., 2017), the latter providing the archaeological context of early modern humans in eastern Africa (Tryon and Faith, 2013;Tryon, 2019). Past work in the Lake Victoria Basin has documented the expansion of Serengeti-like grasslands across the region in the late Pleistocene (e.g., Tryon et al., 2010Tryon et al., , 2012Tryon et al., , 2016Faith et al., 2015;Garrett et al., 2015), likely in response to increased aridity and desiccation of the lake (e.g., Beverly et al., 2015aBeverly et al., , 2017Beverly et al., , 2020. This interpretation has been heavily influenced by the fossil faunas, including inferences based on the dominance of R. atopocranion, which was assumed to have had an affinity for open grassland habitats similar to extant alcelaphins (e.g., Faith et al., 2011;. ...
... Analyses of ancient soils, associated fossil taxa, and bathymetric reconstructions suggest that open and grassy habitats were widespread throughout much of the late Pleistocene in the Lake Victoria Basin. The region was considerably drier than modern times from ∼100-36 ka, which probably resulted in the complete desiccation of Lake Victoria and an expansion of a Serengeti-like ecosystem across the basin Beverly et al., 2015aBeverly et al., , b, 2017Beverly et al., , 2020Faith et al., 2015). ...
... In the absence of younger sediments around Lake Victoria and a more clear understanding of when Rusingoryx made its last appearance in the basin, it is difficult to pin down the reasons for its demise. However, we note that given its clear affinity for open habitats, it is likely that Rusingoryx inhabited the grassy plains that were exposed by the reduction of Lake Victoria due to the dry conditions that persisted throughout the ∼100-36 ka interval Beverly et al., 2015aBeverly et al., , b, 2017Beverly et al., , 2020Faith et al., 2015). ...
Rusingoryx atopocranion is an extinct alcelaphin bovid from the late Pleistocene of Kenya, known for its distinctive hollow nasal crest. A bonebed of R. atopocranion from the Lake Victoria Basin provides a unique opportunity to examine the nearly complete postcranial ecomorphology of an extinct species, and yields data that are important to studying paleoenvironments and human-environment interaction. With a comparative sample of extant African bovids, we used discriminant function analyses to develop statistical ecomorphological models for 18 skeletal elements and element portions. Forelimb and hin-dlimb element models overwhelmingly predict that R. atopocranion was an open-adapted taxon. However, the phalanges of Rusingoryx are remarkably short relative to their breadth, a morphology outside the range of extant African bovids, which we interpret as an extreme open-habitat adaptation. It follows that even recently extinct fossil bovids can differ in important morphological ways relative to their extant counterparts, particularly if they have novel adaptations for past environments. This unusual phalanx morphology (in combination with other skeletal indications), mesowear, and dental enamel stable isotopes, demonstrate that Rusingoryx was a grassland specialist. Together, these data are consistent with independent geological and paleontological evidence for increased aridity and expanded grassland habitats across the Lake Victoria Basin.
... Additionally, the refilling of Lake Victoria shortly after ~36 ka (ref. 35), which had been desiccated for much of the LGP, would have also impacted dust regimes, potentially altering the 87 Sr/ 86 Sr values measured around parts of the lake today. ...
... The Pleistocene archaeology, palaeontology and geology of these localities have been investigated by the Lake Victoria Prehistory Project since 2009, continuing to today. Rusinga is an island in Lake Victoria that was connected to the mainland during the LGP when Lake Victoria was desiccated 35 , with Pleistocene exposures yielding abundant faunal remains alongside Middle Stone Age (MSA) artefacts. Specimens have been collected across multiple stratigraphic intervals spanning ~100-36 ka (ref. ...
Eastern Africa is home to the largest terrestrial migrations on Earth. Though these migratory systems have been well studied for decades, little is known of their antiquity and evolutionary history. Serially sampled strontium stable isotopes (⁸⁷Sr/⁸⁶Sr) from tooth enamel can be used to track migration in mammals. Here we analyse ⁸⁷Sr/⁸⁶Sr for 79 bovid and equid individuals representing 18 species from four localities in Kenya to characterize prehistoric migratory systems during the Last Glacial Period (115–11.7 ka). Of the species analysed, 16 lack definitive evidence for migration, including blue wildebeest (Connochaetes taurinus), Thomson’s gazelle (Eudorcas thomsonii) and plains zebra (Equus quagga), which are long-distance migrants today in the Greater Serengeti Ecosystem and historically in the Athi-Kapiti Plains. Only two species, the extinct wildebeests Rusingoryx atopocranion and Megalotragus sp., were migratory. These findings suggest a possible alternative narrative about ecosystem dynamics during the Last Glacial Period and shed light on the behaviour of both extant and extinct species at this time. In particular, these results indicate that migratory behaviour in extant species either emerged during the Holocene or was more spatiotemporally constrained in the past. Our results contribute to a growing body of evidence suggesting that the structure and function of geologically recent large mammal communities in eastern Africa differed considerably from those observed in the present day.
... Its formation can be attributed to the reverse flow resulting from the damming of rivers flowing westward, brought about by the uplifting of the western section of the East African Rift System (EARS; Danley et al., 2012;Johnson et al., 2000). Since its formation, the lake has undergone alterations over time, but there is no evidence of further tectonic modifications since the middle Pleistocene (Beverly et al., 2020). It is located at an altitude of 1135 m above sea level (m asl), and it has a maximum water depth of 68 m (Johnson et al., 1996;Fig. ...
... The southward shift of the Afrotropical rainbelt and reorganization of the monsoon circulation, in combination with a weakening of regional rainfall systems at ca. 17,000 cal yr BP, altered the climate at lower elevation sites causing aridity, which played an important role in Tanzania, Ghana, the Niger-Sanaga and Congo watersheds (Stager et al., 2011). At this time, Lake Victoria was experiencing a desiccation phase, as evidenced by seismic data and the presence of a palaeo-vertisol (Beverly et al., 2020;Johnson et al., 1996;Stager et al., 2011). In the surrounding region, grassland savanna became the dominant landscape feature, alongside the coexistence of Afromontane vegetation, until temperatures started to gradually rise by ca. 2 • C before 15,500 cal yr BP in eastern Africa (Berke et al., 2012;Figs. ...
... Substantial changes in vegetation structure are implied by the Late Pleistocene faunas, which are dominated by alcelaphin antelopes, equids, and other open-habitat grazers, collectively indicating an expansion of semi-arid grasslands Tryon et al., 2016). Estimates of annual precipitation derived from geochemical analysis of paleosols (∼750-1,000 mm/yr) imply a reduction in rainfall relative to the present-day (∼1,400 mm/yr) (Beverly et al., 2015a(Beverly et al., , 2017, with a recent water-budget model indicating that this would lead to desiccation of Lake Victoria (Beverly et al., 2020). Thus, with respect to the portion of the Late Pleistocene sequence studied thus far (∼100-36 ka), Lake Victoria is likely to have been substantially reduced, if not completely desiccated, with at least the eastern margin of the basin dominated by grassland vegetation. ...
... The mechanisms responsible for the recent range contractions of white rhinoceros and southern reedbuck are uncertain. Human impacts have been suggested as playing a (Robertshaw et al., 1983), but this demands further evaluation given the limited evidence for anthropogenic impacts on Africa's faunas deeper in time (e.g., Faith, 2014;Faith et al., 2020). ...
We report on the Late Pleistocene (36-12 ka) mammals from Kibogo in the Nyanza Rift of western Kenya, providing (1) a systematic description of the mammal remains, (2) an assessment of their paleoenvironmental implications, and (3) an analysis of the biogeographic implications of non-analog species associations. Kibogo has yielded one of the largest paleontological assemblages from the Late Pleistocene of eastern Africa, and it is dominated by grassland ungulates (e.g., equids and alcelaphin antelopes), including an assortment of extralimital (e.g., Equus grevyi, Ceratotherium simum, Redunca arundinum) and extinct species (Syncerus antiquus, Damaliscus hypsodon, Megalotragus sp.). The composition of the fauna, in conjunction with the soils and topography of the region, indicate the local presence of edaphic grassland situated within a broader environment that was substantially grassier and likely drier than at present. In contrast to non-analog faunas from higher latitudes (e.g., North America and western Eurasia), the climatic niches of non-analog species associations strongly overlap, indicating that non-analog climate regimes during the Late Pleistocene of eastern Africa are not necessary to account for the former association of presently allopatric species. The Kibogo faunas add to a growing body of evidence implying that the composition of present-day African herbivore communities is distinct from those of the geologically recent past.
... The majority (90%) of stations show a mean Deuterium (D) excess value higher than the global average of 10 computed by [50]. The high D-excess values (>10) observed in local precipitation, along with findings from studies by [10,68,69], suggest local atmospheric moisture recycling processes within the region. This localized effect can be attributed to the influence of the Great Lakes (particularly Lake Victoria) on local precipitation. ...
This study characterized groundwater resources for the Nakivale sub-catchment of the transboundary Victoria Basin in Uganda using classical hydrochemical and stable isotopic approaches. Groundwater in the study area is essential for domestic, agricultural, and industrial uses. As a sub-domain of the larger Victoria Basin, it also plays a crucial role in shaping the hydrological characteristics of this vital transboundary basin, both in terms of quality and quantity fronts. This makes its sustainable management and development vital. The predominant groundwater type is Ca-SO4, with other types including Ca-HCO3, Na-Cl, Na-HCO3, and Ca-Mg-SO4-Cl. Hydrochemical facies analysis highlights the importance of rock–water interactions in controlling groundwater chemistry, mainly through incongruent chemical weathering of Ca-rich plagioclase feldspars and the oxidation of sulfide minerals, such as pyrite, which are prevalent in the study area. Groundwater recharge is primarily influenced by the area’s topography, with recharge zones characterized by lineament networks, located in elevated areas. Stable isotope analyses indicate that groundwater mainly originates from local precipitation, while tritium data suggest the presence of both recent and older groundwater (likely over 20 years old). The study’s comprehensive approach and findings contribute significantly to the understanding of groundwater systems in the region, thus providing valuable insights for policymakers and stakeholders involved in water resource management and development strategies.
... The type of flooding of greatest concern was associated with flooding of land areas by rising lake water levels. Basically due to the increase in rainfall over Lake Victoria (from which it receives about 80% of water, depending on inter-annual variability [41,42]), but also due to increased inflows from its tributaries, the majority of the simulations projected that water levels would rise. An increase from 20 cm to 70 cm was projected under ssp126 around 2050 and ssp370 in the distant future, respectively and with 70% to 90% model agreement ( figure B16). ...
Understanding people's perceptions of climate change and associated environmental risks is paramount in assessing how individuals respond to climate change. Awareness of the consequences of climate change determines the present and future behaviours and expectations, as well as the actions taken to mitigate the likely impacts. We surveyed the perceived and expected climate change consequences of experts and community members in the Lake Victoria basin in East Africa, compared them with hydro-meteorological observations and projections, and established that some perceived trends, such as increasing temperature or rainfall intensity, correspond with meteorological observations. However, the perceived increase in drought occurrence (believed to be a recent consequence), was not substantiated by the meteorological data. It was only in the northwestern region that drought frequency increased since the year 2000, while the rest of the basin did not experience such a trend. Community members were concerned about the already noticeable consequences of climate change on their livelihoods through agriculture or fishing, while experts were mainly focused on the amplification of hazards such as floods and droughts. This divergence may imply that experts underestimate the consequences that society is already facing. Nevertheless, both groups expect that climate change will undoubtedly lead to the deterioration of human well-being by affecting food security, increasing poverty, and increasing the incidence of disease. This is a serious concern that requires immediate attention. Such insights into people's climate change perceptions can help policy-makers, researchers, and community members to better tailor adaptation solutions acceptable to the local context. Effective governance is essential to enable people to adapt to climate change and other challenges, including those resulting from the impacts of globalisation, demographic trends, and the degradation and scarcity of resources.
... The majority (90%) of stations show a mean Deuterium (D) excess value higher than the global average of 10 computed by [47]. The high D-excess values (>10) observed in local precipitation, along with findings from studies by [7,62,63], suggest local atmospheric moisture recycling processes within the region. This localized effect can be attributed to the influence of the Great lakes (particularly lake Victoria) on local precipitation. ...
This study characterized groundwater resources for the Nakivale sub-catchment of the transboundary Victoria basin in Uganda using classical hydrochemical and stable isotopic approaches. Groundwater in the study area is essential for domestic, agricultural, and industrial uses. As a sub-domain of the larger Victoria Basin, it also plays a crucial role in shaping the hydrological characteristics of this vital transboundary basin, both in terms of quality and quantity fronts. This makes its sustainable management and development vital. The predominant groundwater type is Ca-SO4, with other types including Ca-HCO3, Na-Cl, Na-HCO3, and Ca-Mg-SO4-Cl. Hydrochemical facies analysis highlights the importance of rock-water interactions in controlling groundwater chemistry, mainly through incongruent chemical weathering of Ca-rich plagioclase feldspars and the oxidation of sulfide minerals, such as pyrite which are prevalent in the study area. Ground-water recharge is primarily influenced by the area’s topography, with recharge zones characterized by lineament networks, located in elevated areas. Stable isotope analyses indicate that groundwater mainly originates from local precipitation, while tritium data suggests the presence of both recent and older groundwater (likely over 20 years old). The study’s comprehensive approach and findings contribute significantly to the understanding of the region's groundwater systems, thus providing valuable insights for policymakers and stakeholders involved in water resource management and development strategies.
... Modern Lake Victoria is a hydrologically open lake and responds sensitively to rainfall variations (Beverly et al., 2020;Yin and Nicholson, 1998). The hydrological budget is controlled by direct precipitation (>80% of incoming water) and lake surface evaporation (~70% of outgoing water) (Yin and Nicholson, 1998). ...
The Lake Victoria ecosystem is emblematic of the catastrophic effects that human activities, particularly cultural eutrophication, can have on freshwater biodiversity. However, little is known about the long-term spatial and temporal pattern of aquatic primary paleo-production (PPaq) and producer communities in Lake Victoria and how these patterns relate to past climate variability, landscape evolution, lake hydrology, mixing regimes, nutrient cycling, and biodiversity dynamics in the past 17 kyr. We use sediments from four well-dated cores along a transect from offshore to nearshore sites, and exploit XRF element scanning and hyperspectral imaging data, TC, TN, bSi, δ 13 C and δ 15 N, and sedimentary pigments to investigate paleolimnological variability and change. Our findings demonstrate that changes in PP aq and algal communities during the past 17 kyr were closely related to hydroclimatic changes, lake mixing, and nutrient availability. During the wetland phase (16.7-14.5 cal ka BP), PPaq levels remained generally low, while chromophytes and chlorophytes dominated the algal community. Following the rapid lake level rise (~14.2 cal ka BP) during the early African Humid Period (AHP), PPaq levels steadily increased, accompanied by a shift towards cyanobacteria and chromophytes. During the Holocene, our results suggest repeated short-lived arid intervals (~10.5, ~9, 7.8-7.2, ~4, and 3.2-3.0 cal ka BP) and two distinct periods of enhanced lake mixing associated with high PP aq and high diatom productivity: the first one between 11 and 9 cal ka BP, which coincided with the maximum of the AHP (high precipitation, high wind, enhanced mixing), and the second, less pronounced one, between 7 and 4 cal ka BP. Between these two periods (i.e. 9-7 cal ka BP) we observe reduced diatom productivity, relatively low PPaq , and high C/N ratios, suggesting conditions with more stable lake stratification, likely associated with reduced wind strength, and some nutrient limitation (N and P). Finally, the drier conditions around the end of the AHP (ca. 4 cal ka BP) and during the late Holocene were associated with decreasing lake mixing and increasing dominance of cyanobacteria. Given our reconstruction of PP aq over the past 17 kyr, we conclude that the levels in the 20th century are unprecedentedly high, consistent with the massive human-mediated impact on the Lake Victoria ecosystem including biodiversity loss.
... [Photos of Victoria pi and sc by Frans Witte (HEST), al by Vianny Natugonza (NaFIRRI), Lake Edward pa by Nathan Vranken, and Lake Kivu cichlids by Guy Periat. All other photos by O. S.] most recently from at least 19,000 to 16,000 years ago (22)(23)(24). It is unclear how any one lineage could retain this large admixture variation and the exceptional radiation propensity associated with it through this long desiccation phase. ...
Although some lineages of animals and plants have made impressive adaptive radiations when provided with ecological opportunity, the propensities to radiate vary profoundly among lineages for unknown reasons. In Africa’s Lake Victoria region, one cichlid lineage radiated in every lake, with the largest radiation taking place in a lake less than 16,000 years old. We show that all of its ecological guilds evolved in situ. Cycles of lineage fusion through admixture and lineage fission through speciation characterize the history of the radiation. It was jump-started when several swamp-dwelling refugial populations, each of which were of older hybrid descent, met in the newly forming lake, where they fused into a single population, resuspending old admixture variation. Each population contributed a different set of ancient alleles from which a new adaptive radiation assembled in record time, involving additional fusion-fission cycles. We argue that repeated fusion-fission cycles in the history of a lineage make adaptive radiation fast and predictable.
... In addition to environmental changes, preservation or sampling biases associated with middle and later Holocene lake level fluctuations could have also influenced the observed lack of Late Phase lakeshore sites in the northeastern basin. Lake Victoria is a shallow lake that Beverly et al. (2019) argue dried out completely during particularly arid periods in the terminal Pleistocene ~18-14 cal ka. Although lake level data is not available for the middle and later Holocene, it is possible that the shoreline retreated with increased aridity ~5-4 ka. ...
Kansyore pottery-using groups of the northeastern Lake Victoria Basin represent one of only a few examples of ‘complex’ hunter-gatherers in Africa. Archaeologists link evidence of specialized fishing, a seasonal land-use cycle between lake and riverine sites, and intensive investment in ceramic production to behavioral complexity after 9 thousand years ago (ka). However, a gap in the Kansyore radiocarbon record of the region between ~7 and 4.4 cal ka limits explanations of when and why social and economic changes occurred. This study provides the first evidence of lakeshore occupation during this temporal break at the only well-studied Kansyore site in eastern Uganda, Namundiri A. Within the context of other sites in nearby western Kenya, radiometric and faunal data from the site indicate a move from the lake to a greater reliance on riverine habitats with middle Holocene aridity ~5–4 cal ka and the arrival of food producers to the region after ~3 cal ka.
... Other human activities such as dam construction (Avery and Tebbs, 2018), ground water pumping for urban and agricultural pursuits (Gaye and Tindimugaya, 2019) and pollution/eutrophication (Twesigye, 2015) have also reduced water levels in recent decades (Cohen, 2018). Forecast projections from Great Lakes predict significant changes in hydrology, which could shift resources and negatively impact the economic livelihoods of the region's inhabitants (Beverly et al., 2020). ...
Due to its position in the trough of the East African Rift Valley, the Great Lakes region has experienced significant effects from changes to earth’s climate during the Holocene. Annually migrating atmospheric pressure systems bring oceanic moisture inland, causing precipitation mainly in the highlands that charges ground water systems in the lowlands. Although the Holocene has been a relatively stable climatic epoch for most of the planet, the lacustrine region of eastern Africa has experienced significant hydrologic variability. The African Humid Period brought significantly higher rainfall to the northern two-thirds of the continent, including the most of the lacustrine zone, but had a minimal impact on lake levels in the southern aspect of the region. Lake records show that after 2000 BCE, the northern region experienced lower average precipitation while lake levels rose on balance across the southern region. However, high-resolution records consistently demonstrate that there is significant variability in the tempo and spatial application of precipitation and vegetation in East African lakes. Additionally, since the advent of the Common Era, there are increasingly obvious impacts of human landscape disturbance on the region’s ecology, affecting lake levels and biodiversity.
Northeastern Lake Victoria partially lies within the east-west trending Nyanza Rift. Many Miocene deposits identified as lacustrine have been reinterpreted as terrestrial, although parts of sedimentary sequences at Karungu, Maboko Island and Rusinga Island (Kulu Formation) are still recognised as lake deposits. Several palaeolakes have been identified around the base of volcanoes. The Pleistocene fluvio-lacustrine Kanjera Beds were once used to define a ‘Kanjeran pluvial’, but pluvial concepts were subsequently rejected. Lake deposits have also been recognised in part of the Rawi Formation at Kanam, west of Kanjera. Lake Victoria formed as a result of reversal of river flows caused by the uplift of the western branch of the East African Rift System. Subsequent uplift and tilting shifted the Lake Victoria depositional centre ~50 km eastwards from its original location, flooding many former river valleys. Seismic reflection data indicate up to 60 m of Pleistocene and Holocene sediment below the lake, which, given likely sedimentation rates, suggests that deposition started at least by 400 ka. Erosional periods are also indicated by seismic profiles, which have been attributed to drier periods and episodes of lake desiccation. Modelling indicates that the lake can potentially switch between modern lake elevations and complete desiccation in just a few centuries.
Lake Victoria fills a sag basin and is the largest freshwater body in Africa by surface area with the Winam Gulf occupying the east-west trending Nyzanza Rift in Kenya. Direct rainfall onto the lake accounts for 82% of water inputs and is controlled by topography and seasonal movements of the Inter Tropical Convergence Zone and Congo Air Boundary. The lake is very dilute compared with other large East African water bodies with a pH of 8.0–8.6. Stratification develops periodically through the year with eutrophication having lowered hypolimnetic oxygen levels. Photosynthesis in surface water increased in recent decades with algal concentrations five times greater than in the 1950s and nitrogen acting as a limiting nutrient. Phytoplankton compositions have changed in response to anthropogenic pollution with diatoms generally giving way to cyanobacteria. In contrast, zooplankton (copepods, cladocerans, rotifers) have experienced less compositional change. Major changes in fish reflect climate change and stratification, anthropogenic eutrophication and deliberate introductions of predator species. Diatomaceous sediments dominate on the lake floor with variable organic contents. Vivianite-rich nodules are present in some areas. Seismic data indicates about 60 m of later Quaternary sediments with multiple erosion surfaces are present below the lake floor. Cores record late Pleistocene desiccation.
The Lake Victoria basin of Equatorial Africa was formed as a central depression between the eastern and western branches of the East African Rift System (EARS). Lake Victoria itself, at 68,800 km2, has the largest surface area of freshwater of any lake in Africa. It is also a major source for the White Nile, a waterway that connects the central and northern portions of the continent. On geological timescales, Lake Victoria is ephemeral, and at various intervals during the Pleistocene it has dried up, in the process replaced by more open, grassy habitats similar to those found in the Serengeti today. These contrasting conditions in the past serve to underscore archaeological and biogeographic evidence for the social and environmental dynamism of the region, dynamism that likely included periodic intervals of the expansion and contraction of the ranges of plant and animal populations that included early Homo sapiens.
The interplay of factors militating against water resources has created complicated situations with far-reaching ecological consequences. Increase in human population around the world, especially in Africa, has increased pressure on water resources on one hand. On the other hand, the impacts of climate change and attendant consequences of desertification in the Sahel savanna have increased demand for water resources. Lake Chad, sandwiched between Nigeria, Niger, Chad, and Cameroun, is fast diminishing, with a reported loss of at least 85% of its water content between 1963 and 2007. In South Africa, increasing demand for water; occasionally aggravated by drought has made this resource to be dubbed “liquid gold.” This has led to increasing government expenditures to ensure adequate water supply to all populaces. In Algeria, very high temperatures combined with low precipitation creates a perennial water crisis, with several attendant consequences. Decrease in water supply coupled with growing population, industrial activities, increases demand on the available water bodies creating a greater tendency to further contaminate existing water sources. From the west to east, north to south, Africa is plagued with acute water crisis and continual contamination of existing water resources. This chapter reviews the water resources of Africa, in the context of its growing human population and attendant demands within households, as well as growing agricultural, and industrial activities. Water consumption is often accompanied by the discharge of wastewater, unfortunately, many urban communities in Africa lack municipal wastewater treatment plants, and hence, over 80% of wastewater is discharged into receiving water bodies (rivers, streams, and lakes) without treatment, leading to contamination of receiving water bodies. Sources of contaminants and impacts on biota have been reviewed in the light of published reports. The development of bacterial resistant strains and associated economic consequences has been pointed out. The physiological modulatory effects of organic contaminants on aquatic organisms have been highlighted as the initiation point toward biodiversity loss. This chapter concludes by proffering suggestions toward remedying already polluted waters as well as preventing further pollution of water resources to ensure both conservation of aquatic bodies and biodiversity for posterity.KeywordsWater resourcesWater sustainabilityPollutionEmerging contaminantsClimate change
Fire regimes differ across tropical and subtropical biomes depending on multiple parameters whose interactions and levels of importance are poorly understood, particularly at multidecadal and longer time scales. In the catchment of Lake Victoria, savanna, rainforest, and Afromontane vegetation have interspersed over the last 17,000 years, which may have influenced the fire regime and vice versa. However, climate and humans are most often the primary drivers of fire regime changes, and analysing their respective roles is critical for understanding current and future fire regimes. Besides a handful of radiocarbon dates on grassy charcoal, the timescales of published studies of Lake Victoria sediment chronologies rely mostly on dates of bulk sediment, and chronological disagreements persist, mainly due to variation between estimations of the ¹⁴C reservoir effect. Here, we provide independent ¹⁴C chronologies for three Late Glacial and Holocene lacustrine sediment cores from various water depths and compare them with the biostratigraphy to establish a new chronological framework. We present the first continuous sedimentary charcoal records from Lake Victoria; these suggest that fire activity varied substantially during the past 17,000 years. Our new pollen records reveal the long-term vegetation dynamics. The available evidence suggests that before human impact increased during the Iron Age (ca. 2400 yr BP), biomass burning was linked to climate and vegetation reorganizations, such as warming, drying, and the expansion of rainforests and savannas. Our results imply that climate can trigger substantial fire regime changes and that vegetation responses to climate change can co-determine the fire regime. For instance, biomass burning decreased significantly when the rainforest expanded in response to increasing temperatures and moisture availability. Such insights into the long-term linkages between climate, vegetation, and the fire regime may help to refine ecosystem management and conservation strategies in a changing global climate.
The archaeological record of Late Pleistocene Africa is characterized by behavioral diversity and change, notably the technological shift from the Middle Stone Age (MSA) to Later Stone Age (LSA). Recent research shows the MSA-LSA transition was a spatially and temporally complex process. Understanding this transition requires a composite record of archaeological sites from precise chronological and stratigraphic contexts within multiple regions. Here we present excavation and analysis of two open-air Late Pleistocene sites in chronological and geographic association: Anderea’s Farm 1 (GrJe-8) and Kapsarok 1 (GrJe-9), from the Nyanza Rift, Kenya. Volcanic ash correlations of artifact-bearing sediments provide ages of ∼ 45–36 ka for Anderea’s Farm 1 (GrJe-8) and ∼ 50 ka for Kapsarok 1 (GrJe-9). Locally procured lavas were used to produce different stone tools by disparate technological methods. Lithic production at Anderea’s Farm 1 focused on the manufacture of short irregular flakes using expedient and discoidal methods, and tools are dominated by heavy-duty types. In contrast, Kapsarok 1 is characterized by elongated and convergent blanks produced using hierarchical core technologies. Viewed together, Kapsarok 1 and Anderea’s Farm 1 emphasizes high diversity in Late Pleistocene technology of the Victoria Basin. We argue these different technologies are most parsimoniously interpreted as expressions of a broad and flexible behavioral repertoire. Further, our results emphasize how excavation and analysis of open-air archaeological sites in secure chronological and stratigraphic contexts provides the means to sample the necessary range of human behaviours across a landscape commensurate with past forager geographic ranges.
That humans originated from Africa is well-known. However, this is widely regarded as a chance outcome, dependant simply on where our common ancestor shared the land with where the great apes lived. This volume builds on from the 'Out of Africa' theory, and takes the view that it is only in Africa that the evolutionary transitions from a forest-inhabiting frugivore to savanna-dwelling meat-eater could have occurred. This book argues that the ecological circumstances that shaped these transitions are exclusive to Africa. It describes distinctive features of the ecology of Africa, with emphasis on savanna grasslands, and relates them to the evolutionary transitions linking early ape-men to modern humans. It shows how physical features of the continent, especially those derived from plate tectonics, set the foundations. This volume adequately conveys that we are here because of the distinctive features of the ecology of Africa.
Site-specific habitat reconstructions in the form of faunal enamel stable carbon and oxygen isotope data allow for a finer assessment of the context of Homo sapiens in eastern Africa. To date, these studies have focused on a small collection of sites within a constrained spatiotemporal scope. Here, I analyse a compilation of faunal stable isotopes from the Kibish Formation and Porc Epic Cave, Ethiopia, and Rusinga and Mfangano Islands, Karungu, Lukenya Hill, and Panga ya Saidi, Kenya. New data for primate and notably Homo sapiens at Porc Epic are presented. Faunal isotope data indicate that the Lake Victoria and northern Lake Turkana basins were dominated by open grasslands between ~ 105 ka and 50 ka. Sites near the Ethiopian Rift and closer to the coast were at least in part buffered from the environmental changes that occurred further inland. In the following period, ~ 49 ka – 20 ka, inland sites see more wooded conditions while Panga ya Saidi at the coast becomes drier. This compilation provides evidence for spatial and temporal trends in local habitats necessary for understanding the mechanisms through which human populations exchanged genes, ideas, and behaviours during the Late Pleistocene.
The margins of Paleolake Otero in southern New Mexico, USA, contain one of the largest concentrations of fossilized late Pleistocene (Rancholabrean) megafauna trackways in North America. These fossil footprints include tracks of Ice Age proboscideans, ground sloth, dire wolf, and camelids, as well as humans. Biomechanical interpretations of these fossil footprints suggest that prehistoric people in the basin regularly interacted with the megafauna. However, these trackway studies employ a geomorphic context that assumes an unlikely static landscape that changed very little after the human–megafauna interaction occurred during much of the terminal Pleistocene to the latest Holocene. In this study, we present a new lacustrine paleoclimate record from the western margin of Paleolake Otero to demonstrate that the lake underwent six developmental phases as lake levels waxed and waned at the end of the last Ice Age, reflecting a dynamic shoreline. We also reconcile how different factors have complicated the currently proposed timing of these human–megafauna trackway intersections; namely, the lake is now thought to have persisted during the terminal Pleistocene longer than previously thought, and multiple older pre-Ice Age trackways are thought to have been exhumed by Holocene wind erosion. Finally, we propose an alternative model that states that the human trackways are not contemporaneous with nearby megafauna trackways, but simply reflect humans crossing over re-exhumed, and much older, trackway surfaces.
Modern human evolution in Africa over the last ∼300 kyr is complex, with a variety of behavioral and biological changes appearing at different times and places. Explaining this pattern, as well as its relationship to paleoenvironmental circumstances, requires chronological and stratigraphic control of the paleoanthropological record. This study employs tephrostratigraphy, the chemical correlation of volcanic ashes (tephras), to provide chronostratigraphic context for modern human evolution in the Middle Pleistocene–Holocene of equatorial East Africa. This work geographically expands the eastern Lake Victoria Basin tephrostratigraphic framework over an area >16,000 km². Geochemical comparisons show that most eastern Lake Victoria Basin tephras are derived from the volcanoes of the Central Kenyan Rift. New tephra correlations in the eastern Lake Victoria Basin incorporate 11 terrestrial localities throughout the Nyanza Rift as well as the V95–1P sediment core from the Ugandan waters of Lake Victoria. The spatial expansion of the eastern Lake Victoria Basin tephrostratigraphy also extends the previously documented ∼94–36 ka chronology of the tephra sequence to encompass most of the last ∼240 kyr. Tephra correlations presented here provide new ages for previously excavated, but undated, archaeological sites including Songhor (GqJe-1) = >94 ka, Simbi (GrJe-2) = 45–36 ka, and Muguruk (GqJc-1) where the correlation of a ∼49–36 ka tuff may help constrain the age for some or all of the site’s multiple archaeological levels. The tephrostratigraphy presented here thus constitutes a necessary step in expanding a Middle Pleistocene–Holocene chronostratigraphic framework across equatorial East Africa, which is fundamental to future studies on modern human evolution and behavioral change through time.
Lake Victoria is the largest lake in Africa and one of the two major sources of the Nile river. The water level of Lake Victoria is determined by its water balance, consisting of precipitation on the lake, evaporation from the lake, inflow from tributary rivers and lake outflow, controlled by two hydropower dams. Due to a scarcity of in situ observations, previous estimates of individual water balance terms are characterized by substantial uncertainties, which means that the water balance is often not closed independently. In this first part of a two-paper series, we present a water balance model for Lake Victoria, using state-of-the-art remote sensing observations, high-resolution reanalysis downscaling and outflow values recorded at the dam. The uncalibrated computation of the individual water balance terms yields lake level fluctuations that closely match the levels retrieved from satellite altimetry. Precipitation is the main cause of seasonal and interannual lake level fluctuations, and on average causes the lake level to rise from May to July and to fall from August to December. Finally, our results indicate that the 2004–2005 drop in lake level can be about half attributed to a drought in the Lake Victoria Basin and about half to an enhanced outflow, highlighting the sensitivity of the lake level to human operations at the outflow dam.
The impact of changing environments on the evolution and dispersal of Homo sapiens is highly debated, but few data are available from equatorial Africa. Lake Victoria is the largest freshwater lake in the tropics and is currently a biogeographic barrier between the eastern and western branches of the East African Rift. The lake has previously desiccated at ~17 ka and again at ~15 ka, but little is known from this region prior to the Last Glacial Maximum. The Pleistocene terrestrial deposits on the northeast coast of Lake Victoria (94–36 ka) are ideal for paleoenvironmental reconstructions where volcaniclastic deposits (tuffs), fluvial deposits, tufa, and paleosols are exposed, which can be used to reconstruct Critical Zones (CZ) of the past (paleo-CZs). The paleo-CZ is a holistic concept that reconstructs the entire landscape using geologic records of the atmosphere, hydrosphere, lithosphere, biosphere, and pedosphere (the focus of this study). New paleosol-based mean annual precipitation (MAP) proxies from Karungu, Rusinga Island, and Mfangano Island indicate an average MAP of 750 ± 108 mm year⁻¹ (CALMAG), 800 ± 182 mm year⁻¹ (CIA-K), and 1,010 ± 228 mm year⁻¹ (PPM1.0) with no statistical difference throughout the 11 m thick sequence. This corresponds to between 54 and 72% of modern precipitation. Tephras bracketing these paleosols have been correlated across seven sites, and sample a regional paleo-CZ across a ~55 km transect along the eastern shoreline of the modern lake. Given the sensitivity of Lake Victoria to precipitation, it is likely that the lake was significantly smaller than modern between 94 and 36 ka. This would have removed a major barrier for the movement of fauna (including early modern humans) and provided a dispersal corridor across the equator and between the rifts. It is also consistent with the associated fossil faunal assemblage indicative of semi-arid grasslands. During the Late Pleistocene, the combined geologic and paleontological evidence suggests a seasonally dry, open grassland environment for the Lake Victoria region that is significantly drier than today, which may have facilitated human and faunal dispersals across equatorial East Africa.
Around 200,000 yr ago, Homo sapiens emerged in Africa. By 40 ka, Homo sapiens had spread throughout Eurasia, and a major competing species, the Neanderthals, became extinct. The factors that drove our species "out of Africa" remain a topic of vigorous debate. Existing research invokes climate change as either providing opportunities or imposing limits on human migration. Yet the paleoclimate history of northeast Africa, the gateway to migration, is unknown. Here, we reconstruct temperature and aridity in the Horn of Africa region spanning the past 200,000 yr. Our data suggest that warm and wet conditions from 120,000 to 90,000 yr ago could have facilitated early waves of human migration toward the Levant and Arabia, as supported by fossil and lithic evidence. However, the primary out-of-Africa event, as constrained by genetic studies (ca. 65-55 ka), occurred during a cold and dry time. This complicates the climatemigration relationship, suggesting that both "push" and "pull" factors may have prompted Homo sapiens to colonize Eurasia.
An observed drying trend and projected increase in rainfall place East Africa in an apparent climate paradox. It is argued that the observational evidence, backed by climate modeling experiments, support the case that the recent regional scale drying trend in East Africa was dominated by an abrupt decline in March–May rainfall around 1998–1999 and associated with a shift of Pacific decadal variability (PDV) to its cold phase. While it is possible a warming trend in the western tropical Pacific contributed to the severity of recent droughts, the timing of the recent decline is closely tied to PDV. Thus, the paradox is seen to result from a mismatch of timescales on one hand (recent drying versus long-term wettening) and from taking climate change projections at face value on the other. The inability of coupled models to capture fundamental aspects of the current climate undermines confidence in the future climate projections in East Africa. Given substantial errors in coupled climate models' ability to simulate the current climate, it remains an open question as to whether or not the future climate in East Africa will become wetter as a result of anthropogenic climate change.
Significance
Tropical African lakes are well-known to house exceptionally biodiverse assemblages of fish and other aquatic fauna, which are thought to be at risk in the future. Although the modern assemblages are well-studied, direct evidence of the origin of this incredible wealth of species and the mechanisms that drive speciation are virtually unknown. We use a long sedimentary record from Lake Malawi to show that over the last 1.2 My both large-scale climatic and tectonic changes resulted in wet–dry transitions that led to extraordinary habitat variability and rapid diversification events. This work allows us to understand the environmental context of aquatic evolution in the most biodiverse tropical lake.
Paleosols (fossil soils) are abundant in the sedimentary record and reflect, at least in part, regional paleoclimate. Paleopedology thus offers a great potential for elucidating high resolution, deep-Time paleoclimate records. However, many fossil soils did not equilibrate with climate prior to burial and instead dominantly express physical and chemical features reflective of other soil forming factors. Current models that use elemental oxides for climate reconstruction bypass the issue of soil-climate equilibration by restricting datasets to narrow ranges of soil properties, soil-forming environments and mean annual precipitation (MAP) and mean annual temperature (MAT). Here we evaluate a data-driven paleosol-paleoclimate model (PPM1.0) that uses subsoil geochemistry to test the ability of soils from wide-ranging environments to predict MAP and MAT as a joint response with few initial assumptions. The PPM1.0 was developed using a combined partial least squares regression (PLSR) and a nonlinear spline on 685 mineral soil B horizons currently forming under MAP ranging from 130 to 6900 mm and MAT ranging from 0 to 27 °C. The PLSR results on 11 major and minor oxides show that four linear combinations of these oxides (Regressors 1-4), akin to classic oxide ratios, have potential for predicting climate. Regressor 1 correlates with increasing MAP and MAT through Fe oxidation, desilication, base loss and residual enrichment. Regressor 2 correlates with MAT through temperature-dependent dissolution of Na-and K-bearing minerals. Regressor 3 correlates with increasing MAP through decalcification and retention of Si. Regressor 4 correlates with increasing MAP through Mg retention in mafic-rich parent material. The nonlinear spline model fit on Regressors 1 to 4 results in a Root Mean Squared Error (RMSEMAP) of 228 mm and RMSEMAT of 2.46 °C. PPM1.0 model simulations result in Root Mean Squared Predictive Error (RMSPEMAP) of 512mmand RMSPEMAT of 3.98 °C. The RMSE values are lower than some preexisting MAT models and show that subsoil weathering processes operating under a wide range of soil forming factors possess climate prediction potential, which agrees with the state-factor model of soil formation. The nonlinear, multivariate model space of PPM1.0 more accurately reflects the complex and nonlinear nature of many weathering processes as climate varies. This approach is still limited as it was built using data primarily from the conterminous USA and does not account for effects of diagenesis. Yet, because it is calibrated over a broader range of climatic variable space than previous work, it should have the widest array of potential applications. Furthermore, because it is not dependent on properties that may be poorly preserved in buried paleosols, the PPM1.0 model is preferable for reconstructing deep time climate transitions. In fact, previous studies may have grossly underestimated paleo-MAP for some paleosols.
African climate is generally considered to have evolved towards progressively drier conditions over the past few million years, with increased variability as glacial-interglacial change intensified worldwide. Palaeoclimate records derived mainly from northern Africa exhibit a 100,000-year (eccentricity) cycle overprinted on a pronounced 20,000-year (precession) beat, driven by orbital forcing of summer insolation, global ice volume and long-lived atmospheric greenhouse gases. Here we present a 1.3-million-year-long climate history from the Lake Malawi basin (10°-14° S in eastern Africa), which displays strong 100,000-year (eccentricity) cycles of temperature and rainfall following the Mid-Pleistocene Transition around 900,000 years ago. Interglacial periods were relatively warm and moist, while ice ages were cool and dry. The Malawi record shows limited evidence for precessional variability, which we attribute to the opposing effects of austral summer insolation and the temporal/spatial pattern of sea surface temperature in the Indian Ocean. The temperature history of the Malawi basin, at least for the past 500,000 years, strongly resembles past changes in atmospheric carbon dioxide and terrigenous dust flux in the tropical Pacific Ocean, but not in global ice volume. Climate in this sector of eastern Africa (unlike northern Africa) evolved from a predominantly arid environment with high-frequency variability to generally wetter conditions with more prolonged wet and dry intervals.
The effect of changing environment on the evolution of Homo sapiens is heavily debated, but few data are available from equatorial Africa prior to the last glacial maximum. The Karungu deposits on the northeast coast of Lake Victoria are ideal for paleoenvironmental reconstructions and are best studied at the Kisaaka site near Karunga in Kenya (94 to N33 ka) where paleosols, fluvial deposits, tufa, and volcaniclastic deposits (tuffs) are exposed over a ~2 km transect. Three well-exposed and laterally continuous paleosols with intercalated tuffs allow for reconstruction of a succession of paleocatenas. The oldest paleosol is a smectitic paleo-Vertisol with saline and sodic properties. Higher in the section, the paleosols are tuffaceous paleo-Inceptisols with Alfisol-like soil characteristics (illuviated clay). Mean annual precipitation (MAP) proxies indicate little change through time, with an average of 764 ± 108 mm yr −1 for Vertisols (CALMAG) and 813 ± 182 to 963 ± 182 mm yr −1 for all paleosols (CIA-K). Field observations and MAP proxies suggest that Karungu was significantly drier than today, consistent with the associated faunal assemblage, and likely resulted in a significantly smaller Lake Victoria during the late Pleistocene. Rainfall reduction and associated grassland expansion may have facilitated human and faunal dispersals across equatorial East Africa.
Although the African Great Lakes are important regulators for the East African climate, their influence on atmospheric dynamics and the regional hydrological cycle remains poorly understood. This study aims to assess this impact by comparing a regional climate model simulation that resolves individual lakes and explicitly computes lake temperatures to a simulation without lakes. The Consortium for Small-Scale Modelling model in climate mode (COSMO-CLM) coupled to the Freshwater Lake model (FLake) and Community Land Model (CLM) is used to dynamically downscale a simulation from the African Coordinated Regional Downscaling Experiment (CORDEX-Africa) to 7-km grid spacing for the period of 1999–2008. Evaluation of the model reveals good performance compared to both in situ and satellite observations, especially for spatiotemporal variability of lake surface temperatures (0.68-K bias), and precipitation (2116 mm yr 21 or 8% bias). Model integrations indicate that the four major African Great Lakes almost double the annual precipitation amounts over their surface but hardly exert any influence on precipitation beyond their shores. Except for Lake Kivu, the largest lakes also cool the annual near-surface air by 20.6 to 20.9 K on average, this time with pronounced downwind influence. The lake-induced cooling happens during daytime, when the lakes absorb incoming solar radiation and inhibit upward turbulent heat transport. At night, when this heat is released, the lakes warm the near-surface air. Furthermore, Lake Victoria has a profound influence on atmospheric dynamics and stability, as it induces circular airflow with over-lake convective inhibition during daytime and the reversed pattern at night. Overall, this study shows the added value of resolving individual lakes and realistically representing lake surface temperatures for climate studies in this region.
Increasing atmospheric carbon dioxide concentrations are
expected1 to cause major changes in the world's climate over
the next 50-100 yr. The impact of such changes on water resources,
through changing precipitation and evaporation, will, however, be
complicated by the direct effects of increasing CO2 on
vegetation. In controlled environment experiments, higher CO2
levels cause the stomata of plants to close down, decreasing their rate
of transpiration and increasing their water use efficiency2.
Reduced evapotranspiration would make more water available as runoff and
could tend to offset the effects of any CO2-induced
reductions in precipitation or enhance the effects of precipitation
increases. We consider here, in a simple but revealing analysis, the
relative sensitivity of runoff to these two processes, changes in
precipitation and changes in evapotranspiration. We show that, for low
runoff ratios, small changes in precipitation may cause large changes in
runoff. The magnitude and direction of these changes is, however,
strongly dependent on the magnitude of the direct CO2 effect
on plant evapotranspiration.
This paper presents the Lake Victoria water budget for the period 1950-2004 and findings of a study on potential climate change impact on the lake’s Hydrology through the 21st Century. The mass balance components are computed from measured and simulated data. A2 and B2 emission scenarios of the Special Report on Emissions Scenarios are considered in the climate change assessment. Results show that rainfall and Evaporation by far exceed catchment inflow and outflow. Rainfall over the lake exceeds evaporation by a factor of 0.1 whereas outflow exceeds inflow by a factor of 0.27. Due to climate change, increase in temperature of 4-5oC and 2-3oC are expected by the end of the 21st century under the A2 and B2 emission scenarios respectively. There is very significant downward trend in the lake Net Basin Supply reducing by up to 50%) by the end of the Century. Towards the end of the 21st century, the lake is likely to experience more frequent and prolonged droughts implying lower lake levels
This study tests diurnal climatology assumptions made in the MT-CLIM model by examining two microclimate variables driven by diurnal atmospheric dynamics: incident solar radiation (in kilojoules per square metre), and humidity, expressed as vapor pressure deficit, VPD (in kilopascals). The relative VPD humidity comparison was used to test our hypothesis that night minimum temperatures can function as a surrogate for dew-point temperatures. VPD was chosen as the humidity measure for these tests since plants are more directly sensitive to this measure than relative humidity. For the observed vs. estimated vapor pressure deficit models, we obtained coefficients of determination (R2) ranging from 0.66 to 0.84. Incident solar radiation is calculated in the model using an algorithm that relates diurnal temperature amplitude to atmospheric transmissivity, coupled with a potential radiation model to compute diffuse and direct radiation. Correlations for incident solar radiation models indicate generally good agreement, with coefficients of determination ranging from R2 = 0.82 to 0.89. These results suggest that MT-CLIM may be a useful way to provide the climatology that many ecological/hydrological models require, particularly for larger scale spatial modeling applications where precise meteorology may not be as important as a good general characterization of the regional climatology.
The cichlid fishes of the East African Great Lakes are the largest extant vertebrate radiation identified to date. These lakes and their surrounding waters support over 2,000 species of cichlid fish, many of which are descended from a single common ancestor within the past 10 Ma. The extraordinary East African cichlid diversity is intricately linked to the highly variable geologic and paleoclimatic history of this region. Greater than 10 Ma, the western arm of the East African rift system began to separate, thereby creating a series of rift basins that would come to contain several water bodies, including the extremely deep Lakes Tanganyika and Malawi. Uplifting associated with this rifting backponded many rivers and created the extremely large, but shallow Lake Victoria. Since their creation, the size, shape, and existence of these lakes have changed dramatically which has, in turn, significantly influenced the evolutionary history of the lakes' cichlids. This paper reviews the geologic history and paleoclimate of the East African Great Lakes and the impact of these forces on the region's endemic cichlid flocks.
The degree of chemical weathering in soils increases with mean annual precipitation (P; mm) and mean annual temperature (T; C). We have quantified these relationships using a database of major-element chemical analyses of 126 North American soils. The most robust relationship found was between P and the chemical index of alteration without potash (CIA-K): with . Another strong relationship was found between P and 0.0197(CIA-K) 2 P p 221.12e Rp 0.72 the molecular ratio of bases/alumina (B): with . A Mollisol-specific relationship 2 P p 259.34 ln (B) 759.05 R p 0.66 was found relating P to the molar ratio of calcium to aluminum (C) as follows: with P p 130.93 ln (C) 467.4 . Relationships between weathering ratios and T are less robust, but a potentially useful one was found 2 R p 0.59 between T and the molecular ratio of potash and soda to alumina (S) where with 2 T p 18.516(S) 17.298 R p . Our data also showed that most Alfisols can be distinguished from Ultisols by a molecular weathering ratio of 0.37 bases/alumina of !0.5 or by a chemical index of alteration without potassium !80. Application of these data to a sequence of Eocene and Oligocene paleosols from central Oregon yielded refined paleoprecipitation and paleotem-perature estimates consistent with those from other pedogenic and paleobotanical transfer functions for paleoclimate.
Lingering debate among evolutionary biologists over whether or not Lake Victoria dried out during the late Pleistocene focuses
on perceived conflicts between biological and geological evidence for the age of its endemic species. This article reviews
and updates the geophysical and paleoecological evidence for lake-wide desiccation and describes the environmental conditions
that aquatic species likely experienced during the low stand. Lake Victoria was at its lowest between 18,000 and 14,000 calendar
years ago, and it dried out at least once during that time. There is no evidence of remnant ponds or marshes persisting within
the desiccated basin. If such features existed, then they would have been small, shallow, turbid, and/or saline, and therefore
markedly different from the lake to which today’s species are adapted. The existence of Lake Victoria’s diverse endemic biota
must be reconciled with the incontrovertible geophysical and paleoecological evidence of a ca. 15,000year age for the lake,
and not vice versa.
Palaeoclimatic estimates of mean annual rainfall in the equatorial highlands of Central East Africa have been established for the last 40 kyr. The values are inferred from nine fossil pollen sequences, collected from six peat bogs located between 2° and 4°S latitude, in the forest belt, from 1800 to 2240 m a.s.l. The transformation of pollen data into climatic parameters is achieved by the best analogues statistical method, using a modern pollen data set of East and Central Africa and calibration by meteorological data. The climatic reconstructions are first performed for each individual sequences. They are transformed into time-series using 88 of the 125 available radiocarbon dates (including 41 AMS dates). The synthesis of the results is presented as a single curve, illustrating the precipitation values obtained for 682 dated stratigraphic layers plotted on a radiocarbon timescale. The precipitation changes are presented at about a century resolution during the Holocene, and about a millennium for the glacial period.
Climate variability during the present interglacial, the Holocene, has been rather smooth in comparison with the last glacial. Nevertheless, there were some rather abrupt climate changes. One of these changes, the desertification of the Saharan and Arabian region some 4 - 6 thousand years ago, was presumably quite important for human society. It could have been the stimulus leading to the foundation of civilizations along the Nile, Euphrat and Tigris rivers. Here we argue that Saharan and Arabian desertification was triggered by subtle variations in the Earth's orbit which were strongly amplified by atmosphere- vegetation feedbacks in the subtropics. The timing of this transition, however, was mainly governed by a global interplay between atmosphere, ocean, sea ice, and vegetation.
We present here a new solution for the astronomical computation of the insolation quantities on Earth spanning from Myr to 250 Myr. This solution has been improved with respect to La93 (Laskar et al. [CITE]) by using a direct integration of the gravitational equations for the orbital motion, and by improving the dissipative contributions, in particular in the evolution of the Earth–Moon System. The orbital solution has been used for the calibration of the Neogene period (Lourens et al. [CITE]), and is expected to be used for age calibrations of paleoclimatic data over 40 to 50 Myr, eventually over the full Palaeogene period (65 Myr) with caution. Beyond this time span, the chaotic evolution of the orbits prevents a precise determination of the Earth's motion. However, the most regular components of the orbital solution could still be used over a much longer time span, which is why we provide here the solution over 250 Myr. Over this time interval, the most striking feature of the obliquity solution, apart from a secular global increase due to tidal dissipation, is a strong decrease of about 0.38 degree in the next few millions of years, due to the crossing of the resonance (Laskar et al. [CITE]). For the calibration of the Mesozoic time scale (about 65 to 250 Myr), we propose to use the term of largest amplitude in the eccentricity, related to , with a fixed frequency of /yr, corresponding to a period of 405 000 yr. The uncertainty of this time scale over 100 Myr should be about , and over the full Mesozoic era.
External climate forcings-such as long-term changes in solar insolation-generate different climate responses in tropical and high latitude regions. Documenting the spatial and temporal variability of past climates is therefore critical for understanding how such forcings are translated into regional climate variability. In contrast to the data-rich middle and high latitudes, high-quality climate-proxy records from equatorial regions are relatively few, especially from regions experiencing the bimodal seasonal rainfall distribution associated with twice-annual passage of the Intertropical Convergence Zone. Here we present a continuous and well-resolved climate-proxy record of hydrological variability during the past 25,000 years from equatorial East Africa. Our results, based on complementary evidence from seismic-reflection stratigraphy and organic biomarker molecules in the sediment record of Lake Challa near Mount Kilimanjaro, reveal that monsoon rainfall in this region varied at half-precessional ( approximately 11,500-year) intervals in phase with orbitally controlled insolation forcing. The southeasterly and northeasterly monsoons that advect moisture from the western Indian Ocean were strengthened in alternation when the inter-hemispheric insolation gradient was at a maximum; dry conditions prevailed when neither monsoon was intensified and modest local March or September insolation weakened the rain season that followed. On sub-millennial timescales, the temporal pattern of hydrological change on the East African Equator bears clear high-northern-latitude signatures, but on the orbital timescale it mainly responded to low-latitude insolation forcing. Predominance of low-latitude climate processes in this monsoon region can be attributed to the low-latitude position of its continental regions of surface air flow convergence, and its relative isolation from the Atlantic Ocean, where prominent meridional overturning circulation more tightly couples low-latitude climate regimes to high-latitude boundary conditions.
The Earth's climate system is highly nonlinear: inputs and outputs are not proportional, change is often episodic and abrupt, rather than slow and gradual, and multiple equilibria are the norm. While this is widely accepted, there is a relatively poor understanding of the different types of nonlinearities, how they manifest under various conditions, and whether they reflect a climate system driven by astronomical forcings, by internal feedbacks, or by a combination of both. In this paper, after a brief tutorial on the basics of climate nonlinearity, we provide a number of illustrative examples and highlight key mechanisms that give rise to nonlinear behavior, address scale and methodological issues, suggest a robust alternative to prediction that is based on using integrated assessments within the framework of vulnerability studies and, lastly, recommend a number of research priorities and the establishment of education programs in Earth Systems Science. It is imperative that the Earth's climate system research community embraces this nonlinear paradigm if we are to move forward in the assessment of the human influence on climate.
The processes that control climate in the tropics are poorly understood. We applied compound-specific hydrogen isotopes (δD)
and the TEX86 (tetraether index of 86 carbon atoms) temperature proxy to sediment cores from Lake Tanganyika to independently reconstruct
precipitation and temperature variations during the past 60,000 years. Tanganyika temperatures follow Northern Hemisphere
insolation and indicate that warming in tropical southeast Africa during the last glacial termination began to increase ∼3000
years before atmospheric carbon dioxide concentrations. δD data show that this region experienced abrupt changes in hydrology
coeval with orbital and millennial-scale events recorded in Northern Hemisphere monsoonal climate records. This implies that
precipitation in tropical southeast Africa is more strongly controlled by changes in Indian Ocean sea surface temperatures
and the winter Indian monsoon than by migration of the Intertropical Convergence Zone.
Lake Victoria is the largest lake in Africa and harbors more than 300 endemic species of haplochromine cichlid fish. Seismic reflection profiles and piston cores show that the lake not only was at a low stand but dried up completely during the Late Pleistocene, before 12,400 carbon-14 years before the present. These results imply that the rate of speciation of cichlid fish in this tropical lake has been extremely rapid.
The common explanation for the progression of the rainy season over Africa is the seasonal excursion of the ITCZ. The ITCZ paradigm stems from a time when tropical rainfall was assumed to be associated mainly with localized convection. Its development was also linked to the emergence of midlatitude frontal concepts. The paradigm has numerous shortcomings, including the diversity of definitions and the large number of parameters used to identify the ITCZ. A historical look at the concept shows that its use over Africa has long been controversial, with many eminent tropical meteorologists harshly criticizing its applicability over this continent. However, the seasonal excursion of the ITCZ remains the classical explanation for African rainy seasons, especially in the equatorial region. This article underscores the shortcomings of the paradigm in equatorial Africa by examining various aspects of the circulation associated with the spatial and temporal patterns of rainfall during the equatorial rainy seasons. The overall conclusion is that a deeper understanding of the seasonal cycle in the equatorial regions of Africa still needs to be developed.
This book presents an account of the hydrology of the whole Nile basin, dealing with each tributary in turn but drawing attention to links between reaches. The Nile is shown to be a set of very different tributaries which came together by geological accident. Nevertheless, evidence from one part of the basin often throws light on a different area. Recent changes are discussed, in particular the dramatic change of regime of Lake Victoria and other lakes that occurred post-1961. The relationship between hydrology and vegetation affects the important wetlands of the White Nile Basin and discussion of this relationship includes the effect of increased lake flows. The authors draw on records collected throughout the basin to paint a detailed hydrological picture of the Nile. The book is illustrated with over 100 diagrams and photographs.
Late Pleistocene sedimentary, biogeochemical, and fossil data from the Lake Victoria basin (the largest lake in Africa) suggest that its reduction or desiccation during periods of increased aridity repeatedly facilitated the dispersal of C4 grassland ecosystems across the basin. Archaeological evidence from Middle Stone Age and Later Stone Age sites suggest that human groups diffused into the basin during intervals of declining lake levels, likely tracking the movement of the dense and predictable resources of shoreline environments, as well as the dense but less predictable C4 grass grazing herbivores. Repeated cycles of lake expansion and contraction provide a push–pull mechanism for the isolation and combination of populations in Equatorial Africa that may contribute to the Late Pleistocene human biological variability suggested by the fossil and genetic records. Latitudinal differences in the timing of environmental change between the Lake Victoria basin and surrounding regions may have promoted movements across, within, and possibly out of Africa.
The "long rains" season of East Africa has recently experienced a series of devastating droughts, whereas the majority of climate models predict increasing rainfall for the coming decades. This has been termed the East African climate paradox and has implications for developing viable adaptation policies. A logical framework is adopted that leads to six key hypotheses that could explain this paradox. The first hypothesis that the recent observed trend is due to poor quality data is promptly rejected. An initial judgment on the second hypothesis that the projected trend is founded on poor modeling is beyond the scope of a single study. Analysis of a natural variability hypothesis suggests this is unlikely to have been the dominant driver of recent droughts, although it may have contributed. The next two hypotheses explore whether the balance between competing forcings could be changing. Regarding the possibility that the past trend could be due to changing anthropogenic aerosol emissions, the results of sensitivity experiments are highly model dependent, but some show a significant impact on the patterns of tropical SST trends, aspects of which likely caused the recent long rains droughts. Further experiments suggest land-use changes are unlikely to have caused the recent droughts. The last hypothesis that the response to CO2 emissions is nonlinear explains no more than 10% of the contrast between recent and projected trends. In conclusion, it is recommended that research priorities now focus on providing a process-based expert judgment of the reliability of East Africa projections, improving the modeling of aerosol impacts on rainfall, and better understanding the relevant natural variability.
Construction of the Jonglei Canal began in 1977 to reduce absorption and evaporation from the White Nile, and to provide water for irrigation and hydroelectric power in the wetland area of southern Sudan. Based on scientific studies carried out in the early 1950s and in 1978 and 1983, the book aims to present a multi-disciplinary survey of the complex interrelated hydrological, ecological, biological and human problems involved with the scheme. Construction was halted in 1983 by civil war, and the book is intended to provide a foundation upon which further research can proceed with minimum delay should political circumstances allow work on the canal and development of the area to begin again. -after Publisher
This paper presents new calculations of Lake Victoria's water balance. Evaporation is estimated using both the Penman formula and the energy balance approach, and sensitivity studies are performed to determine the influence of input data on the estimates. Rainfall over the lake is estimated from catchment rainfall using a relationship between the two that was derived using satellite data. The results, using the reference period 1956-1978, indicate that mean annual rainfall over the lake is 1791 mm, compared to mean annual evaporation of 1551 mm. When compared with lake level changes, tributary inflow, and discharge during this period, there is a resultant imbalance of 19 mm. Adding this amount to the calculated evaporation, we are able to reproduce with great accuracy the lake level changes during the period 1956-1978 utilizing precipitation estimates of this study plus measured inflow and discharge. Sensitivity studies show that the discrepancy in the balance of 19 mm is considerably smaller than the error in evaporation calculations that can be introduced by uncertainties in the input data. Of particular concern is cloudiness. The diurnal cycle of cloudiness is quite different over the lake than at shoreline stations and the total cloud cover over the lake is probably lower than at these stations. A change from 50% cloudiness to 30% can increase evaporation by about 30%. Thus, this study underscores the need for adequate cloud data, sufficient to resolve the diurnal cycle, as well as direct estimates of lake rainfall in assessing the lake's water balance.
The African Humid Period of the early to mid-Holocene (12,000–5000 years ago) had dramatic ecological and societal consequences, including the expansion of vegetation and civilization into the “green Sahara.” While the humid period itself is well documented throughout northern and equatorial Africa, mechanisms behind observed regional variability in the timing and magnitude of the humid period remain disputed. This paper presents a new hydrogen isotope record from leaf waxes (δDwax) in a 15,000-year sediment core from Lake Tana, Ethiopia (12°N, 37°E) to provide insight into the timing, duration, and intensity of the African Humid Period over northeastern Africa. δDwax at Lake Tana ranges between −80‰ and −170‰, with an abrupt transition from D-enriched to D-depleted waxes between 13,000–11,500 years before present (13–11.5 ka). A similarly abrupt transition from D-depleted to D-enriched waxes occurs ca 8.5–8 ka and is followed by generally D-enriched waxes throughout the late Holocene. Trends in δDwax covary with changes in Northern Hemisphere summer insolation and reflect increased precipitation at Lake Tana during the AHP; however, the transition from D-depleted to D-enriched waxes occurs earlier at Lake Tana (ca 8 ka, vs 5 ka) than in many other regional records, and the amplitude of D-depletion during the AHP is larger at Lake Tana as well. We attribute this early enrichment to a reduction of moisture derived from westerly sources (the Congo Basin and Atlantic Ocean) which we suggest are D-depleted relative to moisture sourced from the east (Indian Ocean) and the north (Red Sea and Mediterranean Sea). Our new record highlights the importance of both the northward migration of the tropical rain belt as well as east-west migration of the Congo Air Boundary to precipitation source and amount during the African Humid Period.
Improved tables for the albedo of water as a function of latitude for
latitudes from 0 to 90 deg and for 10 and 30 deg latitude belts are
presented. Water albedos averaged over all radiation received at all
solar elevation angles, as a function of month, and yearly averages are
calculated from the Fresnel equation and from the data set of
Grishchenko (1959) in which diffuse radiation is taken into account. The
albedos calculated on the basis of Grishchenko's data are found to be 2
to 4% higher at low latitudes and up to 20% lower near the pole than
albedos calculated by Sivkov (1952) for midday at the middle of each
month and presently in general use.
Vertisols are clayey, shrink-swell soils that are widely recognized in the rock record, thus generating the need to better understand the dynamics of elemental concentrations on the development of weathering indexes for climate interpretations. We assessed the weathering performance of the four major base-forming oxides (CaO, MgO, Na2O, K2O) along a modern Vertisol climosequence spanning a strong precipitation gradient, and discovered that the concentration of bulk soil CaO and MgO yields the strongest correlation to mean annual precipitation (MAP). Based on this finding, we introduce the CALMAG weathering index, defined as Al2O3 / (Al2O3 + CaO + MgO) × 100, which improves rainfall estimates for Vertisols relative to the well-established CIA-K (chemical index of alteration minus potassium) weathering index. Rather than documenting the hydrolysis of weatherable minerals common in many other soil orders, in Vertisols CALMAG principally tracks the fl ux of calcium and magnesium sourced from calcium carbonate, detrital clay, and exchangeable Ca2+ and Mg2+. Application to two Mesozoic paleosols reveals that in drier climates CIA-K yields higher MAP estimates than CALMAG, but that the reverse is true in wetter climates. This work improves paleorainfall estimates from Vertisols and suggests that a family of weathering indexes is needed for different paleosol types.
We propose a generally applicable formula for estimating evaporation rate from open water bodies which utilizes readily available land-based meteorological data. We follow the well-known aerodynamic approach in which evaporation rate is modelled as the product of a vapour pressure deficit between the water surface and upwind air and a wind function which depends on wind speed. Importantly, such a technique should account for the size of a given water body as evaporation rate is known to decrease with distance downwind from the shore due to the increasing entrainment of water vapour. Our area-dependent relationship is based on a compilation of site-specific wind functions reported for individual water bodies covering a large range of sizes from 0.07 m2 to 33.5 km2. The proposed wind function (mm d−1 kPa−1) is f(U2) = (2.36 + 1.67U2)A−0.05 where U2 is wind speed (m s−1) measured over land at a height of 2 m and A is the surface area of the water body (m2). The areal dependence of our relationship was found to be similar to that proposed in other experimental and theoretical studies. Our proposed area-dependent wind function might be expected to be a best estimate in the absence of more specific site related information. From the scatter of reported site-specific wind functions around our proposed relationship, we develop an estimate of its uncertainty. We also explore the propagation of this wind function uncertainty through aerodynamic and combination type evaporation estimation techniques. We use Rushy Billabong, a 5 ha pond in Australia, as a case study and show how uncertainty can be reduced by more than 60% by using a combination equation approach.
The relative timings of the last deglacial warming in the Southern and Northern hemispheres are not well constrained, but are a crucial component in understanding the mechanisms of deglaciation1. A clearer picture of the degree of interhemispheric synchrony has been obscured by a dearth of high-resolution temperature records that can be tied to the absolute calendar timescale. Moreover, the quantification of tropical temperatures during the last glacial cycle is controversial2-8. Here we apply the alkenone method of sea surface temperature reconstruction9,10 to several high-resolution sediment cores recovered from the tropical Indian Ocean between 20° N and 20° S. The inferred initial sea surface temperature warming ~15,000 calendar years ago at 20° S is in phase with Northern Hemisphere sea (this study) and air11 temperature changes, but lags Antarctic warming12-14 by several millennia. This finding, along with the results of recent modelling studies15,16, provides strong support for the idea that changes in the ocean's global thermohaline circulation were not the only cause of interhemispheric climate teleconnection during the last deglaciation.
The Toro-Ankole volcanics in the northern sector of the Western Branch of the East African Rift are generally assumed to be of Quaternary age. New field observations and comparative KAr and ArAr analyses on whole rock samples and phenocrysts of perpotassic lavas reveal the presence of excess Ar and led to the conclusion that apparently no volcanics have been erupted prior to 50 ka.The present geochronological data, compared with previous ones for the Virunga and South Kivu volcanic fields, suggest that a Late Quaternary pulse of volcanic activity occured in the three areas, with the emission of perpotassic, mildly potassic or sodic alkaline magmas. The genetic implications of such a synchronous activity with wide compositional variation are briefly discussed with respect to basement age and assumed lithosphere characteristics.
Lake Victoria water levels provide an important indicator of climate; since 1954 these levels have been maintained natural by agreement, but after recent increased releases require adjustment to retain a naturalised series. It is possible to compute the impact of recent over-abstraction on declining water levels. This impact has been significant: of the order of 0.6 m.RésuméLes niveaux d'eau du Lac Victoria constituent un indicateur important du climat; depuis 1954, ces niveaux sont maintenus naturels par convention, mais nécessitent d'être corrigés en une série naturalisée suite à une récente augmentation des prélèvements. Il est possible de calculer l'impact des sur-prélèvements récents sur les niveaux d'eau. L'impact est significatif: de l'ordre de 0.6 m.
Past research on the climate response to orbital forcing has emphasized the glacial-interglacial variations in global ice volume, global-mean temperature, and the global hydrologic cycle. This emphasis may be inappropriate in the tropics, where the response to precessional forcing is likely to be somewhat independent of the glacial-interglacial variations, particularly in variables relating to the hydrologic cycle. To illustrate this point, we use an atmospheric general circulation model coupled to a slab ocean model, performing experiments that quantify the tropical climates response to (1) opposite phases of precessional forcing, and (2) Last Glacial Maximum boundary conditions. While the glacially-forced tropical temperature changes are typically more than an order of magnitude larger than those arising from precessional forcing, the hydrologic signals stemming from the two forcings are comparable in magnitude. The mechanisms behind these signals are investigated and shown to be quite distinct for the precessional and glacial forcing. Because of strong dynamical linkages in the tropics, the model results illustrate the impossibility of predicting the local hydrologic response to external forcing without understanding the response at much larger spatial scales. Examples from the paleoclimate record are presented as additional evidence for the importance of precessional signals in past variations of the tropical climate.
The Cenozoic evolution of the Nile basin reflects a complex interaction between tectonic, volcanic and climatic events. The Ethiopian and Ugandan headwaters of the Nile attain elevations in excess of 2 km, while the watersheds rise to over 5 km. The Ethiopian tributaries of the Nile (notably the Blue Nile/Abbai and the Atbara/Tekezze) provide the bulk of the flood discharge and sediment load, but the Ugandan tributaries, via the White Nile, provide the reliable dry season discharge responsible for maintaining perennial flow in the main Nile. The hydrologi-cal differences between the Blue and White Nile rivers reflect their very different geomorphic histories, reflecting the Cenozoic tectonic and volcanic evolution of their respective headwaters.
The beginning of the Bølling-Allerød warm period was marked in Greenland ice by an abrupt rise in δ18O, an abrupt drop in dust rain, and an abrupt increase in atmospheric methane content. The surface waters in the Norwegian Sea underwent a simultaneous abrupt warming. At about this time, a major change in the pattern of global rainfall occurred. Lake Victoria (latitude 0°), which prior to this time was dry, was rejuvenated. The Red Sea, which prior to this time was hypersaline, freshened. Lake Lahontan, which prior to this time had achieved its largest size, desiccated. Whereas the chronologic support for the abruptness of the hydrologic changes is firm only for the Red Sea, in keeping with evidence obtained well away from the northern Atlantic in the Santa Barbara Basin and the Cariaco Trench, the onset and end of the millennial-duration climate events were globally abrupt. If so, the proposed linkage between the size of African closed basin lakes and insolation cycles must be reexamined.
An energy-balanced hydrologic model is used to quantitatively assess atmosphere–water budget relationships across the Lake Malawi catchment, a hydrologically-open lake within the East African Rift System. The model first simulates the historical lake-level record over the last 100 years using climate station and vegetation data as inputs. Atmospheric conditions required to sustain equilibrium water balance are then estimated at known critical lake-levels: modern (700 m maximum water depth), basin closure (696 m maximum water depth), 500 m, 350 m, 200 m, and 150 m maximum water depth. The critical low lake stages were determined from analysis of seismic-reflection and deep lake drill-core data. The model predicts modern precipitation rate to be 955 mm/yr, which is consistent with observed climate station precipitation records. The minimum lowstand observed in geophysical records is 150 m water depth (550 m below present lake-level), and occurred about 95,000 years before present. The precipitation rate required to sustain equilibrium conditions at this low lake stage is 557 mm/yr, assuming modern Lake Malawi temperature and vegetation, and 374 mm/yr using modern temperature and vegetation data from the Little Karoo Basin, an analogue for the Malawi paleo-environment during severe arid intervals that resulted in major lake lowstands. The latter result is consistent with the range of precipitation measured from the Little Karoo Basin (100 to 500 mm/yr), and from interpretations of drill-core data sets (Cohen et al., 2007). The time required to drop lake-level from its modern maximum to the most severe low lake stage determined from paleoclimate data sets (from 700 m to 150 m maximum water depth) is less than 2500 years, even when accounting for additional water volume loss stored as groundwater. A lake-level fall of this magnitude reduces the lake surface area by 94% and reduces the total lake volume by 99%.
Seismic-reflection data from crater lake Challa (Mt. Kilimanjaro, equatorial East Africa) reveal a ∼ 210-m thick sedimentary infill containing distinct seismic-stratigraphic signatures of late-Quaternary lake-level fluctuations. Extrapolation of a well-constrained age model on the cored upper part of the sequence suggests that these lake-level fluctuations represent a detailed and continuous record of moisture-balance variation in equatorial East Africa over the last 140 kyr. This record indicates that the most severe aridity occurred during peak Penultimate glaciation immediately before ∼ 128 kyr BP (coeval with Heinrich event 11) and during a Last Interglacial ‘megadrought’ period between ∼ 114 and ∼ 97 kyr BP; in comparison, Last Glacial Maximum (LGM) aridity was modest. It was preceded by ∼ 75 000 years of relatively stable and moist climate conditions interrupted by eleven short-lived dry spells, five of which match the timing of Heinrich events 2 to 6. Climate history near the East African equator reflects variation in the precessional forcing of monsoon rainfall modulated by orbital eccentricity, but precession-driven moisture fluctuations were less extreme than those observed in northern and southern tropical Africa. The near-continuous moist climate from ∼ 97 to 20.5 kyr BP recorded in the Lake Challa record contrasts with the trend towards greater aridity after ∼ 70 kyr BP documented in equatorial West Africa. This long period of moist glacial climate and a short, relatively modest LGM drought can be attributed to greater independence of western Indian Ocean monsoon dynamics from northern high-latitude glaciation than those in the tropical Atlantic Ocean. This rather persistent moist glacial climate regime may have helped maintain high biodiversity in the tropical forest ecosystems of the Eastern Arc mountains in Tanzania.
Variations in the temporal and spatial distribution of solar radiation caused by orbital changes provide a partial explanation for the observed long-term fluctuations in African lake levels. The understanding of such relationships is essential for designing climate-prediction models for the tropics. Our assessment of the nature and timing of East African climate change is based on lake-level fluctuations of Lake Naivasha in the Central Kenya Rift (0°55′S 36°20′E), inferred from sediment characteristics, diatoms, authigenic mineral assemblages and 17 single-crystal 40Ar/39Ar age determinations. Assuming that these fluctuations reflect climate changes, the Lake Naivasha record demonstrates that periods of increased humidity in East Africa mainly followed maximum equatorial solar radiation in March or September. Interestingly, the most dramatic change in the Naivasha Basin occurred as early as 146 kyr BP and the highest lake level was recorded at about 139–133 kyr BP. This is consistent with other well-dated low-latitude climate records, but does not correspond to peaks in Northern Hemisphere summer insolation as the trigger for the ice-age cycles. The Naivasha record therefore provides evidence for low-latitude forcing of the ice-age climate cycles.
Between 15,000 and 18,000 years ago, large amounts of ice and meltwater entered the North Atlantic during Heinrich stadial 1. This caused substantial regional cooling, but major climatic impacts also occurred in the tropics. Here, we demonstrate that the height of this stadial, about 16,000 to 17,000 years ago (Heinrich event 1), coincided with one of the most extreme and widespread megadroughts of the past 50,000 years or more in the Afro-Asian monsoon region, with potentially serious consequences for Paleolithic cultures. Late Quaternary tropical drying commonly is attributed to southward drift of the intertropical convergence zone, but the broad geographic range of the Heinrich event 1 megadrought suggests that severe, systemic weakening of Afro-Asian rainfall systems also occurred, probably in response to sea surface cooling.