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Article

Sharon E. Nicholson

This article provides an in-depth look at all aspects of the climate of the Sahel, including the pervasive dust in the Sahelian atmosphere. Emphasis is on two aspects: West African monsoon and the region’s rainfall regime. This includes an overview of the prevailing atmospheric circulation at the surface and aloft and the relationship between this and the rainfall regime. Aspects of the rainfall regime that are considered include its unique characteristics, its changes over time, the storm systems that produce rainfall, and factors governing its variability on interannual and decadal time scales. Variability is examined on three time scales: millennial (as seen is the paleo records of the last 20,000 years), multi-decadal (as seen over the last few centuries as seen from proxy data and, more recently, in observations), and interannual to decadal (quantified by observations from the late 19th century and onward). A unique feature of Sahel climate is that is rainfall regime is perhaps the most sensitive in the world and this sensitivity is apparent on all of these time scales.

Article

For several decades, the Sahelian countries have been facing continuing rainfall shortages, which, coupled with anthropogenic factors, have severely disrupted the great ecological balance, leading the area in an inexorable process of desertification and land degradation. The Sahel faces a persistent problem of climate change with high rainfall variability and frequent droughts, and this is one of the major drivers of population’s vulnerability in the region. Communities struggle against severe land degradation processes and live in an unprecedented loss of productivity that hampers their livelihoods and puts them among the populations in the world that are the most vulnerable to climatic change. In response to severe land degradation, 11 countries of the Sahel agreed to work together to address the policy, investment, and institutional barriers to establishing a land-restoration program that addresses climate change and land degradation. The program is called the Pan-Africa Initiative for the Great Green Wall (GGW). The initiative aims at helping to halt desertification and land degradation in the Sahelian zone, improving the lives and livelihoods of smallholder farmers and pastoralists in the area and helping its populations to develop effective adaptation strategies and responses through the use of tree-based development programs. To make the GGW initiative successful, member countries have established a coordinated and integrated effort from the government level to local scales and engaged with many stakeholders. Planning, decision-making, and actions on the ground is guided by participation and engagement, informed by policy-relevant knowledge to address the set of scalable land-restoration practices, and address drivers of land use change in various human-environmental contexts. In many countries, activities specific to achieving the GGW objectives have been initiated in the last five years.

Article

The Republic of Mali comprises a very diverse population spread over a vast territory composed of a large part of the southern Sahara, the Sahel, and the savannah. One of the world’s great rivers, the Niger, runs through much of the national territory, reaching its northern apex near Timbuktu. For over a millennium, this territory has allowed empires and kingdoms to flourish alongside decentralized societies. These include the empires of Ghana, Mali and Songhay, as well as any number of smaller states, trading diasporas, and nomadic and semi-nomadic communities. The territory of Mali has long been a hub in African commercial and intellectual circuits, notably those linking the societies of the Maghreb (or North Africa) to those bordering the Atlantic. In the 19th century, as elsewhere in Muslim Africa, new and explicitly Islamic states emerged in western and central Mali. They did not endure more than a few decades, as the territory was colonized by France in the late 19th century. The Republic of Mali claimed its independence in 1960 and rapidly developed greater autonomy from French neo-colonialism than did most of its neighbors. Mali has maintained an out-sized diplomatic and cultural role on the African continent and beyond under a socialist government from 1960 to 1968, military government through 1991, and a vibrant democracy in the decades since. However, since 2011, the country has been increasingly beset by violent conflicts between nonstate actors, the national government, and foreign forces including the French. Thus, in historical perspective, Mali’s geographic position and its environment have proven conducive to the production of expansive, diverse, and mutually dependent communities that have produced radically distinct and often fragile states.

Article

Mineral dust is the most important natural aerosol type by mass, with northern Africa the most prominent source region worldwide. Dust particles are lifted into the atmosphere by strong winds over arid or semiarid soils through a range of emission mechanisms, the most important of which is saltation. Dust particles are mixed vertically by turbulent eddies in the desert boundary layer (up to 6km) or even higher by convective and frontal circulations. The meteorological systems that generate winds strong enough for dust mobilization cover scales from dust devils (~100m) to large dust outbreaks related to low- and high-pressure systems over subtropical northern Africa (thousands of kilometers) and include prominent atmospheric features such as the morning breakdown of low-level jets forming in the stable nighttime boundary layer and cold pools emanating from deep convective systems (so-called haboobs). Dust particles are transported in considerable amounts from northern Africa to remote regions such as the Americas and Europe. The removal of dust particles from the atmosphere occurs through gravitational settling, molecular and turbulent diffusion (dry deposition), as well as in-cloud and sub-cloud scavenging (wet deposition). Advances in satellite technology and numerical dust models (including operational weather prediction systems) have led to considerable progress in quantifying the temporal and spatial variability of dust from Africa, but large uncertainties remain for practically all stages of the dust cycle. The annual cycle of dustiness is dominated by the seasonal shift of rains associated with the West African monsoon and the Mediterranean storm track. In summer, maximum dust loadings are observed over Mauritania and Mali, and the main export is directed toward the Caribbean Sea, creating the so-called elevated Saharan Air Layer. In winter the northeasterly harmattan winds transport dust to the tropical Atlantic and across to southern America, usually in a shallower layer. Mineral dust has a multitude of impacts on climate and weather systems but also on humans (air pollution, visibility, erosion). Nutrients contained in dust fertilize marine and terrestrial ecosystems and therefore impact the global carbon cycle. Dust affects the energy budget directly through interactions with short- and long-wave radiation, with details depending crucially on particle size, shape, and chemical composition. Mineral dust particles are the most important ice-nuclei worldwide and can also serve as condensation nuclei in liquid clouds, but details are not well understood. The resulting modifications to cloud characteristics and precipitation can again affect the energy (and water) budget. Complicated responses and feedbacks on atmospheric dynamics are known, including impacts on regional-scale circulations, sea-surface temperatures, surface fluxes and boundary layer mixing, vertical stability, near-surface winds, soil moisture, and vegetation (and therefore again dust emission). A prominent example of such complex interactions is the anti-correlation between African dust and Atlantic hurricane activity from weekly to decadal timescales, the causes of which remain difficult to disentangle. Particularly in the early 21st century, research on African dust intensified substantially and became more interdisciplinary, leading to some significant advances in our understanding of this fascinating and multifaceted element of the Earth system.

Article

Niger, Mali, Mauritania, and Chad are some of least researched countries in sub-Saharan Africa. Since independence from France in 1960 these four countries have experienced two distinct yet interrelated struggles: the struggle for statehood and the struggle for democracy. Each country has experienced violent conflict between the central authorities in the capitals and security challengers on the peripheries. Prominent examples are the Tuareg uprisings in Niger and Mali, the various rebel insurgencies in Chad, and the conflict between black Africans and Arabs in Mauritania. The emergence of jihadi-Salafi groups in the West African sub-region affects all four countries and poses a particularly strong security challenge to Mali. All these conflicts are unresolved. The liberalization of the political sphere in the late 1980s and early 1990s has led to considerable political diversity across the Sahel. In Niger and Mali meaningful multiparty competition and basic civil liberties have taken root despite many setbacks. Civil society is strong and in the past has successfully mobilized against autocratic tendencies. In Mauritania and Chad, democratic institutions exist on paper as autocratic rulers have managed to stay in office. The national armed forces remain the preeminent political actors. Civil society is not strong enough to achieve political change for the better. Stagnant living conditions, social immobility, the ongoing war against Islamic terrorism, and weak accountability mechanisms remain the most important political challenges for the Sahel.

Article

Madina Thiam

Over centuries, a variety of decentralized societies and centralized states have formed in territories across the western Sahel and southwest Sahara, and along the Niger and Senegal river valleys. Women have played central yet often unacknowledged roles in building these communities. By the late 11th century, some were rulers, as tombstones from the Gao region seem to suggest. A travelogue describing the Mali empire, and a chronicle from Songhay, tell stories of women who plotted political dissent or staged rebellions in the 14th–16th centuries. By and large, everyday women’s reproductive and productive labor sustained their families, and structured life in agricultural, pastoral, fishing, or trading communities. In the 1700s in Segu, women brewed mead, cultivated crops, dyed textiles, and participated in the building of fortifications. In Masina in the 1800s, girls attended qurʾanic school, and a woman was the custodian of the caliph’s library. Women also suffered great violence stemming from conflicts, forced displacement, and slavery. By the end of the 19th century, they made up a considerable portion (at times the majority) of enslaved individuals in the region. After the European conquest and creation of the French Soudan colony, the French administration imposed an export-oriented wage economy, in which women worked to supply crops and sustain infrastructure projects. From the regions of Kayes, Kita, and Nioro, many migrated to groundnut- or gold-producing regions of Senegambia. While women’s labor and migrations were seldom accounted for in administrative records, their attempts to leave unhappy marriages or escape enslavement do appear in court records. However, colonial domination was gendered: the administration ultimately shunned women’s emancipation efforts, seeking to channel its rule by reinforcing patriarchal authority in communities. In 1960, the Republic of Mali achieved independence. Under the democratic and military governments that followed, women built pan-African and transnational alliances. In 1991 and beyond, they fought to achieve more rights, and greater political power and representation. Their labor and migrations have continued to sustain a large portion of the economy. Post-2011, they have been both active participants in, and victims of, the conflicts that have engulfed the country, suffering displacement, loss of livelihood, and sexual violence, for which many have yet to receive justice.

Article

Accurate projections of climate change under increasing atmospheric greenhouse gas levels are needed to evaluate the environmental cost of anthropogenic emissions, and to guide mitigation efforts. These projections are nowhere more important than Africa, with its high dependence on rain-fed agriculture and, in many regions, limited resources for adaptation. Climate models provide our best method for climate prediction but there are uncertainties in projections, especially on regional space scale. In Africa, limitations of observational networks add to this uncertainty since a crucial step in improving model projections is comparisons with observations. Exceeding uncertainties associated with climate model simulation are uncertainties due to projections of future emissions of CO2 and other greenhouse gases. Humanity’s choices in emissions pathways will have profound effects on climate, especially after the mid-century. The African Sahel is a transition zone characterized by strong meridional precipitation and temperature gradients. Over West Africa, the Sahel marks the northernmost extent of the West African monsoon system. The region’s climate is known to be sensitive to sea surface temperatures, both regional and global, as well as to land surface conditions. Increasing atmospheric greenhouse gases are already causing amplified warming over the Sahara Desert and, consequently, increased rainfall in parts of the Sahel. Climate model projections indicate that much of this increased rainfall will be delivered in the form of more intense storm systems. The complicated and highly regional precipitation regimes of East Africa present a challenge for climate modeling. Within roughly 5º of latitude of the equator, rainfall is delivered in two seasons—the long rains in the spring, and the short rains in the fall. Regional climate model projections suggest that the long rains will weaken under greenhouse gas forcing, and the short rains season will extend farther into the winter months. Observations indicate that the long rains are already weakening. Changes in seasonal rainfall over parts of subtropical southern Africa are observed, with repercussions and challenges for agriculture and water availability. Some elements of these observed changes are captured in model simulations of greenhouse gas-induced climate change, especially an early demise of the rainy season. The projected changes are quite regional, however, and more high-resolution study is needed. In addition, there has been very limited study of climate change in the Congo Basin and across northern Africa. Continued efforts to understand and predict climate using higher-resolution simulation must be sustained to better understand observed and projected changes in the physical processes that support African precipitation systems as well as the teleconnections that communicate remote forcings into the continent.

Article

Rasmus Fensholt, Cheikh Mbow, Martin Brandt, and Kjeld Rasmussen

In the past 50 years, human activities and climatic variability have caused major environmental changes in the semi-arid Sahelian zone and desertification/degradation of arable lands is of major concern for livelihoods and food security. In the wake of the Sahel droughts in the early 1970s and 1980s, the UN focused on the problem of desertification by organizing the UN Conference on Desertification (UNCOD) in Nairobi in 1976. This fuelled a significant increase in the often alarmist popular accounts of desertification as well as scientific efforts in providing an understanding of the mechanisms involved. The global interest in the subject led to the nomination of desertification as focal point for one of three international environmental conventions: the UN Convention to Combat Desertification (UNCCD), emerging from the Rio conference in 1992. This implied that substantial efforts were made to quantify the extent of desertification and to understand its causes. Desertification is a complex and multi-faceted phenomenon aggravating poverty that can be seen as both a cause and a consequence of land resource depletion. As reflected in its definition adopted by the UNCCD, desertification is “land degradation in arid, semi-arid[,] and dry sub-humid areas resulting from various factors, including climate variation and human activities” (UN, 1992). While desertification was seen as a phenomenon of relevance to drylands globally, the Sahel-Sudan region remained a region of specific interest and a significant amount of scientific efforts have been invested to provide an empirically supported understanding of both climatic and anthropogenic factors involved. Despite decades of intensive research on human–environmental systems in the Sahel, there is no overall consensus about the severity of desertification and the scientific literature is characterized by a range of conflicting observations and interpretations of the environmental conditions in the region. Earth Observation (EO) studies generally show a positive trend in rainfall and vegetation greenness over the last decades for the majority of the Sahel and this has been interpreted as an increase in biomass and contradicts narratives of a vicious cycle of widespread degradation caused by human overuse and climate change. Even though an increase in vegetation greenness, as observed from EO data, can be confirmed by ground observations, long-term assessments of biodiversity at finer spatial scales highlight a negative trend in species diversity in several studies and overall it remains unclear if the observed positive trends provide an environmental improvement with positive effects on people’s livelihood.

Article

The study of West Africa has contributed to the expansion of comparative arid-lands floodplain prehistory, from both the data collection (cultural and historical) and the theoretical aspects. The neoevolutionary approach that often pictures Africa as a backward continent has been successfully challenged. In the Middle Senegal Valley and in the Inland Niger Delta, research on their societies’ complexity done along these two subcontinent’s floodplains has described new processes (including urbanization) that were not previously featured in the archaeological literature. The two floodplains, because of their ecological diversity, with the richness of their ecological diversity, attracted Saharan populations affected by drought at the end of the second millennium and the first millennium BC. However, after their initiation occupation the two areas took different trajectories in complexity and settlement organization. Large complex settlements have been found at Jenne-jeno and in the Ile a Morphil that illustrate whole new trajectories of civilization. These forms of complexity, found in areas with historically known polities, were not included in the range of possibilities predicted by standard complexity theories regarding civilizational development. Ethnographic and historical data, reveal the existence of societies with a central authority embedded within and balanced by a diffuse, segmented and heterarchical power structure; often as a strategy to resist the individual consolidation of power. These societies exhibit evidence of horizontal differentiation and consensus-based decision making. All these types of organization are characterized by the presence of several sources of power vested in corporate entities, such as lineages, age groups, cults and secret societies.

Article

The Sahel or Sahil is in a sense the “coast” of the Sahara and its cities major “ports” in trade circuits linking long-standing regional exchange in the products of different ecozones to the markets of the Mediterranean through the trans-Saharan trade. Despite botanical diversity and the capacity to support high concentrations of humans and livestock, the productivity of this region depends upon a single unpredictable annual rainy season. Long- and short-term fluctuations in aridity have required populations specializing in hunting, farming, fishing, pastoralism, gold mining, and trade to be mobile and to depend upon one another for their survival. While that interdependence has often been peaceful and increasingly facilitated through the shared idiom of Islam, it has also taken more coercive forms, particularly with the introduction of horses, guns, and a dynamic market in slaves. Although as an ecozone the region stretches all the way to the Red Sea, the political Sahel today comprises Senegal, Mauritania, Mali, Burkina Faso, Niger, and Chad—all former French colonies. France’s empire was superimposed upon the existing dynamics in the agropastoral meeting ground of the desert edge. Colonial requirements and transportation routes weakened the links between the ecozones so crucial to the success of states and markets in the region. Despite the abolition of slavery in 1905, France tacitly condoned the persistence of servile relations to secure requisitions of labor, food, and livestock. Abolition set off a very gradual shift from slavery to other kinds of labor patterns which nonetheless drew upon preexisting social hierarchies based upon religion, caste, race, and ethnicity. At the same time, gender and age gained in significance in struggles to secure labor and status. “Black Islam” (Islam noir), both invented and cultivated under French rule, was further reinforced by the bureaucratic logic of the French empire segregating “white” North Africa and “black” sub-Saharan Africa from one another. Periodic drought and famine in the region has prompted a perception of the Sahel as a vulnerable ecological zone undergoing desertification and requiring intervention from outside experts. Developmentalist discourse from the late colonial period on has facilitated the devolution of responsibilities and prerogatives that typically belong to the state to nongovernmental bodies. At the same time, competition over political authority in the fragmented postcolonial states of the Sahel has often reinscribed and amplified status and ethnic differences, pitting Saharan populations against the governments of desert edge states. External and internal radical Islamic movements entangled with black market opportunists muddy the clarity of the ideological and political stakes in ways that even currently (2018) further destabilize the region.

Article

The Sahel of Africa has been identified as having the strongest land–atmosphere (L/A) interactions on Earth. The Sahelian L/A interaction studies started in the late 1970s. However, due to controversies surrounding the early studies, in which only a single land parameter was considered in L/A interactions, the credibility of land-surface effects on the Sahel’s climate has long been challenged. Using general circulation models and regional climate models coupled with biogeophysical and dynamic vegetation models as well as applying analyses of satellite-derived data, field measurements, and assimilation data, the effects of land-surface processes on West African monsoon variability, which dominates the Sahel climate system at intraseasonal, seasonal, interannual, and decadal scales, as well as mesoscale, have been extensively investigated to realistically explore the Sahel L/A interaction: its effects and the mechanisms involved. The Sahel suffered the longest and most severe drought on the planet in the 20th century. The devastating environmental and socioeconomic consequences resulting from drought-induced famines in the Sahel have provided strong motivation for the scientific community and society to understand the causes of the drought and its impact. It was controversial and under debate whether the drought was a natural process, mainly induced by sea-surface temperature variability, or was affected by anthropogenic activities. Diagnostic and modeling studies of the sea-surface temperature have consistently demonstrated it exerts great influence on the Sahel climate system, but sea-surface temperature is unable to explain the full scope of the Sahel climate variability and the later 20th century’s drought. The effect of land-surface processes, especially land-cover and land-use change, on the drought have also been extensively investigated. The results with more realistic land-surface models suggest land processes are a first-order contributor to the Sahel climate and to its drought during the later 1960s to the 1980s, comparable to sea surface temperature effects. The issues that caused controversies in the early studies have been properly addressed in the studies with state-of-the-art models and available data. The mechanisms through which land processes affect the atmosphere are also elucidated in a number of studies. Land-surface processes not only affect vertical transfer of radiative fluxes and heat fluxes but also affect horizontal advections through their effect on the atmospheric heating rate and moisture flux convergence/divergence as well as horizontal temperature gradients.