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[EN]The greening of the Sahara, associated with the African Humid Period (AHP) between ca. 14,500 and 5,000 y ago, is arguably the largest climate-induced environmental change in the Holocene; it is usually explained by the strengthening and northward expansion of the African monsoon in response to orbital forcing. However, the strengthened monsoon in Early to Middle Holocene climate model simulations cannot sustain vegetation in the Sahara or account for the increased humidity in the Mediterranean region. Here, we present an 18,500-y pollen and leaf-wax δD record from Lake Tislit (32° N) in Morocco, which provides quantitative reconstruction of winter and summer precipitation in northern Africa. The record from Lake Tislit shows that the northern Sahara and the Mediterranean region were wetter in the AHP because of increased winter precipitation and were not influenced by the monsoon. The increased seasonal contrast of insolation led to an intensification and southward shift of the Mediterranean winter precipitation system in addition to the intensified summer monsoon. Therefore, a winter rainfall zone must have met and possibly overlapped the monsoonal zone in the Sahara. Using a mechanistic vegetation model in Early Holocene conditions, we show that this seasonal distribution of rainfall is more efficient than the increased monsoon alone in generating a green Sahara vegetation cover, in agreement with observed vegetation. This conceptual framework should be taken into consideration in Earth system paleoclimate simulations used to explore the mechanisms of African climatic and environmental sensitivity.

Proxy reconstructions from the mid-Holocene (MH: 6,000 years ago) indicate an intensification of the West African Monsoon and a weakening of the South American Monsoon, primarily resulting from orbitally-driven insolation changes. However, model studies that account for MH orbital configurations and greenhouse gas concentrations can only partially reproduce these changes. Most model studies do not account for the remarkable vegetation changes that occurred during the MH, in particular over the Sahara, precluding realistic simulations of the period. Here, we study precipitation changes over northern Africa and South America using four fully coupled global climate models by accounting for the Saharan greening. Incorporating the Green Sahara amplifies orbitally-driven changes over both regions, and leads to an improvement in proxy-model agreement. Our work highlights the local and remote impacts of vegetation and the importance of considering vegetation changes in the Sahara when studying and modeling global climate.

The ‘Greening’ and subsequent desertification of the Sahara during the early to mid-Holocene is a dramatic example of natural climate change. We analyse a suite of simulations with a newly palaeo-conditioned configuration of the HadCM3 coupled model that is able to capture an abrupt desertification of North Africa during this time. We find that this model crosses a threshold of moisture availability for vegetation at around 6000 years before present. The resultant rapid reduction in vegetation cover acts to reduce precipitation through moisture recycling and surface albedo feedbacks. Precursor drying events which are not directly forced also indicate that the model is close to a critical moisture level. Similar precursor-like events appear in a Holocene record from the East of the continent, hinting that the natural system may resemble some of the properties of this model simulation. The overall response is not fundamentally altered by the inclusion of solar irradiance variations or volcanic eruptions. The simulated timing of the abrupt transition is mostly controlled by orbital forcing and local positive feedbacks, but it is also modulated to some extent by the state of the atmosphere and ocean. Comparisons with proxy records across North Africa show good agreement with the model simulations, although the simulations remain overly dry in the East. Overall, a threshold response may present a useful model of the real transition, but more high-resolution palaeoclimate records would help to discriminate among the predictions of climate models.

Proxy records have shown that the Mid-Holocene was a period of humid conditions across West Africa, with an enhanced West African Monsoon (WAM) and vegetated conditions in areas currently characterized by desert, often referred to as the Green Sahara. However, General Circulation Models regularly struggle with recreating this strengthened Mid-Holocene monsoon in West Africa. Vegetation feedbacks has long been viewed as an essential process modulating the monsoon variability in West Africa, and simulations using prescribed vegetation to recreate a Green Sahara have shown a strengthened WAM and increased rainfall. However, simulations with prescribed vegetation in Sahara represent an idealized vegetation cover and do not take any environmental heterogeneity into account. Furthermore, this only represents a one-directional forcing by the vegetation on the climate rather than the full vegetation feedback. To address this, we have simulated the Mid-Holocene (~ 6 ka) climate using the Earth System Model EC-Earth3-Veg. The results show that coupled dynamic vegetation reproduces an apparent enhancement of the WAM, with the summer rainfall in the Sahel region increasing by 15% compared to simulations with a prescribed modern vegetation cover. Vegetation feedbacks enhance the warming of the Sahara region, deepens the Sahara Heat Low, results in increased rainfall and strengthens monsoonal flow across West Africa. However, the enhancement is still below what can be viewed in proxy reconstructions, highlighting the role of model limitation and biases and the importance of investigating other processes, such as the interactive aerosol-albedo feedback.

Background Little is known about the peopling of the Sahara during the Holocene climatic optimum, when the desert was replaced by a fertile environment. Results In order to investigate the role of the last Green Sahara in the peopling of Africa, we deep-sequence the whole non-repetitive portion of the Y chromosome in 104 males selected as representative of haplogroups which are currently found to the north and to the south of the Sahara. We identify 5,966 mutations, from which we extract 142 informative markers then genotyped in about 8,000 subjects from 145 African, Eurasian and African American populations. We find that the coalescence age of the trans-Saharan haplogroups dates back to the last Green Sahara, while most northern African or sub-Saharan clades expanded locally in the subsequent arid phase. Conclusions Our findings suggest that the Green Sahara promoted human movements and demographic expansions, possibly linked to the adoption of pastoralism. Comparing our results with previously reported genome-wide data, we also find evidence for a sex-biased sub-Saharan contribution to northern Africans, suggesting that historical events such as the trans-Saharan slave trade mainly contributed to the mtDNA and autosomal gene pool, whereas the northern African paternal gene pool was mainly shaped by more ancient events.

Abstract Green Sahara periods (GSPs) represent episodes during which the present-day Sahara was transformed into a savannah in response to intensification of the West African monsoon (WAM). Although GSPs might have dramatically altered the size, structure, and connectivity of human populations in Africa and nearby regions of Asia, their significance for human evolution remains unknown due to the problems involved in gauging the penetration of the WAM over the Sahara at evolutionary timescales. Here I reanalyse monsoon run-off and dust records back to 3 million years ago from Eastern Mediterranean ODP Site 967, and assimilate them with North African palaeoenvironmental data to substantiate penetration of the WAM front during GSPs to latitudes beyond 28°N. These results, coupled with demographic and ecological data for modern hunter-gatherers, point to a significant expansion of human populations during GSPs compared with background desert conditions. Given the clustering of GSPs around long-term maxima in the eccentricity of the Earth’s orbit, I propose that recurrent periods of human population expansion driven by GSPs led to an increased number of favourable mutations. Along with environmental factors favourable for triggering epigenetic changes, this might have led to the rise in enhanced phenotypic plasticity that underpins the speciation of hominin lineages at times of high climate variability envisaged by the variability selection hypothesis. Clustering of GSPs around the Pliocene/Pleistocene boundary, simultaneously with a protracted period of wetter conditions in East Africa and the Sinai Peninsula, further suggests that the initial colonization of Eurasia by hominins occurred circa 2.6 Ma, much earlier than typically considered.

Lake Chad, the largest freshwater lake of north-central Africa and one of the largest lakes of Africa, is the relict of a giant Quaternary lake (i.e., Megalake Chad) that developed during the early- to mid-Holocene African Humid Period. Over the drylands of the Sahara Desert and the semi-arid Sahel region, remote sensing (optical satellite imagery and digital elevation models) proved a successful approach to identify the paleo-shorelines of this giant paleo-lake. Here we present the first attempt to estimate the isostatic response of the lithosphere due to Megalake Chad and its impact on the elevation of these paleo-shorelines. For this purpose, we use the open source TABOO software (University of Urbino, Italy) and test four different Earth models, considering different parameters for the lithosphere and the upper mantle, and the spatial distribution of the water mass. We make the simplification of an instantaneous drying-up of Megalake Chad, and compute the readjustment related to this instant unload. Results (i.e., duration, amplitude, and location of the deformation) are then discussed in the light of four key areas of the basin displaying prominent paleo-shoreline morpho-sedimentary features. Whatever the Earth model and simplification involved in the simulations, this work provides a strong first-order evaluation of the impact on hydro-isostasy of Megalake Chad. It demonstrates that a water body similar to this megalake would induce a significant deformation of the lithosphere in the form of a vertical differential uplift at basin-scale reaching up to 16 m in the deepest part of the paleo-lake, and its shorelines would then be deflected from 2 m (southern shorelines) to 12 m (northern shorelines), with a maximum rate of more than 1 cm y−1. As such, any future study related to the paleo-shorelines of Megalake Chad, should integrate such temporal and spatial variation of their elevations.

The rock shelter of Tan Zoumaitek, Tassili-n-Ajjer, Central Sahara, Algeria, famous for its paintings of humans and mammals, is found to contain as well several pictures of objects that are most likely medusae. Ii is argued that these can be identified as the afrotropical Limnocnida tanganjicae GUNTHER. The duration of their isolation, after the end of the early Holocene humid period, may be contemporaneous (c. 8000 years) with that of the extant relicts of that species in Mauretania and Chad. These figures are the oldest images of a cnidarian currently known.

Paleoclimatic records indicate a decline of vegetation cover in the Western Sahara at the end of the African Humid Period (about 5,500 years before present). Modelling studies have shown that this phenomenon may be interpreted as a critical transition that results from a bifurcation in the atmosphere-vegetation system. However, the stability properties of this system are closely linked to climate variability and depend on the climate model and the methods of analysis. By coupling the Planet Simulator (PlaSim), an atmosphere model of intermediate complexity, with the simple dynamic vegetation model VECODE, we assess previous methods for the detection of multiple equilibria, and demonstrate their limitations. In particular, a stability diagram can yield misleading results because of spatial interactions, and the system’s steady state and its dependency on initial conditions are affected by atmospheric variability and nonlinearities. In addition, we analyse the implications of climate variability for the abruptness of a vegetation decline. We find that a vegetation collapse can happen at different locations at different times. These collapses are possible despite large and uncorrelated climate variability. Because of the nonlinear relation between vegetation dynamics and precipitation the green state is initially stabilised by the high variability. When precipitation falls below a critical threshold, the desert state is stabilised as variability is then also decreased.

Historically, human uses of land have transformed and fragmented ecosystems 1 , 2 , degraded biodiversity 3 , 4 , disrupted carbon and nitrogen cycles 5 , 6 and added prodigious quantities of greenhouse gases (GHGs) to the atmosphere 7 , 8 . However, in contrast to fossil-fuel carbon dioxide (CO 2 ) emissions, trends and drivers of GHG emissions from land management and land-use change (together referred to as ‘land-use emissions’) have not been as comprehensively and systematically assessed. Here we present country-, process-, GHG- and product-specific inventories of global land-use emissions from 1961 to 2017, we decompose key demographic, economic and technical drivers of emissions and we assess the uncertainties and the sensitivity of results to different accounting assumptions. Despite steady increases in population (+144 per cent) and agricultural production per capita (+58 per cent), as well as smaller increases in emissions per land area used (+8 per cent), decreases in land required per unit of agricultural production (–70 per cent) kept global annual land-use emissions relatively constant at about 11 gigatonnes CO 2 -equivalent until 2001. After 2001, driven by rising emissions per land area, emissions increased by 2.4 gigatonnes CO 2 -equivalent per decade to 14.6 gigatonnes CO 2 -equivalent in 2017 (about 25 per cent of total anthropogenic GHG emissions). Although emissions intensity decreased in all regions, large differences across regions persist over time. The three highest-emitting regions (Latin America, Southeast Asia and sub-Saharan Africa) dominate global emissions growth from 1961 to 2017, driven by rapid and extensive growth of agricultural production and related land-use change. In addition, disproportionate emissions are related to certain products: beef and a few other red meats supply only 1 per cent of calories worldwide, but account for 25 per cent of all land-use emissions. Even where land-use change emissions are negligible or negative, total per capita CO 2 -equivalent land-use emissions remain near 0.5 tonnes per capita, suggesting the current frontier of mitigation efforts. Our results are consistent with existing knowledge—for example, on the role of population and economic growth and dietary choice—but provide additional insight into regional and sectoral trends. Trends in the rate of region- and sector-specific land-use greenhouse gas emissions in 1961–2017 show an acceleration of about 20% per decade after 2001.