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The Climate-Environment-Society Nexus in the Sahara from Prehistoric Times to the Present Day
Page 1 The Climate-Environment-Society Nexus in Sahara from Prehistoric Times to the Present NICK BROOKS, ISABELLE CHIAPELLO, SAVINO LERNIA, NICK DRAKE, MICHEL LEGRAND, MOULIN AND JOSEPH The Sahara is a key region for studies of archaeology, human-environment interaction, biogeochemical cycles, and global climate change. With a few notable exceptions, the region the subject of very little international scientific research, a fact that is remarkable given Sahara’s proximity to Europe, the developmental issues facing its growing population, region’s sensitivity to climate change and the Sahara’s potential for influencing climate through the export of airborne mineral dust. This article seeks to address human- environment interaction in the Sahara from an interdisciplinary perspective, focusing on implications of Saharan environmental variability and change for human populations within and outside of the region on timescales ranging from decades to millennia. article starts by addressing past climatic changes and their impacts on human populations, before moving on to consider present day water resources and rainfall variability in longer-term context; the possibility of a ‘greening’ of the southern Sahara as suggested some climate models is also discussed. The role of the Sahara as the world’s largest of airborne mineral dust is addressed in some detail, as are the impacts of dust on climate, ecosystems and human health, as well as the implications of future changes in climate dust production and the role of the Sahara in the Earth system. The article ends with a discus- sion and synthesis that explores the lessons that may be learnt from a study of the physical social sciences in the Sahara, in particular focusing on what the signature of past and socio-cultural changes can tell us about human responses and adaptations to climatic environmental change – a matter of great relevance to researchers and policy makers alike the context of anthropogenic climate change or ‘global warming’. Nick Brooks is Assistant Director of the Saharan Studies Programme at the University of East Anglia, and senior Research Associate at the Tyndall Centre for Climate Change Research. He has a background in physical sciences and climatology, and has been working in the field of Saharan geoarchaeology since 1999. Isabelle Chiapello is a CNRS Research Fellow at the University of Lille, where her recent work included analysis of the factors controlling dust production and transport in the Sahara and Sahel. Savino di Lernia is Professor of Ethnoarchaeology at the University of Rome ‘La Sapienza’, and of the Italian-Libyan Archaeological Mission in the Acacus and Messak (Libyan Sahara). Nick Drake is Reader in the Department of Geography at King’s College, London, specialising in remote sensing and zone geomorphology. Michel Legrand is Professor of Atmospheric Sciences at the Laboratoire d’Atmosphérique at the Université des Sciences et Technologies de Lille, specialising in the remote sensing atmospheric aerosols. Cyril Moulin is a researcher at the CEA/CNRS Laboratoire des Sciences du Climat de l’Environnement in Gif-sur-Yvette, specialising in African climate and dust production and transport. Joseph Prospero is a Distinguished Faculty Scholar, Professor of Marine and Atmospheric Chemistry, and Director of the Cooperative Institute for Marine and Atmospheric Studies at the Rosenstiel of Marine and Atmospheric Science, University of Miami. He has worked extensively in the field of atmos- pheric chemistry, and on the transport of African dust to the Caribbean and North America. The Journal of North African Studies, Vol.10, No.3–4 (September–December 2005) pp.253–ISSN 1362-9387 print=1743-9345 DOI: 10.1080=13629380500336680 # 2005 Taylor &
Page 2 The purpose of this paper is to review the state of the environmental sciences in Sahara and to consider their wider relevance in a variety of contexts. In particular focus on the links between the physical and social sciences, and interdisciplinary between different research areas. A major aim of the paper is to demonstrate relevance of Saharan research in a number of fields for wider global issues such climate change and human adaptation. The paper is partly structured as a “history” of the Sahara, on the one hand in order to address a range of issues relating prehistoric, historic, contemporary and future climatic and environmental change, on the other to illustrate the relevance of palaeoenvironmental, archaeological historical information for studies of twenty first century human-interaction. We define the Sahara approximately as the region of northern African average annual rainfall is currently below 100 mm (Figure 1). This should be interpreted loosely given the large fluctuations of rainfall at the of the desert on multiple timescales; the Sahara may be viewed as expanding and con- tracting as a response to global and regional changes in climate that modulate at its periphery. In the recent geological past the Sahara thus defined has much further south than at present, and has shrunk to such an extent that it disappeared. Any consideration of the past or future evolution of the Sahara therefore also consider its periphery, particularly at its southernmost edge FIGURE ISOHYETS REPRESENTING MEAN ANNUAL RAINFALL IN MM OVER NORTHERN FOR THE TWENTIETH Source: From the dataset of New et al. (2000) and obtained from the Climatic Research Unit. The Sahara defined here loosely as the region where mean annual rainfall is below 100 mm. THE JOURNAL OF NORTH AFRICAN
Page 3 the largest variations occur in response to fluctuations in the strength and position the monsoon. The paper begins with a brief review of our knowledge of how the environment has changed in the past, with a particular focus on the early pluvial (approximately six to ten thousand years before present) and the desiccation of the region. These changes are viewed within the context of climate change, a topic currently of serious concern to scientists and policy tasked with developing strategies to mitigate the severity of, and promote to, human-induced climate change in the twenty-first century. This discussion of changes in the Saharan environment frames the remainder of the paper, which with the legacy of the past, and what we can learn from it. The discussion of the physical aspects of past environmental change is by a review of how these changes affected human societies. In particular, of adaptation and their consequences are considered. Moving on from discussions of the past, the present-day environment of Sahara is considered in terms of groundwater resources, and the mobilisation transport of mineral dust from Saharan sources. Water resources are within the context of settlement and development. Mineral dust is discussed some length due to the importance of the Sahara as a global dust source, and impact of dust on human health, ecological productivity and global climate. Ground- water and dust sources are considered within the context of past changes in Saharan environment, particularly fluvial activity, which have determined nature and distribution. Current and future climate variability are also considered within the context existing rainfall variability and potential future changes in rainfall associated changes in the African monsoon. Finally, issues of environmental change and variability, adaptation and develop- ment are synthesised. The lack of international research in the Sahara when with other parts of the world is addressed, and the case for cross-disciplinary in the Sahara is presented. Past Environments of the The history of the northern African land mass is one of dramatic changes in physical environment, of oscillations between arid phases during which much the sub-continent is effectively uninhabitable, and humid episodes that the desert regions of the Sahara into a fertile landscape of lakes and savannah. timescales of tens of thousands of years these changes are driven by glacial cycles, with glacial conditions in the northern hemisphere being associated with cold, conditions over northern Africa. 2,3,4,5, During the last glacial maximum (LGM) some 21 thousand calendar years before present (21 ka), the Sahara desert a much larger area than today, as apparent from the dating of fossil dunes some south of the present extent of mobile dunes. A combination of factors leads increased aridity during periods of glaciation, including reduced THE CLIMATE-ENVIRONMENT-SOCIETY NEXUS IN THE
Page 4 moisture availability, decreased solar heating of the land surface, and large-changes in atmospheric and oceanic circulation. 8, The Over the past 1.65 million years, approximately corresponding to the period, there have been some 17 glacial cycles, each lasting approximately and it is the generally held view that glacials are arid and interglacials humid. However, determining the effects of climate change on the Sahara problematic due to the paucity of organic deposits. Organic remains, such as palaeo- lake sediments, provide important information regarding arid-humid transitions. However, these deposits are rapidly deflated by the wind during arid periods few ancient deposits have managed to endure the multiple episodes of aridity. Because of these factors the number of lacustrine in the that have been dated to the last interglacial or older is no more than five. However, it is possible to glean some useful information from these studies. Sediments Lake provide one of the longest records, with evidence for episodes at 380, 240, 128, 118(þ27 2 20), 74(þ23 2 16), 47(þ17 2 13), 30 and from 14 + 1.7 to about 3 ka. This record of humidity conforms with view that interglacials are associated with humid periods but also indicates that epi- sodic humidity occurs during glacial cycles. A similar picture emerges if we these lacustrine episodes to the few others identified in the Sahara. A large existed in the Basin of the Tunisian Chotts at 150, 92, 75(þ7 2 6) 42.4 + 2ka, there is evidence for lacustrine conditions at 75 + 7 ka in Algeria, and there are indications of humidity between 71 and 87 ka in Egypt and northern Sudan. Some of these humid episodes are synchronous two or more regions of the Sahara (e.g. 128, 74 and 42 2 47 ka), while are not (e.g. 155 ka), although this is not unexpected due to the paucity of the lacus- trine record and uncertainties in dating. Encouragingly, studies of alluviation on northern fringes of the Sahara agree with the data from the central Sahara. Libya was subjected to fluvial activity indicative of a wetter climate at 125 + 15, 76 + 4, 42 + 5.1, 23.2 + 2.8 and 12.5 + 1.5 ka, while in southern Tunisia and subsequent alluviation occurred at 47 + 12 and from about 8+2 ka. Thus in north and central Sahara there is clear evidence for brief humid episodes during glacials, as well as in interglacials. The Early Holocene Humid Summer insolation over northern Africa reached a maximum at the beginning of Holocene period, around 10 ka, by which time humid conditions had established in the Sahara. This so-called Holocene Climatic Optimum saw a of the Sahara as the monsoon rain belt shifted hundreds of kilometres to the north, leading to the formation of numerous lakes in areas that are now hyper-arid, the development of a mosaic of savannah and woodland throughout much of Sahara. 21,22,23, It is during this period that the Sahara was reoccupied by populations, who initially survived through hunting and gathering, exploiting THE JOURNAL OF NORTH AFRICAN
Page 5 abundant humid climate fauna and flora of the then-moist Sahara, as discussed more detail below. The Holocene Climatic Optimum was, however, interrupted by a number of episodes, some of which appear to have been associated with transient changes climate at the global or hemispheric scale. The most important such events episodes of cooling in the North Atlantic, recurring on millennial timescales. et al. identify a number of such Atlantic cooling events dated at 11.1, 10.3, 9.4, 8.1, 5.9, 4.2, 2.8 and 1.4 ka. The 8.1 ka event coincides, within the envelope uncertainty associated with the dating of such episodes, with a period of lasting the order of centuries in the Sahara. The 5.9 ka event occurs around time of a dry episode evident in many, although not all, Saharan lake records, marking the beginning of a shift towards more permanent aridity. 27, The Desiccation of the A number of data indicate that aridity increased in the Sahara after an arid occurring around 6 ka. The subsequent desiccation was not a smooth, process characterised by a linear response of the monsoon to steadily solar heating or insolation. Instead, it appears that desiccation occurred in a fashion, with one or more episodes of abrupt drying. 30,31, It is most likely that drying of the Sahara was the result of a complex interaction between declining insolation, transient climatic perturbations such as the cold arid episode at 6 ka, and dynamic vegetation-atmosphere feedbacks involving the retention recycling of moisture. The earlier 8 ka cold arid episode occurred at a time when solar insolation sufficiently strong to drive a recovery in the monsoon and in vegetation cover, this transient climatic perturbation had passed. However, insolation was weaker by 6 ka, and may have been insufficient to drive a full recovery of coupled vegetation-monsoon system after it had been disrupted by the cold episode identified around this . This transient climatic perturbation well have been the trigger for the final desiccation of the Sahara, occurring at time when the monsoon system was at least partially sustained through retention and recycling by vegetation systems that originally developed as a of strong insolation driving a vigorous monsoon. Evidence for abrupt towards 5 ka in a number of Saharan 35,36,37,38, suggests the final of remaining vegetation systems around this time, perhaps as solar crossed a threshold below which vegetation-atmosphere interactions could longer sustain rainfall, or possibly as a result of further transient perturbations which vegetation could not recover in a low-insolation regime. By around 5 ka desiccation associated with the southward retreat of the system was well established; this process culminated in the formation of the present- day Sahara Desert. The timing and speed of this process of desertification varied place to place, and was mediated by geography, topography, hydrogeology and nature of regional climatic systems. For example, in the Wadi Tanezzuft, in Libyan Fezzan, the depletion of soil water reserves was not completed until 3.5 ka, and fluvial activity persisted until around 2.7 apparently, this was THE CLIMATE-ENVIRONMENT-SOCIETY NEXUS IN THE
Page 6 to the combined effects of the large rainfall catchment area (the Tassili Mountains.) and of the dune systems bordering the fluvial valley, which acted as water reservoirs. The climatic and associated environmental changes described above were restricted to the Sahara. Abundant evidence indicates that the broad pattern of conditions in the early to middle Holocene, interrupted by arid episodes and by a process of desiccation starting around 6 ka and accelerating around 5 ka, prevailed throughout many subtropical and extra-tropical northern regions. Our understanding of past environmental change in the Sahara represents but one component in our wider understanding of global climatic environmental change which, when coupled with results from other regions, enables us to develop a deeper understanding of the Earth System and of responses to environmental change. Such an understanding is necessary if we to confront issues such as long-term climate variability and anthropogenically- driven climate change, processes which can have a profound impact socio-economic development. Linking Past Environmental and Cultural The Sahara occupies a privileged position in studies of human-environment inter- action, as a consequence of the clear climatic signals in Saharan records, and the associated evidence of cultural change during key transition between arid and humid conditions. In many other geographical regions has been bedevilled by arguments over environmental determinism, leading to reluctance to consider the role of environmental change in mediating cultural trajec- tories. In contrast, archaeologists working in the Sahara have by and large the central role of environmental change, without falling into the trap of other important factors in the development of human societies. In one sense, may view the Sahara as a laboratory of human response to environmental change, given the overwhelming nature of the climatic changes that have affected region throughout the relatively archaeologically accessible Holocene. As such, Sahara can tell us much about processes of adaptation, the understanding of is crucial for the formulation of policies designed to address the impacts of climate change, particularly in the developing world. Human Occupation of the Sahara in the Human occupation of the Sahara has been limited by the availability of water, little or no occupation away from the Mediterranean coast and the Nile during arid phases associated with glacial epochs. A notable hiatus in occupation, at least in the central Saharan regions, appears to be related to the Quaternary glacial: evidence for Upper Palaeolithic sites is very rare, and the clear evidence of human occupation during the Pleistocene (the section of Quaternary predating the beginning of the Holocene some 10,000 years ago) back to the Aterian, from 90 to 60 ka. A human presence around 30 ka in Wadi al-Shati region in Libya has also been hinted at, although the evidence equivocal. Nevertheless, the spread of modern humans during the THE JOURNAL OF NORTH AFRICAN
Page 7 was facilitated by the existence of migration corridors, in particular the Nile Valley, which water was available even during the least hospitable phases of this epoch. Evidence of Pleistocene occupation in the central Sahara is almost composed of scatters of lithic tools, most of them deprived of any faunal or stratigraphic context. Even in the exceptional contexts of the Acacus mountains, well preserved Pleistocene sites have not been identified, although Uan Uan have yielded assemblages of worked stone, dated via optically stimu- lated luminescence (OSL) and thermoluminescence (TL) methods. The archaeologi- cal situation is much more fortunate in the Eastern Sahara, thanks to research Wendorf and associates, for example in Wadi Kubbanya, Bir Tarfawi and Sahara. 46, Leaving aside technological developments and site organisation, one the most important issues to be placed on the archaeological agenda is the of modern humans in Africa north of the equator. The Sahara, together with Nubian corridor, is believed to be a major area for the dispersal of modern humans, but coordinated international research in this field is lacking. Furthermore, it is essential to build a detailed scheme of Late Quaternary trends, as a prerequisite to analyse and track possible Homo sapiens paths. Recent archaeological research emphasises the role of the Aterian as a possible technological indicator of the presence of modern humans in Sahara. 48, ‘Continuistic’ views, based on (single and isolated) radiocarbon determi- nations pointed in the past to a very late presence of Aterian sites in Morocco other North African contexts, suggesting a direct relation between Aterian and Upper Palaeolithic North African contexts. Recent research, specifically Egypt and Libya, pushes back this technological phase to 140–60 ka ago, underlines, at least in the central Sahara, a break in human occupation related the expansion of the ‘Ogolian’ desert. Human-Environment Interaction in the Sahara in the Early-Mid The archaeological evidence indicates that the latest phase of human occupation the central Sahara commenced at the beginning of the Holocene, around or after 10 ka. 50, These early Holocene communities consisted of groups of hunter- gatherers, which in many cases practiced a fairly sedentary lifestyle, exploiting abun- dant locally available food resources in the form of wild fauna and flora. 52,53, It is worth noting that the scant palaeo-anthropological evidence (from Uan and Uan Muhuggiag in the central Sahara of Libya) points to sub-Saharan affinities. This fits with more recent human remains from the Egyptian oasis, which indicate similar affinity on the basis of dental analysis. These findings support the of a northwards movement of human populations as they followed the monsoon rains, which strengthened and penetrated further north into the Sahara at the beginning the Holocene. The gap between the beginning of the humid period in the after the last glacial maximum (ca. 15–13 ka) and the appearance of the Holocene occupation sites might be interpreted as a consequence of the time for vegetation and fauna to recolonise hyperarid environments. More cautiously, the first genetic data on Saharan palaeo-populations also a sub-Saharan affinity. 58, Evidence for a southern provenance of the first THE CLIMATE-ENVIRONMENT-SOCIETY NEXUS IN THE
Page 8 Saharans might also be seen in from rock art, although the subjective nature of interpretations must be recognised: in the and Acacus depictions figures with what appear to be black African features have been interpreted indicating the possible presence of populations originating in sub-Saharan regions. Dating of this material is controversial, but an Early Holocene attribution for so-called “Round Head” style of painting, as usually claimed in the literature, 62, may reinforce the archaeological and palaeo-anthropological evidence. It is during the early Holocene that we find the earliest (although still controversial) evidence for cattle domestication in the eastern Sahara, the part of North Africa at this time. 64, Reviewing the literature on the of cattle domestication in Africa, Marshall and Hildebrand argue that this was local development based on the exploitation of indigenous species, and driven the desire of hunter-gatherers to increase the predictability of their food supply, perhaps initially in order to ensure the availability of cattle for slaughter ritual festivals. They suggest that this development happened in the Sahara as a result of the greater environmental variability in this region, and the greater need for intervention to ensure predictability of food supply, compared with other Saharan regions. Citing other work, Marshall and conclude that during the frequent periods of drought affecting the eastern Sahara, cattle would have represented ‘a more reliable resource than plants because populations are maintained through movements that exploit local differences topography, vegetation, and rainfall’. According to and Marshall and Hildebrand, citing numerous sources, cattle pastoralism spread westwards through the central Sahara in an fashion from around 7 ka, where it coexisted with hunting and gathering. Di suggests that the diffusion of cattle-based culture in the Sahara occurred in to short, abrupt dry events during the 8th to 6th millennia before present (BP). 8 ka arid episode described above may have played a key role in the spread pastoralism. Di Lernia and Palombini suggest that the dry interval at 8 ka ‘probably favoured the integration of cattle herding within foraging commu- nities’ in the Libyan Sahara. If this episode also marked a shift from year-to seasonal rainfall as has been suggested by some authors, cattle herding have represented an appropriate adaptation in order to enhance food security in more seasonally variable environment. In the Acacus mountains of Libya, semi-permanent settlements in lowland gave way to seasonal migration during the late 7th millennium BP, when conditions deteriorated during a ‘severe, abrupt dry interval lasting several centu- ries’. This new pattern of subsistence involved increased use of areas in the dry season. Di Lernia and suggest that sheep and (most probably introduced from the western Asia in the early 7th BP) were tended in highland regions from late winter to early spring in order reduce pressure on lowland pasture land, which was set aside for cattle in dry season; in the study areas in the Libyan Fezzan on which they focus, cattle remains are found predominantly around the interdune palaeo-lakes in lowland ergs. THE JOURNAL OF NORTH AFRICAN
Page 9 The nature of the spread of cattle herding through the Sahara is still a issue. According to Schild and Wendorf, this occurred during wet periods, people to move with their livestock westward. Conversely, and 76, argue that migration by cattle-herding groups was stimulated by aridity. They argue that this explanation is more compatible with the record, which consists of scattered and isolated contexts throughout the Sahara. Dates associated with these contexts also suggest discrete episodes of spanning short periods, implying rapid, intermittent movements of small of herders, colonising new and unfamiliar environments as they were forced move in search of water and pasture during arid crises. Social and Cultural Responses to Mid-Late Holocene The process of desertification at around 5 ka appears to have been rapid in at some parts of the Sahara, and was associated with profound changes in societies. Increasingly harsh conditions would have had a profound effect culture and belief systems; it has been suggested that an increased diversity in subject matter of rock engravings and paintings, with a greater emphasis on relating to sexuality and fertility, and on ‘enigmatic’ or fantastical beings, dates the period of climatic deterioration, ‘when concerns over human and animal may have become acute’. In reference to the Fezzan region of Libya, Mattingly write that ‘Human activity was not cut off at a stroke, however, but have become more focussed on specific locations, with the rock art representing more sophisticated dialogue between people and a powerful spirit world, on formalised religion’. It has been suggested that this kind of relationship back to the emergence of the Pastoral cattle cult at 6.4–6.0 ka, when non-slaughtering of precious livestock and the flourishing of an artistic tradition on cattle appear to represent the first evidence of ritual relationships between populations, the physical environment, rainfall and ‘divinities’. 82,83, Human responses to the desiccation of the Sahara were spatially and mediated by geography, resource availability and local responses. In the Libyan Fezzan two types of response may be identified, to di Lernia and Palombini. In higher elevation regions cattle herding almost com- pletely disappeared after 5 ka. This was replaced by highly mobile pastoralism on sheep and goats and involving large-scale year round movement in order to remnant water and pasture, the origins of a nomadic lifestyle that persists to this day. In contrast, lower elevation regions were characterised by increasing settlement relict oases, associated with sedentism and more intensive exploitation of resources. Di Lernia and Palombini characterise the former as a ‘light’ approach landscape exploitation, associated with a relative egalitarianism and sustainable of resources, and the latter as a ‘hard’ and ultimately unsustainable approach to landscape leading to possible degradation of the landscape, conflict (possibly land rights) and increased social stratification. Behavioural adaptations focusing on resource extraction were associated profound changes in social organisation, as indicated by archaeological studies funerary monuments and burial practices. Di Lernia and describe THE CLIMATE-ENVIRONMENT-SOCIETY NEXUS IN THE
Page 10 period centred around 5 ka as the ‘hinge’ between different pastoral cultures in Wadi Tanezzuft in the Libyan Fezzan, which they term Middle and Late Pastoral. is around this time that stone funerary monuments are associated with burials in the Wadi Tanezzuft; previously these had been reserved for ritual burials. This process is reflected in other central Saharan regions. A review of archaeological work by indicates that in southwestern Libya and Niger, the period 6.4–5.9 ka is represented exclusively by faunal burials; 5.8–4.9 by faunal, human and ‘empty’ burials (i.e., cenotaphs); and 4.8 ka onwards by human burials. Di Lernia and interpret this innovation in funerary as being associated with ‘emerging figures within the pastoral group... [representing] the first evidence in the area of a process of increasing social stratification’. Di Lernia et al. (2002) interpret the evolution of funerary monuments practices and settlement patterns as evidence of major changes in population in Wadi Tanezzuft, and by implication in the greater central Sahara region. monuments would have served a dual purpose in a landscape occupied by numbers of pastoralists, on the one hand acting as foci for gatherings of related groups, and on the other serving as markers of boundaries, territories or zones influence, asserting relationships between clan groups and the landscape. In the Fezzan, it appears that climatic desiccation was associated with migration, increased population density, changes in religious beliefs and practices, social stratification and a more territorial approach to the landscape, as well as diver- ging adaptations to facilitate resource extraction in a more hostile landscape. certain localities these changes appear to have provided the preconditions for emergence of complex urban societies and the formation of entities states, catalysed by the final desiccation of most of the landscape soon 3 ka. 89, The onset of conditions equivalent to present aridity might be viewed a threshold that stimulated a step change in social organisation as groups adopted new techniques to access water and utilise scarce productive land. In the Wadi Tanezzuft, the depletion of soil water reserves was not completed about 3.5 ka, and fluvial activity persisted until around 2.7 ka. Further to the in the Wadi al-Hayat, there is evidence that springs dried up around or before 3 ka. These late dates for fluvial and spring activity correspond approximately to early stages of the Garamantian civilisation, which dominated the Fezzan about 1, 700, a period ‘notable ... for the local evolution of urbanism, irrigated agriculture and writing’. The Garamantes are described by the historian Herodotus, writing in the fifth , and later represented a challenge to Roman aspirations in the central Sahara and North Africa. The Garaman- tian ‘capital’ of Garama (or Old Germa, situated within 2 km of the modern town Germa/Jarma) was located in the Wadi al-Hayat (also known as the Wadi al-Ajal), with Garamantian settlements also located in the nearby Wadis, al-Shati, and Tanezzuft. 94,95,96,97, By 3.1 + 0.125 ka the springs had dried up and surface water was either scarce or absent at the base of the escarpment forming the southern boundary the Wadi al-Hayat. 99, However, the water table was probably very near surface at the base of the wadi, as is evidenced by the fact that recent THE JOURNAL OF NORTH AFRICAN
Page 11 excavations uncovered a well in the early Garamantian phase of Old Germa sunk to depth of 70 cm, indicating water at very shallow depths. evidence indicates that irrigated agriculture was introduced soon after the of the springs, in the early part of the first , presumably in to a lack of water caused by the fall in the water table. There is no direct evidence irrigation systems at this time, but the water table was near the surface in Germa and is possible that wells were used to tap this resource. The earliest irrigation systems foggara which were used to tap the elevated water table at the base of the escarpment. Archaeological evidence suggests that they were probably introduced by the final , and definitely before the fourth . Interestingly this corresponds with archaeobotanical evidence for the intensification and of agriculture involving the introduction of a farming system that utilised both and summer crops. Foggara would also have allowed the extensification agriculture at a similar time. In combination these developments could have lead an increase in agricultural production, and there are likely to be strong between this and the rise of the Garamantes as a major political power in central Sahara. The Garamantian culture appears to have been the result of local innovation, outcome of a process of increasing social complexity among the pastoral groups of Fezzan. Referring to the Wadi Tanezzuft, di Lernia et al. write that ‘In a sense, Late Pastoral people became the Garamantes’. This conclusion is by the work of Mattingly et al., who find some of the latest Pastoral lithics pottery in early Garamantian forts along the southern edge of the Wadi al-Hayat. In the Fezzan we thus see the ultimate expression of the human response to deserti- fication in the development of a significant urban civilisation. The emergence of the Garamantian polity, largely driven by changes in availability and geographic serendipity, is not the only example of increased complexity leading to the emergence of what we might call a ‘state-level society’ in a time of increasing aridity. The earlier development of Dynastic Egypt has been interpreted at least in part as a result of social responses to environmental desic- cation. 108,109, Palaeoenvironmental evidence indicates increasing aridity to the and west of the Nile Valley during the 6th millennium BP, with a final desiccation most of this region in the late sixth millennium BP, when the early Dynastic emerged. 111,112,113, Excavations at Hierakonpolis are indicative of attempts to inte- grate animal herds with a sedentary lifestyle at the beginning of the Dynastic period, suggesting that mobile groups were forced or encouraged to settle permanently in Nile Valley. Midant- explicitly links local increases in density with desertification at the end of the Predynastic period. evidence provides abundant support for models of cultural evolution increased social stratification and the concentration of political power as a to increased population densities resulting from a decrease in available land, and associated migration/increased sedentism as a consequence of environ- mental desiccation. To summarise, linked environmental and cultural change in the Sahara is simply a matter of poverty and famine resulting from aridity. During the THE CLIMATE-ENVIRONMENT-SOCIETY NEXUS IN THE
Page 12 Neolithic, in the 7th and 6th millennia BP, the Sahara gave birth to one of the cultures of antiquity, the Saharan cattle-based culture. Material cultural, sites, food security, rock art, ideology and funerary practices are all part of an extra- ordinary culture, the nature of which was driven by a profound relationship with physical environment. This culture had to cope with numerous fluctuations resource availability associated with climatic variability on a range of timescale, some of which were extremely severe. This cultural tradition only came to an with the final desiccation of the Sahara around 5 ka. However, the pastoral of the Holocene Sahara did not vanish into the sand: the migration of pastoral to refugia such as the Nile Valley, the Sahel, and the Saharan highlands and oases made a vital contribution to many subsequent African cultures, Pharaonic Egypt, the Garamantes, the present-day cultures of the Sahel, and the modern day Berber and Tuareg populations. 118,119,120, In a more general sense, we may claim that the essence of this cattle-based culture spread from the Sahara much of sub-Saharan Africa, shaping the history of the entire African continent. Water Resources and Human As has been the case throughout the Quaternary period, water resources are today limiting factor in human settlement and development. Increasing populations associated urbanisation and economic development are placing greater demands water resources throughout the Sahara. These resources may also be affected climate change in the near future, for example with elevated surface further enhancing evaporation, and changes in meteorological patterns resulting changes in the distribution of rainfall. Modern permanent settlements in the Sahara are situated where water is either from rainfall generated as a result of topography or the intrusion of extra- Saharan weather systems (such as in highland and coastal regions), or at where groundwater occurs at or near the surface in local topographic minima. Water resources may therefore be affected by rainfall variability and changes groundwater levels. Groundwater and Human Most inland Saharan settlements rely almost exclusively on groundwater. pumping and irrigation technology has enabled the expansion of agriculture many locations, such as the Wadi al-Hayat in the Libyan Fezzan, which an object lesson in the interaction of water resources, human populations and techno- logical innovation. Here a downward trend in population density started in the with the decline of the Garamantian culture. The cause of demise of the Garamantes is not known with any certainty, although water levels either as a lagged response to the climatic desiccation of the region, as a result of over exploitation by local populations, has been suggested. However, other explanations, such as a decline in trade in the later years of Roman empire, should also be considered. Whatever the cause of the demise the Garamantes, it appears that groundwater levels declined between THE JOURNAL OF NORTH AFRICAN
Page 13 times and the end of the second . In recent decades the socio- economic decline of the Wadi al-Hayat has been reversed through the use of irriga- tion, dependent on the pumping of groundwater from increasingly greater depths. However, increases in population, the expansion of agriculture, and the process urbanisation have resulted in a lowering of groundwater levels since the 1970s, exceeding 20 m in some locations according to local informants. White et al. used satellite imagery and aerial photography to examine changes in distributions between the 1950s and the end of the twentieth century in the al-Hayat, and found a shift in the vegetated zone resulting from the expansion agriculture at the southern edge of the wadi, and the die-back of vegetation in unirri- gated northern areas. The most obvious adverse impacts are on stands of date palms, many of which no longer survive without human intervention, and have given way more drought or salt-tolerant species. Such studies raise questions of sustainability: can settlements in central regions continue to expand given their reliance on fossil water reserves that are longer replenished by rainfall? Furthermore, how sustainable are schemes such the Great Manmade River project in Libya? Water use in Libya has been at some eight times higher than its renewable water resources, an extreme of a situation faced by all the countries of the southern Mediterranean coast means they are likely to become much more dependent on food imports (a means of importing water) in the future. Pressure on water use is likely to exacerbated by climate change; in the southern Mediterranean reductions in of some 20–25 per cent and an increase in average annual temperature of 2– 2.758 C by the 2050s have been forecast by global climate models. Increases temperature will lead to greater evaporation, compounding water scarcity from reductions in rainfall. Such developments are likely to increase pressure fossil water reserves from the Saharan aquifers. Any expansions in the sector will also increase water demand. Given the likely increased use of groundwater, a better understanding of groundwater reserves and dynamics, the nature, capacity and behaviour of the Sahara’s large subterranean aquifers highly desirable. Such aquifers will not reach equilibrium to keep pace with extrac- tion, which will result in the formation of ‘cones of depression’ near areas or areas of intense agricultural activity. Ebraheem et al. conclude that Nubian Sandstone Aquifer under southwest Egypt does not exist in a steady and is still responding to past humid conditions, and estimate that the planned extrac- tion of 1,200 million /year in the East Oweinat area could result in a drawdown up to 200 m relative to 1960s levels within 100 years, with the cone of extending to Dakhla and Kharga oases. Rainfall Given the extreme variability of precipitation even in the wetter parts of the Sahara, the concept of ‘mean annual rainfall’ has little practical meaning. However, it is pointing out that precipitation in some parts of the Sahara is sufficiently high for populations to rely at least partially on rainfall, and to be adversely affected periods when rainfall is anomalously low. describes how the THE CLIMATE-ENVIRONMENT-SOCIETY NEXUS IN THE
Page 14 Ahaggar Tuareg have developed strategies to cope with drought, and how coupled with the social upheavals of Algerian independence led to hardship in 1960s. Drought is a concept that is also familiar to the inhabitants of the far west of Sahara. In the inland regions of the disputed territory of Western Sahara is relatively abundant, sustained by occasional summer rains representing the northerly penetration of the African Monsoon beyond 208 N, and similarly rainfall associated with the Atlantic Westerlies. Despite the relative abundance rainfall and vegetation in this region, the lack of surface water (e.g. in the of oases), as well as the ongoing political conflict with Morocco and the of a resumption of hostilities, mitigates against permanent settlement. Nonetheless, the region is used by nomads whose principal source of water takes the form milk from their animals. The Sahelian region, situated at the southern margin of the Sahara, has experi- enced one of the most persistent and severe changes in climate during the period meteorological records. This consisted of a multi-decadal scale drying commencing in the 1960s and persisting into the 1990s, with severe droughts the early 1970s and 1980s, and some amelioration in recent 132, (Figure 2). It is now well established that rainfall in the Sahel is driven largely by patterns global surface temperature, with the temperature of the Indian Ocean playing a role. While rainfall data for the Sahara are sparse, there is no evidence for comparable trend north of the Sahel-Sahara transition zone in those regions of Sahara where topography results in non-negligible rainfall. For example, annual and summer rainfall totals for Tamanrasset in the highlands of the Sahara (home of the Kel Ahaggar Tuareg mentioned above) reveal a wet period the 1950s, as in the Sahel, but no long-term drying trend in subsequent FIGURE SPATIALLY AGGREGATED ANNUAL RAINFALL ANOMALIES (IN STANDARD DEVIATIONS) REPRESENTING THE REGION 108 – 208 N; 258 W – 308 E, ROUGHLY CORRESPONDING THE SAHELIAN ZONE. ANOMALIES ARE CALCULATED WITH RESPECT TO THE FOR THE ENTIRE SERIES (1901 – 1998) Source: From the dataset of New et al. (2000). THE JOURNAL OF NORTH AFRICAN
Page 15 (Figure 3). Instead, the Tamanrasset rainfall record consists of quasi-variations of some 10–15 years, with extreme variations in annual rainfall from to 160 mm. On average, approximately half of the rainfall at Tamanrasset occurs the summer, although variations between individual years are great. For example, the period from July to September saw no recorded rainfall in 1926 and 1973, whereas all of the recorded rainfall during 1999 occurred in these months. The near to medium term future of the Sahara is uncertain in climatic terms. there are no indications that the region will experience a shift to humid comparable to those existing in the early Holocene, a number of climate studies suggest that anthropogenic climate change may be associated with a strength- ening of the African summer monsoon and an intensification and northerly displace- ment of monsoon rains, leading to wetter conditions in the northern Sahel southern Sahara. 135,136, Other modelling work suggests that the Sahara may northwards, implying that even if rainfall increases in the south of the Sahara, may decline in regions near the Mediterranean coast. The suggestion that the Sahel and the southern margins off the Sahara may become wetter is consistent recent observations indicating a greening of the Sahel explained partly by rainfall and partly by increased vegetation cover due to human activity. 139, However, rainfall still remains below the high levels of the mid-century, and interannual rainfall variability and drought still pose problems for Sahelian societies, as evident from the famine that is unfolding FIGURE TWENTIETH CENTURY ANNUAL RAINFALL TOTALS FOR TAMANRASSET, OVER WHICH SUPERIMPOSED A 5-YEAR MOVING AVERAGE (SOLID LINE) Source: The Climatic Research Unit. THE CLIMATE-ENVIRONMENT-SOCIETY NEXUS IN THE
Page 16 Niger at the time of writing (July 2005). The impact of climate change on climate is likely to remain unclear for some time, until additional and modelled data are available. A shift in the monsoon rain belt would have profound implications for societies. Areas that are not currently viable for human populations would available for pastoralism, and existing marginal areas might become viable rainfed agriculture. However, an intensified monsoon would also be with more frequent flash floods and the likely spread of water borne diseases. Recently high rainfall in the Sahel has enabled locusts to thrive, resulting in devastation of crops and food insecurity in a number of countries in 2004 2005. describes how heavy rainfall in the Sahel in 1953 famine. While an intensification of the monsoon would undoubtedly bring in terms of pastoralism and agriculture, the consequences would not necessarily wholly benign. Extreme rainfall variability on different timescales is a fact of life in on margins of the Sahara; a failure to fully appreciate this fact was a factor in expansion of agriculture into historically marginal areas along the Sahel-boundary in the 1950s and 1960s, which were anomalously wet when with the twentieth century rainfall record as a whole. As sedentary expanded northwards, mobile pastoralists were pushed into more marginal areas. The vulnerability of both populations to drought was significantly increased, FIGURE TWENTIETH CENTURY RAINFALL TOTALS FOR TAMANRASSET FOR THE MONTHS JULY, AUGUST AND SEPTEMBER (THE WETTEST MONTHS IN THE SAHEL), WITH 5-MOVING Source: The Climatic Research Unit. THE JOURNAL OF NORTH AFRICAN
Page 17 and the combination of elevated vulnerability and severe rainfall deficits in the 1970s resulted in widespread famine and the collapse of livelihoods and systems, representing a significant discontinuity in Sahelian developmental trajec- tories from which the region is still recovering. describes the droughts of the 1970s and 1980s as a contributory factor the development of internal conflict in Mali, where it undermined the livelihoods of the semi-nomadic Tuareg, causing large numbers of them to refuge in camps or urban areas where they experienced social and marginalisation. Many Tuareg migrated to neighbouring countries, and young men become involved in conflicts throughout North Africa and the East, in which they acquired considerable military experience, eventually to Mali having to face unemployment and marginalisation, creating the for the ‘Second Tuareg Rebellion’ in 1990. These conditions were exacerbated a history of mistrust between the Tuareg and post-independence governments, lack of available livelihoods and social support networks for returning migrants (a result of previous drought and conflict), continuing drought and competition for resources between nomadic and settles peoples, and the flooding the region with small arms as a result of conflicts in neighbouring countries, particu- larly Western Sahara. describes the conflict between the nomadic Tuareg the settled communities as ‘not so much an ethnic or racial issue as an economic one’ highlighting ‘the economic dimensions of the problem in the north: were fighting for scarce resources and jealously insisting that others were preferred.’ This pattern of marginalisation, drought and conflict has been throughout the Sahel, and demonstrates that climate variability and change combine with other factors to cause conflict, particularly in marginal where populations are facing a number of different environmental, social, and political stresses. Any future increase in rainfall in the northern Sahel and southern Sahara be associated with the risk of unsustainable agricultural expansion if longer-climatic variability is not considered in development policies. A study by Maynard et al. suggests that the Sahel may be less prone drought in a world characterised by low to moderate levels of greenhouse warming, associated with atmospheric greenhouse gas near current values. However, other studies suggest that the greater levels greenhouse warming likely to result from the continued intensive use of fuels into the latter half of the twenty-first century may result in Africa. 149, Any expansion of agriculture and settlement into areas newly productive by a strengthened monsoon may ultimately result in an exacer- bation of vulnerability as in the 1950s and 1960s, followed by drought famine as conditions deteriorate, as they did after the 1960s. The interaction naturally occurring drought hazard and socially constructed vulnerability the twentieth century holds important lessons for economic development agricultural activity in a region where climatic variability on a variety of time- scales is the norm. THE CLIMATE-ENVIRONMENT-SOCIETY NEXUS IN THE
Page 18 Non-hydrological Hazards and While the availability of water must remain the principal consideration in development and settlement expansion, human health and comfort are also by other environmental factors. In the Sahara, strategies for coping with heat, and also with dust storms, are also of great importance. As mentioned above, climate change is likely to result in higher average surface temperatures, which turn will lead to more frequent heat extremes. 151, A further instance of what may be termed “maladaptation” associated population expansion and the associated processes of economic development urbanisation is the move away from traditional architecture. Traditional materials and architectural styles are well adapted to the extreme desert environment, particularly in terms of temperature regulation. Historically, settlements have built of mud brick, an inexpensive and readily available material that is an insulator, and have incorporated convection chimneys to encourage the circulation cool air, and covered walkways to shelter their inhabitants from the sun. The nature of these settlements also affords some protection from dust. However, many these settlements are now being abandoned in favour of generic modern towns construction pays little or no attention to the particular hazards associated with desert environment. Streets are now open to sun and dust (partly to vehicular traffic), and poorly insulated modern buildings fitted with air have replaced the subtle architecture of the traditional towns. In the town Ghadames in western Libya, the inhabitants of a traditional town (also a World Heritage Site) moved to an adjacent, purpose built modern town in 1980s. In the summer many return to the old town, where they maintain their houses and gardens, as it is more comfortable than the modern settlement. The challenge of urbanisation in the Sahara is to blend the most desirable of traditional architecture with modern technology, providing comfort in an environment while ensuring access to modern amenities. Greater use of mud instead of concrete and cement (the production of which is energy intensive) reduce both construction costs and greenhouse gas emissions, assisting adaptation to and mitigation of anthropogenic climate change. Fathy, working the context of development in rural Egypt, has advocated a greater emphasis such traditional building materials and styles in order to enhance development reduce poverty through the use of more affordable materials and the inclusion local people in the design and construction process. The Sahara in the Earth System: Airborne Mineral Water, or rather the lack of it, is not the only ubiquitous feature of the environment with which its inhabitants have had to contend since the onset of desic- cation many millennia ago. The geomorphological processes associated with environmental changes in northern Africa have also resulted in the Sahara the world’s largest source of airborne mineral dust. The mobilisation, and deposition of dust has major impacts not only within the Sahara, but in THE JOURNAL OF NORTH AFRICAN
Page 19 regions outside of northern Africa. Dust mobilised in the Sahara is transported distances and deposited not only within Africa itself but also over the Atlantic, Mediterranean, the Middle East, Europe and the Americas. 154,155,156, In many these regions dust transport and deposition has significant implications for climate, ecology and human health. Given the potential for human impacts on the surface and anthropogenically driven climate change to affect dust production, generation of dust represents another key interface between people and the environment in North Africa, with global implications. Here we consider the role dust in the coupled earth-ocean-atmosphere system, before moving on to address nature of Saharan dust sources and their evolution over time. Finally the impacts human systems (principally on human health) of Saharan dust, and of changes in mobilisation, transport and deposition, are discussed. Dust Impacts on Dust directly affects climate at local and regional scales by modifying the structure of the atmosphere. On the one hand dust reduces the amount of solar radiation reaching the Earth’s surface, causing cooling in the lower atmosphere, On the other hand dust absorbs outgoing longwave radiation, trapping heat in atmosphere and causing warming in much the same way as gases. 158, These two effects act in opposition to one another, and the effect of dust on temperature depends on a variety of factors including the distribution of the dust, the size distribution of the dust particles, their composition, the nature of the underlying surface, and the time of day. Where dust exists in elevated layer overlying less dusty air, the overall result will be a cooling of Earth’s surface and the lower levels of the atmosphere during the daytime. night, only the longwave effect acts, resulting in warmer surface temperatures. Dust thus acts to reduce the daily temperature range at the Earth’s surface. Dust exists in a elevated layer – the Saharan Air Layer (SAL) – overlying humid oceanic air mass over the Atlantic Ocean and over the Sahel during monsoon season. Warming within the dust layer, reinforced by near-cooling during the daytime, acts to reinforce the temperature inversion at the of the SAL and stabilise the atmosphere, inhibiting convective activity. This phenom- enon has been detected over the Sahel, where it has been proposed as a of drought . Atmospheric stabilisation by dust has also observed to inhibit the development of hurricanes over the Atlantic. The of sea surface temperatures by dust over the northern Atlantic may also play a role reinforcing or sustaining the dipolar temperature anomaly (cooling over the hemisphere relative to the southern hemisphere and northern Indian Ocean) associ- ated with drought conditions in the Sahel. 163,164,165, Low altitude cooling also reduce evaporation over the ocean or over moist land surfaces, further the likelihood of rainfall. Simulations using a general circulation that evaporation and precipitation are reduced globally by dust, although they that the local presence of dust increases rainfall over deserts. However, this conclusion is not supported by any observational evidence. THE CLIMATE-ENVIRONMENT-SOCIETY NEXUS IN THE
Page 20 Dust can also affect global climate through its influence on marine and ecosystems. The most widely known such impact is the fertilisation of the ocean iron carried in aeolian dust originating in the world’s deserts. Jickells et al. estimate average global dust production at around 1.7 billion tonnes per year, almost two-thirds of this material originating from North Africa and 26 per cent the dust reaching the oceans. Once deposited in the ocean, iron from dust is believed to enhance biological productivity, which leads to the of atmospheric carbon dioxide that is taken up by phytoplankton and exported deep water and ultimately to the ocean floor to be incorporated in marine sedi- ments. Thus oceanic dust deposition contributes to a reduction in the gas content of the atmosphere, acting to modulate the rate of increase of anthropo- genically driven greenhouse warming. The importance of this effect is a matter debate, however, and further research is necessary in order to quantify the of dust on atmospheric greenhouse gas concentrations. Dust also acts to the production of dimethyl sulphide (DMS) by marine organisms; when emitted the atmosphere, DMS is oxidized to sulfate aerosol, which scatters solar back to space, thereby acting to cool the Earth’s surface, further offsetting or regional surface warming. Nutrients from Saharan dust, such as phosphorous, are also important for terrestrial ecosystems, and are believed to play a vital role in sustaining the rainforest. Modelling studies suggest that the forests of Amazonia are vulnerable large-scale die-back caused by higher temperatures and reduced rainfall from anthropogenic climate change; 173,174, any reduction in the supply of to the forest canopy may conceivably accelerate this process. Die-back of the forests would release large quantities of carbon dioxide into the atmosphere, acceler- ating global greenhouse warming. Finally, dust may affect climate by modifying the reflectance or albedo of the surface. Where dust is deposited over snow fields or ice sheets it reduces the reflec- tance of the surface, increasing the amount of solar radiation absorbed by the which results in heating and melting of the snow and ice. While such a process unlikely to be significant at the global scale today, this mechanism is believed have been important in regulating past glacial cycles, in periods when global transport was much greater than at present. Dust deposition over areas may accelerate the melting of glaciers and snow and ice fields, which already shrinking in many areas as a result of climate change. In certain regions this could impact on water resources, flood hazards and tourism, significant economic consequences. Dust is clearly an important component of the atmosphere, and represents medium via which the Sahara influences global climate through the modulation global biogeochemical cycles. The Sahara, in turn, is sensitive to global change, as demonstrated by its dramatic history of transitions between arid humid conditions. Any change in the Saharan dust cycle is likely to have repercussions for other regions, and perhaps for global climate as a whole. efforts at understanding the dust cycle and the sensitivity of the Sahara to in climate are thus required if we are to improve our capability to model THE JOURNAL OF NORTH AFRICAN
Page 21 predict future global climate change. Any such efforts must start with an of the nature of the dust sources themselves. The Nature of Saharan Dust Dust is not emitted from the Sahara in a uniform or random fashion; the vast of aeolian material is generated from a variety of specific source regions that particular seasonal patterns of activity. These regions are prescribed by a combi- nation of geomorphological and meteorological factors, principally a supply of particles available for erosion, little or no vegetation cover, and wind speeds regularly exceed the threshold value for emissions for the surface in question. Wind erosion may be suppressed by the presence of surface crusts (which common in arid regions) and the presence of obstacles such as bushes, rocks pebbles. The process of emission, once the threshold wind speed has been exceeded, begins with the mobilisation of large particles (in the size range 40–500 mm) are free at the soil surface. These are typically sand particles derived from quartz, often with small clay particles adhering to their surface, and aggregates of clay. The impact of these particles on the ground surface results in their own and the breaking up of aggregate particles within the surface. This sandblasting in the release of particles of dimension less than about 20 mm which are available uplift into the atmosphere and subsequent long-range transport. 178, In recent years the major dust sources in northern Africa have been identified satellite remote sensing, principally using the Total Ozone Mapping (TOMS) Aerosol Index (AI) and the Infra-red Difference Dust Index (IDDI) derived from data acquired by METEOSAT. 181, Global monitoring of using the TOMS AI indicates that the Sahara is by far the worlds largest source, and that the most active source region is the Bodele Depression, described as the worlds largest single dust source, or the ‘dustiest place in world’. 183, The Bodele Depression contains the exposed lake sediments of now desiccated Lake Megachad, and it is these sediments that provide the mobilised as airborne dust. However, the importance of the Bodele Depression also a result of its geographical situation to the southwest of the gap between Tibesti and Ennedi Mountains, which funnels and amplifies the prevailing northeast- erly winds, resulting in a high frequency of events during which surface winds the threshold velocity for sediment mobilisation. In addition, field and observations reveal that the lake sediments are partially covered by mobile dunes, which provide an abundant supply of coarse material with which palaeolake surface is sandblasted. The western Saharan region straddling the borders of Mali, Mauritania Algeria also appears to be particularly active; in the IDDI imagery this region is prominent as the Bodele Depression, and is flanked by additional sources in south-central Algeria and the northern regions of Western (Figure 4). Satellite imagery and field observations indicate that these regions are heterogeneous in nature, consisting of a number of different including but not restricted to dry lake beds. Field observations by two the authors (Drake and Brooks) in the north of Western Sahara, coupled THE CLIMATE-ENVIRONMENT-SOCIETY NEXUS IN THE
Page 22 observations from the Moderate Resolution Imaging Spectrometer (MODIS) identified clay lake beds as sources of airborne dust. An extensive gravel plain strad- dling the border of Western Sahara and Mauritania also appears to be a source of dust; this plain is cut by dry channels and it is likely that the mobilised by the wind in this location derive from both past wind and water action. The coarse resolution of the TOMS and IDDI imagery (in the region of 40 km) means that many of the dust sources indicated by long term averages of both of data remain an enigma. The situation is complicated by disagreement the TOMS and IDDI datasets, which is due at least in part to the lack of of the TOMS AI to low-level dust. The main western source region is more promi- nent, and located further north, in the IDDI than in the TOMS dataset. In the IDDI, Bodele region is linked to the western source area by a zone of intermediate values extending through central Niger and northern Mali (including the of the western maximum in the TOMS data); similar magnitude values are over much of south-central Mauritania near the northern limit of the monsoon rains. number of other regions appear as sources in the IDDI but not in the data. 187, These include a region extending from south to north through centre of northern Sudan and southern Egypt, approximately following the FIGURE MEAN ANNUAL IDDI VALUES OVER AFRICA, REPRESENTING AVERAGE REDUCTION THE BRIGHTNESS TEMPERATURE OF THE EARTH IN KELVIN, AS MEASURED BY INFRA-RED CHANNEL OF METEOSAT. HIGHER VALUES INDICATE GREATER OF INFRA-RED RADIATION BY ATMOSPHERIC AEROSOLS; OVER THE SAHARA DUST IS THE DOMINANT AEROSOL, WHILE HIGH VALUES IN OTHER REGIONS RESULT FROM BIOMASS BURNING. SAHARAN REGIONS WHERE IDDI VALUES CONSISTENTLY HIGH (REPRESENTED BY THE ORANGE AND RED AREAS) INTERPRETED AS REPRESENTING MAJOR DUST SOURCE REGIONS; NOTE THE DEPRESSION IN THE CENTRAL-SOUTHERN SAHARA AND THE REGION OF HIGH IN THE WESTERN SAHARAN THE JOURNAL OF NORTH AFRICAN
Page 23 of the Nile and extending several hundred kilometres either side of it, and a region the northwest of the Tibesti mountains where the borders of Libya, Niger and meet, extending northwards into the central regions of southern Libya. Despite their disagreement, the TOMS and IDDI data clearly indicate that fluvial activity plays an important role in determining the distribution and nature the main Saharan dust sources. Sediments from palaeolake Chad provide the for wind erosion that makes the Bodele Depression such an active dust source, smaller dry lake beds are evident as dust sources in Western Sahara. However, while topographic lows containing fluvial sediments from past humid episodes clearly important in the global dust cycle, a straightforward mapping of sources onto such features appears overly simplistic. For example, the largest enclosed basin, the recently identified Lake MegaFezzan in Libya, appears to generate little or no dust. This palaeolake exhibits very geomorphology to palaeolake Chad, as its sediments are capped by hard calcrete gypsum crusts and large dunes. Clearly, not all palaeolake surfaces act as sources. Neither are all dust sources associated with palaeolakes, as illustrated the emission of dust from the gravel plains of Western Sahara. Instead, sources appear to be heterogeneous in nature, with different sources being at different surface wind speeds. Climatic Versus Human Influences on Dust Source During the 1990s it was suggested by a number of authors that apparent increases the mobilisation and transport of northern African dust over the latter half of the twen- tieth century were the result of land degradation in the Sahel, combined with desiccation. 191,192, However, there is little or no evidence for widespread systema- tic land degradation resulting from human activity in this region, and a number studies have convincingly argued that what has often been interpreted as systematic, regional-scale desertification (the ‘southwards march of the Sahara’) is nothing than the physical expression of an oscillation of the ‘desert boundary’ as the and maximum northerly position of the Inter-Tropical Convergence Zone and associ- ated monsoon air mass varies on interannual and interdecadal timescales. 194,195,196, Historically, the controversy over the role of land degradation in dust comes from the paucity of measurements that would permit the identification of long-term trends in dust mobilisation in the Sahel. Indeed, for many years the location at which dust was measured on a regular basis was Barbados, 5,000 km from the western coast of Africa. Available since 1965, i.e., the beginning of both dramatic population growth in the Sahel and the decline in rain- fall in this region, this dataset would have been suitable for the search for trends production (rather than transport) if its ability to represent processes occurring the African continent were not so questionable. In contrast to this highly record, satellites offer a unique opportunity to monitor atmospheric dust at global scale. However, suitable satellite imagery for dust monitoring has only available since the very end of the 1970s, and thus covers too short a period for to be useful in discriminating between an anthropogenic trend and the ‘natural’ impact of drought in the 1980s and 1990s. Combining satellite imagery and THE CLIMATE-ENVIRONMENT-SOCIETY NEXUS IN THE
Page 24 from the Barbados record may provide a means of addressing this problem in the future. However, as discussed above, assessments of the major dust source indicate that they are overwhelmingly are located in Saharan regions that are to have experienced significant human impacts. 199,200, It appears that variations have been the principal driver of observed changes in dust production. On the one hand variations in rainfall have affected vegetation cover and influenced the susceptibility of certain surfaces to wind erosion, while on the changes in atmospheric processes are likely to have resulted in changes in balance between dust mobilisation and deposition within the Sahel and Sahara. 202, Climate change may play a key role in modulating future dust from the Sahara. Furthermore, Chiapello and have shown that the Atlantic Oscillation (NAO), a large-scale meteorological system that controls the strength of the Trade-Winds during the winter, explains a large part the year-to-year variability in dust transport from the Sahara to the tropical Atlantic. Its influence spreads further since a correlation of the NAO index dust activity in the Bodele depression suggests that the NAO influences emissions as well as transport. It is thus likely that future changes in circulation will strongly affect the dust cycle, a problem that numerical models have to answer in the forthcoming years. Future Dust On long timescales, dust emissions from northern Africa are modulated by large-changes in climate, as demonstrated by the abrupt increase in wind-blown dust depos- ited off West Africa at the end of the last Saharan humid phase around 5.5 ka. While dust sources do not map directly onto palaeolake surfaces, they are associated with fluvial activity, begging the question of how long they may active before their reserves of erodible material are depleted once fluvial has effectively ceased. Wind action is limited to the soil surface, so emission can be sustained in the term only if the superficial soil layer is constantly supplied with fine material. Such supply can be explained in terms of the settling of dust from the atmosphere, disaggregation of larger particles by wind action, and the disturbance of the due to the motion of sand dunes (anthropogenic disturbance, e.g. by vehicles, play a role, but the contribution of such processes has not been quantified and is no evidence for large ‘new’ dust sources, as discussed above). The action of rainfall and associated fluvial activity also represents an efficient of disturbing the land surface and of carrying fine material to basins via of wadis. While such events are rare in the Sahara, the lack of vegetation that single events can disturb and mobilise large amounts of material. Furthermore, rainfall does occur fairly regularly over some of the Saharan highlands, providing means of replenishing erodible material in the adjacent lowlands. Rajot et al. conclude that the vertical flux of emitted dust is not supply limited, even with fraction of silt and clay as low as 1.5 per cent near the surface. This finding THE JOURNAL OF NORTH AFRICAN
Page 25 supported by evidence that areas not likely to be replenished by recent fluvial activity, such as flat gravel plains as described above, are also sources of dust. There thus appears to be little reason to expect the supply of material available wind erosion in the Sahara to be exhausted in the foreseeable future. However, climate change has the potential to alter the amount of dust mobilised in and from the Sahara via its impacts on the land surface. In particular, the possible ‘green- ing’ of the southern Sahara as a consequence of anthropogenic greenhouse as discussed above, may serve to reduce the supply of dust from certain through the stabilisation of the land surface. A strengthening of the Monsoon and resulting expansion of vegetation cover as modelled by and Claussen et al. could conceivably cut off the dust supply from sources to the current monsoon belt, including the Bodele Depression. Given the role dust in the climatic, meteorological and biogeochemical processes discussed in this section, such a partial greening of the Sahara could have significant and regional implications. Impacts of Dust on Human As a component of the Earth system which contributes to the regulation of biogeo- chemical cycles, ecological productivity and climate, variations in dust mobilisation, transport and deposition have implications for human activities. Impacts on productivity may affect fish stocks, with associated economic implications, individual dust events affect transport and day-to-day human activities. Large-influences of dust on climate and the possible role of dust in modulating may potentially be implicated in food insecurity. The role of dust in the removal carbon dioxide from the atmosphere via its influence on marine and terrestrial biologi- cal productivity is likely to play a role in the sensitivity of the global climate anthropogenic greenhouse gas emissions. In order to model and predict climate changes that will require adaptive responses in human societies, dust be better represented in global climate simulations. The representation of dust regional climate models and the incorporation of its effects in regional scenarios future climate is necessary if certain key processes are to be represented accurately; for example, changes in atmospheric dust content over the northern tropical might have profound implications for future trends in hurricane activity. of such trends will be crucial in the development of policies, strategies and to cope with future changes in climate and their impacts. Regardless of its impact on future changes in climate, the present-day impact dust on human health is sufficient grounds for further research into its characteristics, mobilisation, transport and deposition. Mineral aerosols transported large are generally less than 20 mm in dimension, and typically contain large numbers particles of sub-micron dimension. They therefore span the size range for associated with adverse impacts the human respiratory system. Saharan events regularly contribute to air pollution limits being exceeded in the region, and African dust frequently reaches the Americas and north-Europe, where it can contribute to high atmospheric pollutant levels when with aerosol particles from other sources. Variations in concentrations of THE CLIMATE-ENVIRONMENT-SOCIETY NEXUS IN THE
Page 26 whose size is the order of 10 mm (PM10s) in Trinidad are associated with the transport of dust from northern Africa. Very little internationally published research has been carried out into the impacts of airborne dust within northern Africa (although see Laval, Fossati, 1969. ) Nonetheless, research in other parts of the world suggests these impacts are likely to be great. Lung conditions in populations living in or semi-arid regions have been linked with exposure to silica in the Himalayas, the Thar Desert of and the Southwest USA, and the deposition of in the lungs is associated with a widely recognised condition known as ‘desert syndrome’. 219, Dust events originating in the interior desert of Australia been associated with increased incidences of asthma in Brisbane. The for links between dust and asthma in the Caribbean is more equivocal. It has suggested that dust clouds originating in the Sahara are associated with paediatric asthma accident and emergency emissions in Trinidad, where there is widespread belief that the passage of Saharan dust exacerbates rhinitis asthma. However, emerging findings from an ongoing study by one of authors (Prospero) and colleagues appear to challenge this conclusion. There evidence that dust from northern Africa is associated with the long-range of micro-organisms; daily aerosol samples collected throughout 1996–1997 the trade winds reaching Barbados yielded significant concentrations of viable (i.e. culture-forming) bacteria and fungi only when African dust was present. In of the Sahel, dust storms are believed to be responsible for certain potentially sicknesses. There is thus a strong public health case for research into the health effects of in the Sahara, and into ways to ameliorate any such effects. A greater understanding the activity and nature of Saharan dust sources, and of their potential future evolution, is therefore relevant for health policy both inside and outside northern Africa, as as for activities such as climate forecasting. Such an understanding requires compre- hensive campaigns of field work to investigate the geomorphological, and mineralogical nature of the key dust sources and the surface conditions with emission, as well as remote sensing studies of dust sources and transport. fieldwork is currently underway in the Bodele source region and is an early stage in Western Sahara, but there is little such activity in other parts of north- ern Africa beyond the monitoring of dust event frequencies and visibility. Discussion and Research focusing on regions rather than processes is currently somewhat unfashion- able. However, a focus on specific regions such as the Sahara provides an opportunity to investigate linkages between different physical and social and to conduct truly interdisciplinary research. The Sahara in particular has neglected by the research community for a number of reasons, including (i) difficulty of access and security issues, which discourages researchers from conduct- ing field work, (ii) the economic and political marginalisation of the region and associated lack of participation of Saharan countries in international THE JOURNAL OF NORTH AFRICAN
Page 27 programmes, (iii) the difficulty faced by nationals of Saharan countries in outside of the region, (iv) the isolation of individual researchers working on issues within countries outside of the region, and (v) the fact that what little research conducted in the Sahara is often seen as either rather esoteric in nature (e.g. into archaeology and rock art) or only of relevance to a tiny minority of (e.g. ethnographic and anthropological research). The lack of interest in the Sahara among the wider research community is remark- able given the proximity of the region to Europe and the strong historical ties Europe and North Africa, not to mention the importance of a number of nations as oil producers. Indeed, these factors have drawn the attention of the community, particularly in the United States, although this attention is not matched by detailed knowledge of the political reality on the ground. combination of drought with other economic, social and political issues to political instability and conflict at the margins of the Sahara has been described above. Developmental and security issues should be sufficient to foster a greater interest the Sahara in Europe and North America; however, so should the broader related to the Sahara’s role in the Earth system and human well-being as in this paper. The Sahara as a Component of the Earth The neglect of the physical sciences in the Sahara is particularly notable. While number of teams are conducting archaeological and palaeoenvironmental in a variety of Saharan countries, there has been very little work on environmental issues in the Sahara. This fact is thrown into sharp relief by recent investment of millions of US Dollars in research into the transport mineral dust from Chinese sources. Material from these sources transported densely populated parts of East Asia, notably the Korean peninsula, has for regional climate, human health and ecological systems, and also reaches America. However, there is very little funding for, and no concerted of, research into the transport of dust from the Sahara, which produces more than the Chinese sources, providing up to two thirds of the global atmospheric and contributing to air pollution in Africa, Europe, the Caribbean North America. Just as in China, trends in dust storm activity and in the export dust over adjacent regions have been detected for the Sahara. Furthermore, has been implicated in the suppression of rainfall and the possible exacerbation drought in the Sahel, and has been demonstrated to suppress tropical cyclone and hur- ricane formation over the Atlantic. Better understanding and monitoring of dust events thus has the potential to improve seasonal climate forecasts for a marginal region of sub-Saharan Africa, and enhance hurricane forecasting capabili- ties. Long-range forecasts of dust mobilisation and transport based on change scenarios coupled with dynamic representations of processes and vegetation cover may help us forecast climate variability and (specifically droughts and hurricanes), and assist in the development of policies deal with the potential impacts of climate change. THE CLIMATE-ENVIRONMENT-SOCIETY NEXUS IN THE
Page 28 On longer timescales, a better understanding of how the Saharan land responds to climate change in terms of dust production and vegetation should enable us to reduce uncertainties in the representation of the carbon and atmospheric aerosols in global climate models. The incorporation of aerosols into such models will improve the representation of the Earth System should lead to more realistic climate change scenarios, with improved estimates the sensitivity of global climate and of ecosystems to increases in atmospheric green- house gas concentrations and global mean surface temperatures. This in turn will policy makers to assess climate change risks and design climate change and adaptation policies. Current computer models suggest that anticipated in global climate may lead to a greening of the southern Sahara and northern in the near to medium term, a process that may reduce the supply of erodible for aeolian transport. Such a reduction in dust supply could exacerbate warming via adverse impacts on carbon-storing marine and terrestrial ecosystems, representing a positive feedback in the climate system (although these might be offset to a certain extent by increased carbon sequestration in a densely vegetated Sahara). Reduced dust emissions from northern Africa may increase the frequency and/or intensity of Atlantic hurricanes, exacerbating risk in the Caribbean, the southeastern United States, Central America and northern South America. On the other hand reduced atmospheric dust content have beneficial impacts in terms of human health and rainfall in certain regions. While the mechanisms associated with all these processes have been clearly ident- ified, the processes themselves are not understood in a quantitative manner; importance of the associated impacts could be significant, but will remain a of speculation pending further research. Climate Change and Human Adaptation: Lessons from the The past provides us with no exact analogues for the anthropogenic warming that is anticipated over the coming centuries. Nonetheless, studies of Saharan past can teach us many lessons about physical and social systems their interaction. The Holocene climatic optimum occurring between about 10 5 ka, which was associated with humid conditions in the Sahara, might provide with a very approximate model for future climate change, provided such are treated with caution – the mechanisms behind the early Holocene warming monsoon intensification are different from those driving present day climate change. Furthermore, it must be appreciated that global mean are likely to exceed those of the Holocene climatic optimum before the end of twenty-first century. Palaeoenvironmental data can help us develop a better understanding of changes in climate and the connections between changes at high and low latitudes. Such connections are exemplified by the association between cooling in the Atlantic, apparently originating at high latitudes, and arid crises in the Sahara the wider northern hemisphere subtropical and extra-tropical region during warm humid periods (see for a review of these changes and their on human societies). It is plausible that such cooling episodes will result from THE JOURNAL OF NORTH AFRICAN
Page 29 of freshwater from melting ice and increased river runoff in the Atlantic with anthropogenic climate warming. The likelihood of such events being precipi- tated by human-induced climate change is currently a matter of great debate, studies of past climates can yield information about the conditions under such events occur. Alley argues that past events have occurred with little or no exter- nal forcing and that the difficulty current climate models have in reproducing events indicates that they may be more likely to occur in the future than is accepted. The occurrence of such events in the warm early Holocene suggests anthropogenic greenhouse warming may not necessarily mean that such cold, arid episodes will be absent from the foreseeable future. Studies from such as the Sahara can tell us how severe such events must be in order for effects to be felt in low latitudes with potentially adverse consequences for populations. Again, such information can help policy makers develop strategies avoiding ‘dangerous’ climate change associated with the crossing of thresholds in the climate system beyond which we are likely to experience changes in climate. 233,234, As well as yielding information on the workings of the climate system and between climate change and environmental impacts, studies of the past can tell much about how human societies respond to environmental change. While of vulnerability and the nature of human responses to climatic and change are context-specific, it is possible to make some very general (although not uni- versal) observations regarding human responses to such changes. 236, The reinforces the lesson, evident from more recent experiences in the Sahel, that is the key to the long-term sustainability of livelihoods in highly variable environ- ments. The long records of environmental change in northern Africa caution us beware of complacency during periods of abundance associated with increased rain- fall, as these are invariably followed by episodes of scarcity. Social systems in environments must be flexible and responsive; livestock-based pastoralism is more appropriate strategy than large-scale rain-fed agriculture where rainfall scarce and unpredictable. While this observation might be a moot point in hyper-arid regions of the Sahara, it is of great relevance for marginal regions as the Sahel, where developmental pressures and aspirations of economic led to agricultural expansion in the anomalously wet 1950s and early 1960s, a of optimism driven by independence from colonial rule and the prevailing of technological progress. This expansion only served to exacerbate the impacts the subsequent period of drought and desiccation. Although model studies (to certain extent supported by observations) suggest that the northern Sahel and Sahara may become wetter in the near future, the real possibility of occurrence of arid episodes as described above, or of a reversal of the greening trend at high atmos- pheric greenhouse gas concentrations, means that extreme caution should be in the exploitation of climatic amelioration along the Sahel-Sahara boundary. Model- ling studies and palaeoclimatic data caution us that social systems in northern must be prepared to confront extreme environmental variability if they are to survive. Studies of past responses to climatic and environmental change in the paint a picture of largely reactive, ad hoc ‘adaptation’ occurring during times THE CLIMATE-ENVIRONMENT-SOCIETY NEXUS IN THE
Page 30 environmental crisis. The rapid spread of cattle herding suggests migration as a resort to environmental deterioration, rather than a smooth and painless process which human populations respond to changes as they occur without suffering signifi- cant hardship. While this may not appear to be a very controversial conclusion, stands in sharp contrast to current paradigms of adaptation, which promote the devel- opment of resilience and ‘adaptive capacity’ as a means of coping with – the implication being that adaptation is a means of ‘neutralising’ impacts of climate change and thus of avoiding adverse consequences. In the current view of adaptation as something wholly benign is challenged by archaeological record, which demonstrates that adaptation to environmental has generally involved compromises, and is often what we may term sub-optimal’ in that, while it enables human populations to survive in the face of change, often has negative aspects. The archaeological record illustrates that Saharan became more territorial and stratified as they responded to climatic desiccation, response to a reduction in the productivity of the physical environment. these processes culminated in the emergence of urbanisation based on solutions to adaptation, as in the Nile Valley and the Libyan Fezzan, they proved to vulnerable to collapse in the face of later climatic crises or simply unsustainable in longer term. 241, While the factors behind the demise of the Garamantian culture this region are not known, it is possible that declining groundwater levels played role, and that the Garamantes were confronted with limits to their ability to adapt to environmental change. Future Economic Development in the Modern technology enables settlement in previously uninhabitable regions, permits the rehabilitation of areas where settlement and agriculture have been decline, such as the Garamantian heartland in the Fezzan. This transformation (terms of both reality and aspiration) is evident in the local toponymy, with Wadi al-Ajal (usually translated as ‘Valley of Death’) having been renamed Wadi al-Hayat (‘Valley of Life’). Another example of technology enabling access to water is the Great Manmade River, which transports fossil water from aquifers of the central Sahara in southern Libya to the Mediterranean coast, an expanding population and a developing economy with a vital resource. However, water resources in the central Sahara are finite, and estimates of the amount of available, and the time until it runs out, vary considerably. A better understanding the origins of this fossil water, of the dynamics of the aquifers, and of the necessary for their recharge, may contribute to a more sustainable approach water management. An appreciation of the origins and finite nature of water resources, coupled with policies designed to encourage water conservation, might go some way towards extending the lifetime of fossil reserves; designed to foster more ‘responsible’ use of resources are more likely to when underpinned by campaigns of awareness raising, which must ultimately based on a sound scientific understanding of the region’s natural resource base. Water is not the only precious commodity in the Sahara. Oil and tourism much of the economic development in the region. Tourism in the Sahara THE JOURNAL OF NORTH AFRICAN
Page 31 depends on the preservation of natural and cultural heritage, being based on tourists’ interest in landscapes, history and archaeology. This heritage be compromised by a number of factors, including insensitive development and exploration, which threaten a number of sites of scientific and archaeological interest, and have already damaged or destroyed some. Of course tourism itself can significant damage when it is not carefully managed, through souvenir hunting practices such as the wetting of rock paintings in order to temporarily their appearance for photographic purposes. 244,245, Nonetheless, tourism a potentially lucrative, and sustainable, source of income for many communities and nations. Whereas oil reserves will eventually run out, or become significantly less profitable if concerns about climate change lead to a to alternative energy sources, natural and cultural heritage represent an resource if carefully managed. While it may be expedient in the short term to heritage for the sake of oil exploration and production, it is neither necessary nor sen- sible in the long-term. Oil-based development can be complemented by low-tourism that exploits the great interest in natural history and the human past the educated populations of affluent nations, who are constantly seeking destinations. Such tourism can be underpinned by research in the physical social sciences, which furthers our understanding of the natural and cultural heritage. In some parts of the Sahara, local people are educating themselves in subjects such archaeology in order to participate more fully in heritage tourism and assist in management of valuable archaeological sites. In conclusion, we stress that the Sahara is not merely the ‘empty space’ of the imagination. Like other regions, the Sahara is faced with the challenge of ‘development’ in the face of a growing population and an uncertain climatic future. the Sahara, this challenge is compounded by a reliance on non-renewable resources, extreme environmental variability, and a high sensitivity to climate change. The challenge of coping with variability and change is acute at the southern margins of the desert, along the oscillating Sahara-Sahel bound- ary. Here, future development policies must be built around this variability, founded on a profound appreciation of it. There is much that can be learnt traditional resource management practices, based on flexibility and mobility, tried and tested livelihood models and coping strategies should not simply discarded in the name of progress and modernisation. Technological innovation a role to play in development, for example through the development of methods access and deploy scarce water resources, provided the consequences of activities are carefully considered. Economic diversification also has its part play, for example in the form of locally-run and sensitively managed archaeology- based tourism. The physical and social sciences have a key role to play in the future of the greater Saharan region. However, in order that this role be fulfilled, there be more international support for Saharan research, and much more THE CLIMATE-ENVIRONMENT-SOCIETY NEXUS IN THE
Page 32 between members of the international scientific community and their counterparts the Saharan nations. Finally, the role of the Sahara in influencing global requires much more investigation; the potential of Saharan dust to affect the carbon cycle and perhaps accelerate the process of anthropogenic climate means that to ignore the Sahara is to neglect a key component of the Earth system. 1. N. Brooks, Drought in the African Sahel: Long-Term Perspectives and Future prospects, (Norwich: Tyndall Centre Working Paper No. 61. 2004); available at www.tyndall.ac.uk. 2. A. Goudie, Environmental Change, Third Edition, (Oxford: Oxford University Press 1992). 3. F. Gasse and E. Van Campo, ‘Abrupt post-glacial climate events in West Asia and North monsoon domains’, Earth and Planetary Science Letters 126 (1994) pp. 435–456. 4. D. Jolly, S. P. Harrison, B. Damnati and R. Bonnefille, ‘Simulated climate and biomes of during the late Quaternary: Comparison with pollen and lake status data’, Quaternary Reviews 17 (1998) pp. 629–657. 5. R. B. Alley, P. A. Mayewski, T. Sowers, M. Stuiver, K. C. Taylor and P. U. Clark, ‘Holocene instability: A prominent, widespread event 8200 years ago’, Geology 25 (1997) pp. 483–486. 6. U. von Grafenstein, H. Erlenkeuser, J. Muller, J. Jouzel and S. Johnsen, ‘The cold event 8200 ago documented in oxygen isotope records of precipitation in Europe and Greenland’, Dynamics 14 (1998) pp. 73–81. 7. M. R. Talbot, ‘Late Pleistocene rainfall and dune building in the Sahel’, In A. A. Balkema (Ed.), Palaeoecology of Africa 16, (Rotterdam: Balkema 1983), pp. 203–213. 8. J. Maley, ‘Last Glacial Maximum lacustrine and fluviatile Formations in the Tibesti and other mountains, and large-scale climatic teleconnections linked to the activity of the Subtropical Stream’, Global and Planetary Change 26 (2000) pp. 121–136. 9. Goudie (note 2). 10. Ibid. 11. The Pleistocene corresponds to the majority of the Quaternary period, excluding the last 10,000 years, which constitutes the Holocene. 12. Deposits laid down in the beds of ancient, now dry lakes, or palaeolakes. 13. A recently identified large palaoelake in southwestern Libya. 14. N. A. Drake, K. H. White and S. McLaren, ‘The Palaeohydrology and Palaeoclimate of Megafezzan: A Giant Palaeolake in the Fezzan Basin, Libyan Sahara’, Quaternary Reviews (in press). 15. C. Causse, C. Coque, J. Ch. Fontes, F. Gasse, E. Gibert, H. Ben Ouezdou, and K. Zouari, ‘Two levels of continental waters in the southern Tunisian chotts at about 90 and 150 ka’, Geology (1989) pp. 922–925. 16. J. C. Fontes and F. Gasse, ‘PALHYDAF (Palaeohydrology in Africa) program: objectives, methods, major results’, Palaeogeography, Palaeoclimatology, Palaeoecology 84 (1991) pp. 191–215. 17. B. J. Szabo, C. V. Haynes and T. A. Maxwell, ‘Ages of Quaternary pluvial episodes determined uranium-series and radiocarbon dating of lacustrine deposits of Eastern Sahara’, Palaeogeography, Palaeoclimatology, Palaeoecology 113 (1995) pp. 227–242. 18. J. S. Rowan, S. Black, M. G. Macklin, B. J. Tabner, and J. Dore, ‘Quaternary environmental change Cyrenaica evidenced by U-Th, ESR and OSL dating of coastal alluvial fan sequences’, Libyan 31 (2000) pp. 5–16. 19. K. White, N. A. Drake, A. C. Millington and S. Stokes, ‘Constraining the timing of alluvial response to late Quaternary climatic changes, southern Tunisia’, Geomorphology 17 (1996) pp. 295–304. 20. E. Tuenter, S. L. Weber, F. J. Hilgen and L. J. Lourens, ‘The response of the African summer to remote and local forcing due to precession and obliquity’, Global and Planetary Change 36 (2003) pp. 219–235. 21. N. Petit-Maire (ed.) Le Shati, lac pleistocene du Fezzan (Libye) (Paris/Marseille: CNRS 1982). 22. Jolly et al. (note 4). 23. M. Cremaschi, M. Pelfini, L. Arzuffi, V. Di Mauro, M. Santilli and A. Zerboni, ‘A record for the late Holocene in the central Sahara: tree rings of Cypressus dupretiana from Wadi Tanezzuft area (SW Fezzan, Libya)’, in M. Kaennel Dobbertin and U. Braker (eds.) THE JOURNAL OF NORTH AFRICAN
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Page 38 180. R. Washington, M. Todd, N. J. Middleton and A. S. Goudie, ‘Dust-Storm Source Areas by the Total Ozone Monitoring Spectrometer and Surface Observations,’ Annals of the Association American Geographers 93 (2003) pp. 297–313. 181. Brooks and Legrand (note 161). 182. TOMS measures the amount of ultraviolet radiation reflected by the Earth system (i.e. the and overlying atmosphere combined) at different wavelengths; dust is detected using a method accounts for the different reflective properties of different substances at these (Washington et al., 2003 – see endnote 179). The IDDI detects dust using a different based on the reduction of infra-red radiation escaping the Earth’s atmosphere due to the effect of atmospheric aerosols resulting from their blocking of incoming solar radiation, with their absorption of infra-red radiation emitted by the Earth’s surface (Brooks and Legrand, 2000 – see endnote 157). The TOMS AI and the IDDI data are therefore derived from different principles to detect dust, so it should not be surprising that there are differences in resulting data. 183. Prospero and Lamb (note 155). 184. Washington et al. (note 180). 185. Ibid. 186. N. M. Mahowald and J-L. Dufresne, ‘Sensitivity of TOMS aerosol index to boundary layer height: Implications for detection of mineral aerosol sources’, Geophysical Research Letters 31 (2004) L03103. 187. Brooks and Legrand (note 161). 188. A. S. Goudie and N. J. Middleton, ‘Saharan dust storms: nature and consequences’, Earth-Reviews 56 (2001) pp. 179–204. 189. Drake et al. (Note 14). 190. Marticorena and Bergametti (note 178). 191. G. M. N’Tchayi, J. Bertrand, M. Legrand and J. Baudet, ‘Temporal and spatial variations of the atmos- pheric dust loading throughout West Africa over the last thirty years’, Annales Geophysicae 12 (1994) pp. 265–273. 192. I. Tegen and I. Fung, ‘Contribution to the atmospheric mineral aerosol load from land surface modi- fication’, Journal of Geophysical Research 100, no. D9 (1995) pp. 18707–193. I. Tegen, A. A. Lacis and I. Fung, ‘The Influence on climate forcing of mineral dust from soils’, Nature 380 (1996) pp. 419–422. 194. C. J. Tucker, H. E. Dregne and W. W. Newcomb, ‘Expansion and contraction of the Sahara from 1980 to 1990’, Science 253 (1991) pp. 299–301. 195. C. J. Tucker, W. W. Newcomb and H. E. Dregne, ‘AVHRR data sets for determination of spatial extent’, International Journal of Remote Sensing 15 (1994) pp. 3547–3565. 196. S. E. Nicholson and C. J. Tucker, ‘Desertification, drought, and surface vegetation: an from the West African Sahel’, Bulletin of the American Meteorological Society 79 (1998) pp. 815–829. 197. S. D. Prince, E. B. DeColstoun, and L. L. Kravitz, ‘Evidence from rain-use efficiencies does indicate extensive Sahelian desertification’, Global Change Biology 4 (1998) pp. 359–274. 198. Prospero and Lamb (note 155). 199. Brooks and Legrand (note 161). 200. Goudie and Middleton (note 188). 201. Washington et al. (note 180). 202. Brooks and Legrand (note 161). 203. Prospero and Lamb (note 155). 204. I. Chiapello and C. Moulin, ‘TOMS and METEOSAT satellite records of the variability of dust transport over the Atlantic during the last two decades (1979–1997)’, Geophysical Letters 29 (2002) article no. 1176. 205. P. Ginoux, J. M. Prospero, O. Torres and M. Chin, ‘Long-term simulation of global dust with the GOCART model: Correlation with North Atlantic Oscillation’. Environmental and Software 19 (2004) pp. 113–128. 206. P. de Menocal, ‘Abrupt onset and termination of the African Humid Period: rapid climate responses gradual insolation forcing’, Quaternary Science Reviews 19 (2000) pp. 347–361. 207. J.-L. Rajot, S. C. Alfaro, L. Gomes and A. Gaudichet, ‘Soil crusting on sandy soils and its influence wind erosion’, Catena 53 (2003) pp. 1–16. 208. Brovkin (note 135). 209. Claussen et al. (note 136). THE JOURNAL OF NORTH AFRICAN
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Page 40 242. di Lernia and Palombini (note 70). 243. N. Brooks, S. di Lernia, N. Drake, M. Raffin and T. Savage, ‘The geoarchaeology of Sahara: Preliminary results of the first Anglo-Italian expedition in the “free zone”’, Sahara (2003) pp. 63–80. 244. J. Keenan, ‘The theft of Saharan rock art’, Antiquity 74 (2000) pp. 287–288. 245. J. Keenan, ‘The Lesser Gods of the Sahara’, Public Archaeology 2 (2002) pp. 131–150. 246. Brooks et al. (note 177). 247. J. Keenan, personal communication. THE JOURNAL OF NORTH AFRICAN
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I really think archaeologists need to put more focus on the Sahara. There is so much we don't know about this area of the world even though it contains many roots of Egyptian and Nile Valley civilization as well as other civilizations in Africa.
We found world's earliest African mummy-- Uan Muggiag, what else might we find.
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