Download The Climate-Environment-Society Nexus in the

The Climate-Environment-Society Nexus in the
Sahara from Prehistoric Times to the Present Day
NICK BROOKS, ISABELLE CHIAPELLO, SAVINO DI
LERNIA, NICK DRAKE, MICHEL LEGRAND, CYRIL
MOULIN AND JOSEPH PROSPERO
The Sahara is a key region for studies of archaeology, human-environment interaction, global
biogeochemical cycles, and global climate change. With a few notable exceptions, the region is
the subject of very little international scientific research, a fact that is remarkable given the
Sahara’s proximity to Europe, the developmental issues facing its growing population, the
region’s sensitivity to climate change and the Sahara’s potential for influencing global
climate through the export of airborne mineral dust. This article seeks to address humanenvironment interaction in the Sahara from an interdisciplinary perspective, focusing on the
implications of Saharan environmental variability and change for human populations both
within and outside of the region on timescales ranging from decades to millennia. The
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 their
longer-term context; the possibility of a ‘greening’ of the southern Sahara as suggested by
some climate models is also discussed. The role of the Sahara as the world’s largest source
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 for
dust production and the role of the Sahara in the Earth system. The article ends with a discussion and synthesis that explores the lessons that may be learnt from a study of the physical and
social sciences in the Sahara, in particular focusing on what the signature of past environmental
and socio-cultural changes can tell us about human responses and adaptations to climatic and
environmental change – a matter of great relevance to researchers and policy makers alike in
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 a
senior Research Associate at the Tyndall Centre for Climate Change Research. He has a background in the
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 has
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 Director
of the Italian-Libyan Archaeological Mission in the Acacus and Messak (Libyan Sahara). Nick Drake is a
Reader in the Department of Geography at King’s College, London, specialising in remote sensing and arid
zone geomorphology. Michel Legrand is Professor of Atmospheric Sciences at the Laboratoire d’Optique
Atmosphérique at the Université des Sciences et Technologies de Lille, specialising in the remote sensing of
atmospheric aerosols. Cyril Moulin is a researcher at the CEA/CNRS Laboratoire des Sciences du Climat et
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 School
of Marine and Atmospheric Science, University of Miami. He has worked extensively in the field of atmospheric 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.253–292
ISSN 1362-9387 print=1743-9345 online
DOI: 10.1080=13629380500336680 # 2005 Taylor & Francis
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Introduction
The purpose of this paper is to review the state of the environmental sciences in the
Sahara and to consider their wider relevance in a variety of contexts. In particular we
focus on the links between the physical and social sciences, and interdisciplinary links
between different research areas. A major aim of the paper is to demonstrate the
relevance of Saharan research in a number of fields for wider global issues such as
climate change and human adaptation. The paper is partly structured as a “narrative
history” of the Sahara, on the one hand in order to address a range of issues relating to
prehistoric, historic, contemporary and future climatic and environmental change, and
on the other to illustrate the relevance of palaeoenvironmental, archaeological and
historical information for studies of twenty first century human-environment
interaction.
We define the Sahara approximately as the region of northern African where
average annual rainfall is currently below 100 mm (Figure 1). This definition
should be interpreted loosely given the large fluctuations of rainfall at the fringes
of the desert on multiple timescales; the Sahara may be viewed as expanding and contracting as a response to global and regional changes in climate that modulate rainfall
at its periphery. In the recent geological past the Sahara thus defined has extended
much further south than at present, and has shrunk to such an extent that it effectively
disappeared. Any consideration of the past or future evolution of the Sahara must
therefore also consider its periphery, particularly at its southernmost edge where
FIGURE 1
ISOHYETS REPRESENTING MEAN ANNUAL RAINFALL IN MM OVER NORTHERN AFRICA
FOR THE TWENTIETH CENTURY
Source: From the dataset of New et al. (2000) and obtained from the Climatic Research Unit. The Sahara is
defined here loosely as the region where mean annual rainfall is below 100 mm.
THE CLIMATE-ENVIRONMENT-SOCIETY NEXUS IN THE SAHARA
255
the largest variations occur in response to fluctuations in the strength and position of
the monsoon.1
The paper begins with a brief review of our knowledge of how the Saharan
environment has changed in the past, with a particular focus on the early Holocene
pluvial (approximately six to ten thousand years before present) and the subsequent
desiccation of the region. These changes are viewed within the context of global
climate change, a topic currently of serious concern to scientists and policy makers
tasked with developing strategies to mitigate the severity of, and promote adaptation
to, human-induced climate change in the twenty-first century. This discussion of past
changes in the Saharan environment frames the remainder of the paper, which deals
with the legacy of the past, and what we can learn from it.
The discussion of the physical aspects of past environmental change is followed
by a review of how these changes affected human societies. In particular, processes
of adaptation and their consequences are considered.
Moving on from discussions of the past, the present-day environment of the
Sahara is considered in terms of groundwater resources, and the mobilisation and
transport of mineral dust from Saharan sources. Water resources are considered
within the context of settlement and development. Mineral dust is discussed at
some length due to the importance of the Sahara as a global dust source, and the
impact of dust on human health, ecological productivity and global climate. Groundwater and dust sources are considered within the context of past changes in the
Saharan environment, particularly fluvial activity, which have determined their
nature and distribution.
Current and future climate variability are also considered within the context of
existing rainfall variability and potential future changes in rainfall associated with
changes in the African monsoon.
Finally, issues of environmental change and variability, adaptation and development are synthesised. The lack of international research in the Sahara when compared
with other parts of the world is addressed, and the case for cross-disciplinary research
in the Sahara is presented.
Past Environments of the Sahara
The history of the northern African land mass is one of dramatic changes in the
physical environment, of oscillations between arid phases during which much of
the sub-continent is effectively uninhabitable, and humid episodes that transform
the desert regions of the Sahara into a fertile landscape of lakes and savannah. On
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, arid
conditions over northern Africa.2,3,4,5,6 During the last glacial maximum (LGM)
some 21 thousand calendar years before present (21 ka), the Sahara desert covered
a much larger area than today, as apparent from the dating of fossil dunes some 58
south of the present extent of mobile dunes.7 A combination of factors leads to
increased aridity during periods of glaciation, including reduced atmospheric
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moisture availability, decreased solar heating of the land surface, and large-scale
changes in atmospheric and oceanic circulation.8,9
The Pleistocene
Over the past 1.65 million years, approximately corresponding to the Quaternary
period, there have been some 17 glacial cycles, each lasting approximately 100
ka10 and it is the generally held view that glacials are arid and interglacials humid.
However, determining the effects of Pleistocene11 climate change on the Sahara is
problematic due to the paucity of organic deposits. Organic remains, such as palaeolake sediments, provide important information regarding arid-humid transitions.
However, these deposits are rapidly deflated by the wind during arid periods and
few ancient deposits have managed to endure the multiple episodes of Pleistocene
aridity. Because of these factors the number of lacustrine deposits12 in the Sahara
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 of
Lake Megafezzan13 provide one of the longest records, with evidence for lacustrine
episodes at 380, 240, 128, 118(þ27 2 20), 74(þ23 2 16), 47(þ17 2 13), 30 ka
and from 14 + 1.7 to about 3 ka.14 This record of humidity conforms with the
view that interglacials are associated with humid periods but also indicates that episodic humidity occurs during glacial cycles. A similar picture emerges if we compare
these lacustrine episodes to the few others identified in the Sahara. A large lake
existed in the Basin of the Tunisian Chotts at 150, 92, 75(þ7 2 6) and
42.4 + 2ka,15 there is evidence for lacustrine conditions at 75 + 7 ka in southern
Algeria,16 and there are indications of humidity between 71 and 87 ka in southern
Egypt and northern Sudan.17 Some of these humid episodes are synchronous in
two or more regions of the Sahara (e.g. 128, 74 and 42 2 47 ka), while others
are not (e.g. 155 ka), although this is not unexpected due to the paucity of the lacustrine record and uncertainties in dating. Encouragingly, studies of alluviation on the
northern fringes of the Sahara agree with the data from the central Sahara. Northern
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,18 while in southern Tunisia incision
and subsequent alluviation occurred at 47 + 12 and from about 8+2 ka.19 Thus in the
north and central Sahara there is clear evidence for brief humid episodes occurring
during glacials, as well as in interglacials.
The Early Holocene Humid Episode
Summer insolation over northern Africa reached a maximum at the beginning of the
Holocene period, around 10 ka,20 by which time humid conditions had been
established in the Sahara. This so-called Holocene Climatic Optimum saw a greening
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, and
the development of a mosaic of savannah and woodland throughout much of the
Sahara.21,22,23,24 It is during this period that the Sahara was reoccupied by human
populations, who initially survived through hunting and gathering, exploiting the
THE CLIMATE-ENVIRONMENT-SOCIETY NEXUS IN THE SAHARA
257
abundant humid climate fauna and flora of the then-moist Sahara, as discussed in
more detail below.
The Holocene Climatic Optimum was, however, interrupted by a number of arid
episodes, some of which appear to have been associated with transient changes in
climate at the global or hemispheric scale. The most important such events were
episodes of cooling in the North Atlantic, recurring on millennial timescales. Bond
et al.25 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 of
uncertainty associated with the dating of such episodes, with a period of aridity
lasting the order of centuries in the Sahara.26 The 5.9 ka event occurs around the
time of a dry episode evident in many, although not all, Saharan lake records,
marking the beginning of a shift towards more permanent aridity.27,28
The Desiccation of the Sahara
A number of data indicate that aridity increased in the Sahara after an arid episode
occurring around 6 ka.29 The subsequent desiccation was not a smooth, continuous
process characterised by a linear response of the monsoon to steadily declining
solar heating or insolation. Instead, it appears that desiccation occurred in a stepwise
fashion, with one or more episodes of abrupt drying.30,31,32 It is most likely that the
drying of the Sahara was the result of a complex interaction between declining solar
insolation, transient climatic perturbations such as the cold arid episode at around
6 ka, and dynamic vegetation-atmosphere feedbacks involving the retention and
recycling of moisture.33
The earlier 8 ka cold arid episode occurred at a time when solar insolation was
sufficiently strong to drive a recovery in the monsoon and in vegetation cover, once
this transient climatic perturbation had passed. However, insolation was considerably
weaker by 6 ka, and may have been insufficient to drive a full recovery of the
coupled vegetation-monsoon system after it had been disrupted by the Atlantic
cold episode identified around this time34. This transient climatic perturbation may
well have been the trigger for the final desiccation of the Sahara, occurring at a
time when the monsoon system was at least partially sustained through moisture
retention and recycling by vegetation systems that originally developed as a result
of strong insolation driving a vigorous monsoon. Evidence for abrupt desiccation
towards 5 ka in a number of Saharan regions35,36,37,38,39 suggests the final collapse
of remaining vegetation systems around this time, perhaps as solar insolation
crossed a threshold below which vegetation-atmosphere interactions could no
longer sustain rainfall, or possibly as a result of further transient perturbations from
which vegetation could not recover in a low-insolation regime.
By around 5 ka desiccation associated with the southward retreat of the monsoon
system was well established; this process culminated in the formation of the presentday Sahara Desert. The timing and speed of this process of desertification varied from
place to place, and was mediated by geography, topography, hydrogeology and the
nature of regional climatic systems. For example, in the Wadi Tanezzuft, in the
Libyan Fezzan, the depletion of soil water reserves was not completed until about
3.5 ka, and fluvial activity persisted until around 2.7 ka40 apparently, this was due
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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 not
restricted to the Sahara. Abundant evidence indicates that the broad pattern of wet
conditions in the early to middle Holocene, interrupted by arid episodes and followed
by a process of desiccation starting around 6 ka and accelerating around 5 ka,
prevailed throughout many subtropical and extra-tropical northern hemisphere
regions.41 Our understanding of past environmental change in the Sahara thus
represents but one component in our wider understanding of global climatic and
environmental change which, when coupled with results from other regions,
enables us to develop a deeper understanding of the Earth System and of human
responses to environmental change. Such an understanding is necessary if we are
to confront issues such as long-term climate variability and anthropogenicallydriven climate change, processes which can have a profound impact on
socio-economic development.
Linking Past Environmental and Cultural Change
The Sahara occupies a privileged position in studies of human-environment interaction, as a consequence of the clear climatic signals in Saharan palaeoenvironmental
records, and the associated evidence of cultural change during key transition periods
between arid and humid conditions. In many other geographical regions archaeology
has been bedevilled by arguments over environmental determinism, leading to a
reluctance to consider the role of environmental change in mediating cultural trajectories. In contrast, archaeologists working in the Sahara have by and large appreciated
the central role of environmental change, without falling into the trap of neglecting
other important factors in the development of human societies. In one sense, we
may view the Sahara as a laboratory of human response to environmental change,
given the overwhelming nature of the climatic changes that have affected the
region throughout the relatively archaeologically accessible Holocene. As such, the
Sahara can tell us much about processes of adaptation, the understanding of which
is crucial for the formulation of policies designed to address the impacts of future
climate change, particularly in the developing world.
Human Occupation of the Sahara in the Pleistocene
Human occupation of the Sahara has been limited by the availability of water, with
little or no occupation away from the Mediterranean coast and the Nile Valley
during arid phases associated with glacial epochs. A notable hiatus in human
occupation, at least in the central Saharan regions, appears to be related to the Late
Quaternary glacial: evidence for Upper Palaeolithic sites is very rare, and the latest
clear evidence of human occupation during the Pleistocene (the section of the
Quaternary predating the beginning of the Holocene some 10,000 years ago) dates
back to the Aterian, from 90 to 60 ka.42 A human presence around 30 ka in the
Wadi al-Shati region in Libya has also been hinted at, although the evidence is
equivocal.43 Nevertheless, the spread of modern humans during the Pleistocene
THE CLIMATE-ENVIRONMENT-SOCIETY NEXUS IN THE SAHARA
259
was facilitated by the existence of migration corridors, in particular the Nile Valley, in
which water was available even during the least hospitable phases of this epoch.
Evidence of Pleistocene occupation in the central Sahara is almost invariably
composed of scatters of lithic tools, most of them deprived of any faunal association
or stratigraphic context. Even in the exceptional contexts of the Acacus mountains,
well preserved Pleistocene sites have not been identified, although Uan Tabu44 and
Uan Afuda45 have yielded assemblages of worked stone, dated via optically stimulated luminescence (OSL) and thermoluminescence (TL) methods. The archaeological situation is much more fortunate in the Eastern Sahara, thanks to research by
Wendorf and associates, for example in Wadi Kubbanya, Bir Tarfawi and Bir
Sahara.46,47 Leaving aside technological developments and site organisation, one of
the most important issues to be placed on the archaeological agenda is the spreading
of modern humans in Africa north of the equator. The Sahara, together with the
Nubian corridor, is believed to be a major area for the dispersal of anatomically
modern humans, but coordinated international research in this field is lacking.
Furthermore, it is essential to build a detailed scheme of Late Quaternary climatic
trends, as a prerequisite to analyse and track possible Homo sapiens migration
paths. Recent archaeological research emphasises the role of the Aterian complex
as a possible technological indicator of the presence of modern humans in the
Sahara.48,49 ‘Continuistic’ views, based on (single and isolated) radiocarbon determinations pointed in the past to a very late presence of Aterian sites in Morocco and
other North African contexts, suggesting a direct relation between Aterian peoples
and Upper Palaeolithic North African contexts. Recent research, specifically in
Egypt and Libya, pushes back this technological phase to 140 –60 ka ago, and
underlines, at least in the central Sahara, a break in human occupation related to
the expansion of the ‘Ogolian’ desert.
Human-Environment Interaction in the Sahara in the Early-Mid Holocene
The archaeological evidence indicates that the latest phase of human occupation in
the central Sahara commenced at the beginning of the Holocene, around or soon
after 10 ka.50,51 These early Holocene communities consisted of groups of huntergatherers, which in many cases practiced a fairly sedentary lifestyle, exploiting abundant locally available food resources in the form of wild fauna and flora.52,53,54
It is worth noting that the scant palaeo-anthropological evidence (from Uan Afuda
and Uan Muhuggiag in the central Sahara of Libya) points to sub-Saharan affinities.55
This fits with more recent human remains from the Egyptian oasis, which indicate a
similar affinity on the basis of dental analysis.56 These findings support the hypothesis
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 of
the Holocene. The gap between the beginning of the humid period in the Sahara
after the last glacial maximum (ca. 15– 13 ka) and the appearance of the first
Holocene occupation sites might be interpreted as a consequence of the time taken
for vegetation and fauna to recolonise hyperarid environments.57
More cautiously, the first genetic data on Saharan palaeo-populations also indicate
a sub-Saharan affinity.58,59 Evidence for a southern provenance of the first Holocene
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Saharans might also be seen in from rock art, although the subjective nature of such
interpretations must be recognised: in the Tassili60 and Acacus massifs61 depictions of
figures with what appear to be black African features have been interpreted as
indicating the possible presence of populations originating in sub-Saharan regions.
Dating of this material is controversial, but an Early Holocene attribution for the
so-called “Round Head” style of painting, as usually claimed in the literature,62,63
may reinforce the archaeological and palaeo-anthropological evidence.
It is during the early Holocene that we find the earliest (although still somewhat
controversial) evidence for cattle domestication in the eastern Sahara, the driest
part of North Africa at this time.64,65 Reviewing the literature on the development
of cattle domestication in Africa, Marshall and Hildebrand argue that this was a
local development based on the exploitation of indigenous species, and driven by
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 during
ritual festivals.66 They suggest that this development happened in the eastern
Sahara as a result of the greater environmental variability in this region, and hence
the greater need for intervention to ensure predictability of food supply, when
compared with other Saharan regions. Citing other work,67 Marshall and Hildebrand
conclude that during the frequent periods of drought affecting the eastern Sahara,
cattle would have represented ‘a more reliable resource than plants because their
populations are maintained through movements that exploit local differences in
topography, vegetation, and rainfall’.
According to Hassan68 and Marshall and Hildebrand, citing numerous sources,
cattle pastoralism spread westwards through the central Sahara in an uneven
fashion from around 7 ka, where it coexisted with hunting and gathering. Di Lernia
suggests that the diffusion of cattle-based culture in the Sahara occurred in response
to short, abrupt dry events during the 8th to 6th millennia before present (BP).69 The
8 ka arid episode described above may have played a key role in the spread of
pastoralism. Di Lernia and Palombini suggest that the dry interval at approximately
8 ka ‘probably favoured the integration of cattle herding within foraging communities’ in the Libyan Sahara.70 If this episode also marked a shift from year-round
to seasonal rainfall as has been suggested by some authors,71 cattle herding would
have represented an appropriate adaptation in order to enhance food security in a
more seasonally variable environment.
In the Acacus mountains of Libya, semi-permanent settlements in lowland areas
gave way to seasonal migration during the late 7th millennium BP, when climatic
conditions deteriorated during a ‘severe, abrupt dry interval lasting several centuries’.72 This new pattern of subsistence involved increased use of mountainous
areas in the dry season. Di Lernia and Palombini73 suggest that sheep and goats
(most probably introduced from the western Asia in the early 7th millennium
BP) were tended in highland regions from late winter to early spring in order to
reduce pressure on lowland pasture land, which was set aside for cattle in the
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 the
lowland ergs.
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The nature of the spread of cattle herding through the Sahara is still a controversial
issue. According to Schild and Wendorf, this occurred during wet periods, allowing
people to move with their livestock westward.74 Conversely, Hassan75 and di
Lernia76,77 argue that migration by cattle-herding groups was stimulated by aridity.
They argue that this explanation is more compatible with the archaeological
record, which consists of scattered and isolated contexts throughout the Sahara.
Dates associated with these contexts also suggest discrete episodes of migration
spanning short periods,78 implying rapid, intermittent movements of small groups
of herders, colonising new and unfamiliar environments as they were forced to
move in search of water and pasture during arid crises.
Social and Cultural Responses to Mid-Late Holocene Desiccation
The process of desertification at around 5 ka appears to have been rapid in at least
some parts of the Sahara,79 and was associated with profound changes in human
societies. Increasingly harsh conditions would have had a profound effect on
culture and belief systems; it has been suggested that an increased diversity in the
subject matter of rock engravings and paintings, with a greater emphasis on images
relating to sexuality and fertility, and on ‘enigmatic’ or fantastical beings, dates to
the period of climatic deterioration, ‘when concerns over human and animal fertility
may have become acute’.80 In reference to the Fezzan region of Libya, Mattingly and
associates81 write that ‘Human activity was not cut off at a stroke, however, but may
have become more focussed on specific locations, with the rock art representing a
more sophisticated dialogue between people and a powerful spirit world, bordering
on formalised religion’. It has been suggested that this kind of relationship dates
back to the emergence of the Pastoral cattle cult at 6.4 –6.0 ka, when non-utilitarian
slaughtering of precious livestock and the flourishing of an artistic tradition focused
on cattle appear to represent the first evidence of ritual relationships between human
populations, the physical environment, rainfall and ‘divinities’.82,83,84
Human responses to the desiccation of the Sahara were spatially heterogeneous
and mediated by geography, resource availability and local hydrogeological
responses. In the Libyan Fezzan two types of response may be identified, according
to di Lernia and Palombini.85 In higher elevation regions cattle herding almost completely disappeared after 5 ka. This was replaced by highly mobile pastoralism based
on sheep and goats and involving large-scale year round movement in order to exploit
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 in
relict oases, associated with sedentism and more intensive exploitation of local
resources. Di Lernia and Palombini characterise the former as a ‘light’ approach to
landscape exploitation, associated with a relative egalitarianism and sustainable use
of resources, and the latter as a ‘hard’ and ultimately unsustainable approach to the
landscape leading to possible degradation of the landscape, conflict (possibly over
land rights) and increased social stratification.
Behavioural adaptations focusing on resource extraction were associated with
profound changes in social organisation, as indicated by archaeological studies of
funerary monuments and burial practices. Di Lernia and Manzi86 describe the
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period centred around 5 ka as the ‘hinge’ between different pastoral cultures in the
Wadi Tanezzuft in the Libyan Fezzan, which they term Middle and Late Pastoral. It
is around this time that stone funerary monuments are associated with human
burials in the Wadi Tanezzuft; previously these had been reserved for ritual animal
burials. This process is reflected in other central Saharan regions. A review of the
archaeological work by Sivilli87 indicates that in southwestern Libya and northeastern
Niger, the period 6.4 – 5.9 ka is represented exclusively by faunal burials; 5.8 –4.9 ka
by faunal, human and ‘empty’ burials (i.e., cenotaphs); and 4.8 ka onwards exclusively
by human burials. Di Lernia and Manzi88 interpret this innovation in funerary practices
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 and
practices and settlement patterns as evidence of major changes in population in the
Wadi Tanezzuft, and by implication in the greater central Sahara region. Funerary
monuments would have served a dual purpose in a landscape occupied by increasing
numbers of pastoralists, on the one hand acting as foci for gatherings of related social
groups, and on the other serving as markers of boundaries, territories or zones of
influence, asserting relationships between clan groups and the landscape.
In the Fezzan, it appears that climatic desiccation was associated with inward
migration, increased population density, changes in religious beliefs and practices,
social stratification and a more territorial approach to the landscape, as well as diverging adaptations to facilitate resource extraction in a more hostile landscape. In
certain localities these changes appear to have provided the preconditions for the
emergence of complex urban societies and the formation of entities resembling
states, catalysed by the final desiccation of most of the landscape soon after
3 ka.89,90 The onset of conditions equivalent to present aridity might be viewed as
a threshold that stimulated a step change in social organisation as population
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 until
about 3.5 ka, and fluvial activity persisted until around 2.7 ka.91 Further to the east
in the Wadi al-Hayat, there is evidence that springs dried up around or before 3 ka.92
These late dates for fluvial and spring activity correspond approximately to the
early stages of the Garamantian civilisation, which dominated the Fezzan between
about 1,000 BC and AD 700, a period ‘notable . . . for the local evolution of urbanism,
irrigated agriculture and writing’.93 The Garamantes are described by the Greek
historian Herodotus, writing in the fifth century BC , and later represented a regional
challenge to Roman aspirations in the central Sahara and North Africa. The Garamantian ‘capital’ of Garama (or Old Germa, situated within 2 km of the modern town of
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, Barjuj
and Tanezzuft.94,95,96,97,98
By 3.1 + 0.125 ka the springs had dried up and surface water was either very
scarce or absent at the base of the escarpment forming the southern boundary of
the Wadi al-Hayat.99,100 However, the water table was probably very near the
surface at the base of the wadi, as is evidenced by the fact that recent archaeological
THE CLIMATE-ENVIRONMENT-SOCIETY NEXUS IN THE SAHARA
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excavations uncovered a well in the early Garamantian phase of Old Germa sunk to a
depth of 70 cm, indicating water at very shallow depths.101 Archaeobotanical
evidence indicates that irrigated agriculture was introduced soon after the desiccation
of the springs, in the early part of the first millennium BC ,102 presumably in response
to a lack of water caused by the fall in the water table. There is no direct evidence for
irrigation systems at this time, but the water table was near the surface in Germa and it
is possible that wells were used to tap this resource. The earliest irrigation systems are
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 few
centuries BC , and definitely before the fourth century AD .103 Interestingly this roughly
corresponds with archaeobotanical evidence for the intensification and diversification
of agriculture involving the introduction of a farming system that utilised both winter
and summer crops.104 Foggara would also have allowed the extensification of
agriculture at a similar time. In combination these developments could have lead to
an increase in agricultural production, and there are likely to be strong connections
between this and the rise of the Garamantes as a major political power in the
central Sahara.105
The Garamantian culture appears to have been the result of local innovation, the
outcome of a process of increasing social complexity among the pastoral groups of the
Fezzan. Referring to the Wadi Tanezzuft, di Lernia et al. 106 write that ‘In a certain
sense, Late Pastoral people became the Garamantes’. This conclusion is supported
by the work of Mattingly et al.,107 who find some of the latest Pastoral lithics and
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 desertification in the development of a significant urban civilisation.
The emergence of the Garamantian polity, largely driven by changes in water
availability and geographic serendipity, is not the only example of increased social
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 also
been interpreted at least in part as a result of social responses to environmental desiccation.108,109,110 Palaeoenvironmental evidence indicates increasing aridity to the east
and west of the Nile Valley during the 6th millennium BP, with a final desiccation of
most of this region in the late sixth millennium BP, when the early Dynastic state
emerged.111,112,113,114 Excavations at Hierakonpolis are indicative of attempts to integrate 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 the
Nile Valley.115 Midant-Reynes116 explicitly links local increases in population
density with desertification at the end of the Predynastic period. Archaeological
evidence provides abundant support for models of cultural evolution involving
increased social stratification and the concentration of political power as a response
to increased population densities resulting from a decrease in available productive
land, and associated migration/increased sedentism as a consequence of environmental desiccation.117
To summarise, linked environmental and cultural change in the Sahara is not
simply a matter of poverty and famine resulting from aridity. During the Pastoral
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Neolithic, in the 7th and 6th millennia BP, the Sahara gave birth to one of the great
cultures of antiquity, the Saharan cattle-based culture. Material cultural, occupation
sites, food security, rock art, ideology and funerary practices are all part of an extraordinary culture, the nature of which was driven by a profound relationship with the
physical environment. This culture had to cope with numerous fluctuations in
resource availability associated with climatic variability on a range of timescale,
some of which were extremely severe. This cultural tradition only came to an end
with the final desiccation of the Sahara around 5 ka. However, the pastoral societies
of the Holocene Sahara did not vanish into the sand: the migration of pastoral groups
to refugia such as the Nile Valley, the Sahel, and the Saharan highlands and relict
oases made a vital contribution to many subsequent African cultures, including
Pharaonic Egypt, the Garamantes, the present-day cultures of the Sahel, and probably
the modern day Berber and Tuareg populations.118,119,120,121 In a more general sense,
we may claim that the essence of this cattle-based culture spread from the Sahara to
much of sub-Saharan Africa, shaping the history of the entire African continent.
Water Resources and Human Settlement
As has been the case throughout the Quaternary period, water resources are today the
limiting factor in human settlement and development. Increasing populations and
associated urbanisation and economic development are placing greater demands on
water resources throughout the Sahara. These resources may also be affected by
climate change in the near future, for example with elevated surface temperatures
further enhancing evaporation, and changes in meteorological patterns resulting in
changes in the distribution of rainfall.
Modern permanent settlements in the Sahara are situated where water is available
either from rainfall generated as a result of topography or the intrusion of extraSaharan weather systems (such as in highland and coastal regions), or at oases
where groundwater occurs at or near the surface in local topographic minima.
Water resources may therefore be affected by rainfall variability and changes in
groundwater levels.
Groundwater and Human Settlement
Most inland Saharan settlements rely almost exclusively on groundwater. Modern
pumping and irrigation technology has enabled the expansion of agriculture in
many locations, such as the Wadi al-Hayat in the Libyan Fezzan, which provides
an object lesson in the interaction of water resources, human populations and technological innovation. Here a downward trend in population density started in the first
millennium AD with the decline of the Garamantian culture. The cause of the
demise of the Garamantes is not known with any certainty, although declining
water levels either as a lagged response to the climatic desiccation of the region, or
as a result of over exploitation by local populations, has been suggested.122
However, other explanations, such as a decline in trade in the later years of the
Roman empire, should also be considered.123 Whatever the cause of the demise of
the Garamantes, it appears that groundwater levels declined between Garamantian
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times and the end of the second millennium AD .124 In recent decades the socioeconomic decline of the Wadi al-Hayat has been reversed through the use of irrigation, dependent on the pumping of groundwater from increasingly greater depths.
However, increases in population, the expansion of agriculture, and the process of
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.125
used satellite imagery and aerial photography to examine changes in vegetation
distributions between the 1950s and the end of the twentieth century in the Wadi
al-Hayat, and found a shift in the vegetated zone resulting from the expansion of
agriculture at the southern edge of the wadi, and the die-back of vegetation in unirrigated 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 to
more drought or salt-tolerant species.
Such studies raise questions of sustainability: can settlements in central Saharan
regions continue to expand given their reliance on fossil water reserves that are no
longer replenished by rainfall? Furthermore, how sustainable are schemes such as
the Great Manmade River project in Libya? Water use in Libya has been estimated
at some eight times higher than its renewable water resources, an extreme example
of a situation faced by all the countries of the southern Mediterranean coast which
means they are likely to become much more dependent on food imports (essentially
a means of importing water) in the future.126 Pressure on water use is likely to be
exacerbated by climate change; in the southern Mediterranean reductions in rainfall
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.127 Increases in
temperature will lead to greater evaporation, compounding water scarcity resulting
from reductions in rainfall. Such developments are likely to increase pressure on
fossil water reserves from the Saharan aquifers. Any expansions in the tourism
sector will also increase water demand. Given the likely increased use of fossil
groundwater, a better understanding of groundwater reserves and dynamics, and
the nature, capacity and behaviour of the Sahara’s large subterranean aquifers is
highly desirable. Such aquifers will not reach equilibrium to keep pace with extraction, which will result in the formation of ‘cones of depression’ near populated
areas or areas of intense agricultural activity. Ebraheem et al.128 conclude that the
Nubian Sandstone Aquifer under southwest Egypt does not exist in a steady state
and is still responding to past humid conditions, and estimate that the planned extraction of 1,200 million m3/year in the East Oweinat area could result in a drawdown of
up to 200 m relative to 1960s levels within 100 years, with the cone of depression
extending to Dakhla and Kharga oases.
Rainfall Variability
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 worth
pointing out that precipitation in some parts of the Sahara is sufficiently high for local
populations to rely at least partially on rainfall, and to be adversely affected during
periods when rainfall is anomalously low. Keenan129 describes how the Kel
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Ahaggar Tuareg have developed strategies to cope with drought, and how drought
coupled with the social upheavals of Algerian independence led to hardship in the
1960s.
Drought is a concept that is also familiar to the inhabitants of the far west of the
Sahara.130 In the inland regions of the disputed territory of Western Sahara vegetation
is relatively abundant, sustained by occasional summer rains representing the extreme
northerly penetration of the African Monsoon beyond 208 N, and similarly scarce
rainfall associated with the Atlantic Westerlies. Despite the relative abundance of
rainfall and vegetation in this region, the lack of surface water (e.g. in the form
of oases), as well as the ongoing political conflict with Morocco and the possibility
of a resumption of hostilities, mitigates against permanent settlement. Nonetheless,
the region is used by nomads whose principal source of water takes the form of
milk from their animals.
The Sahelian region, situated at the southern margin of the Sahara, has experienced one of the most persistent and severe changes in climate during the period of
meteorological records.131 This consisted of a multi-decadal scale drying trend
commencing in the 1960s and persisting into the 1990s, with severe droughts in
the early 1970s and 1980s, and some amelioration in recent years132,133 (Figure 2).
It is now well established that rainfall in the Sahel is driven largely by patterns of
global surface temperature, with the temperature of the Indian Ocean playing a key
role.134 While rainfall data for the Sahara are sparse, there is no evidence for any
comparable trend north of the Sahel-Sahara transition zone in those regions of the
Sahara where topography results in non-negligible rainfall. For example, both
annual and summer rainfall totals for Tamanrasset in the highlands of the Algerian
Sahara (home of the Kel Ahaggar Tuareg mentioned above) reveal a wet period in
the 1950s, as in the Sahel, but no long-term drying trend in subsequent decades
FIGURE 2
SPATIALLY AGGREGATED ANNUAL RAINFALL ANOMALIES (IN STANDARD DEVIATIONS)
REPRESENTING THE REGION 108 – 208 N; 258 W – 308 E, ROUGHLY CORRESPONDING TO
THE SAHELIAN ZONE. ANOMALIES ARE CALCULATED WITH RESPECT TO THE MEAN
FOR THE ENTIRE SERIES (1901 – 1998)
Source: From the dataset of New et al. (2000).
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(Figure 3). Instead, the Tamanrasset rainfall record consists of quasi-periodic
variations of some 10 –15 years, with extreme variations in annual rainfall from 0
to 160 mm. On average, approximately half of the rainfall at Tamanrasset occurs in
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. While
there are no indications that the region will experience a shift to humid conditions
comparable to those existing in the early Holocene, a number of climate modelling
studies suggest that anthropogenic climate change may be associated with a strengthening of the African summer monsoon and an intensification and northerly displacement of monsoon rains, leading to wetter conditions in the northern Sahel and
southern Sahara.135,136,137 Other modelling work suggests that the Sahara may shift
northwards,138 implying that even if rainfall increases in the south of the Sahara, it
may decline in regions near the Mediterranean coast. The suggestion that the northern
Sahel and the southern margins off the Sahara may become wetter is consistent with
recent observations indicating a greening of the Sahel explained partly by increased
rainfall and partly by increased vegetation cover due to human activity.139,140
However, rainfall still remains below the high levels of the mid-twentieth
century,141 and interannual rainfall variability and drought still pose considerable
problems for Sahelian societies, as evident from the famine that is unfolding in
FIGURE 3A
TWENTIETH CENTURY ANNUAL RAINFALL TOTALS FOR TAMANRASSET, OVER WHICH IS
SUPERIMPOSED A 5-YEAR MOVING AVERAGE (SOLID LINE)
Source: The Climatic Research Unit.
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FIGURE 3B
TWENTIETH CENTURY RAINFALL TOTALS FOR TAMANRASSET FOR THE MONTHS OF
JULY, AUGUST AND SEPTEMBER (THE WETTEST MONTHS IN THE SAHEL), WITH 5-YEAR
MOVING AVERAGE
Source: The Climatic Research Unit.
Niger at the time of writing (July 2005). The impact of climate change on Sahelian
climate is likely to remain unclear for some time, until additional observational
and modelled data are available.
A shift in the monsoon rain belt would have profound implications for Saharan
societies. Areas that are not currently viable for human populations would become
available for pastoralism, and existing marginal areas might become viable for
rainfed agriculture. However, an intensified monsoon would also be associated
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 the
devastation of crops and food insecurity in a number of countries in 2004 and
2005.142 Grolle143 describes how heavy rainfall in the Sahel in 1953 triggered
famine. While an intensification of the monsoon would undoubtedly bring benefits
in terms of pastoralism and agriculture, the consequences would not necessarily be
wholly benign.
Extreme rainfall variability on different timescales is a fact of life in on the
margins of the Sahara; a failure to fully appreciate this fact was a factor in the
expansion of agriculture into historically marginal areas along the Sahel-Sahara
boundary in the 1950s and 1960s, which were anomalously wet when compared
with the twentieth century rainfall record as a whole. As sedentary agriculturalists
expanded northwards, mobile pastoralists were pushed into more marginal desert
areas. The vulnerability of both populations to drought was significantly increased,
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269
and the combination of elevated vulnerability and severe rainfall deficits in the early
1970s resulted in widespread famine and the collapse of livelihoods and social
systems, representing a significant discontinuity in Sahelian developmental trajectories from which the region is still recovering.144
Keita145 describes the droughts of the 1970s and 1980s as a contributory factor in
the development of internal conflict in Mali, where it undermined the pastoral
livelihoods of the semi-nomadic Tuareg, causing large numbers of them to find
refuge in camps or urban areas where they experienced social and economic
marginalisation.146 Many Tuareg migrated to neighbouring countries, and some
young men become involved in conflicts throughout North Africa and the Middle
East, in which they acquired considerable military experience, eventually returning
to Mali having to face unemployment and marginalisation, creating the conditions
for the ‘Second Tuareg Rebellion’ in 1990. These conditions were exacerbated by
a history of mistrust between the Tuareg and post-independence governments, the
lack of available livelihoods and social support networks for returning migrants (as
a result of previous drought and conflict), continuing drought and associated
competition for resources between nomadic and settles peoples, and the flooding of
the region with small arms as a result of conflicts in neighbouring countries, particularly Western Sahara. Keita147 describes the conflict between the nomadic Tuareg and
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: communities
were fighting for scarce resources and jealously insisting that others were not
preferred.’ This pattern of marginalisation, drought and conflict has been repeated
throughout the Sahel, and demonstrates that climate variability and change can
combine with other factors to cause conflict, particularly in marginal environments
where populations are facing a number of different environmental, social, economic
and political stresses.
Any future increase in rainfall in the northern Sahel and southern Sahara will
be associated with the risk of unsustainable agricultural expansion if longer-term
climatic variability is not considered in development policies. A modelling
study by Maynard et al.148 suggests that the Sahel may be less prone to
drought in a world characterised by low to moderate levels of anthropogenic
greenhouse warming, associated with atmospheric greenhouse gas concentrations
near current values. However, other studies suggest that the greater levels of
greenhouse warming likely to result from the continued intensive use of fossil
fuels into the latter half of the twenty-first century may result in global
temperature patterns associated with drought conditions in northern
Africa.149,150 Any expansion of agriculture and settlement into areas made
newly productive by a strengthened monsoon may ultimately result in an exacerbation of vulnerability as in the 1950s and 1960s, followed by drought and
famine as conditions deteriorate, as they did after the 1960s. The interaction of
naturally occurring drought hazard and socially constructed vulnerability during
the twentieth century holds important lessons for economic development and
agricultural activity in a region where climatic variability on a variety of timescales is the norm.
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Non-hydrological Hazards and Urbanisation
While the availability of water must remain the principal consideration in economic
development and settlement expansion, human health and comfort are also affected
by other environmental factors. In the Sahara, strategies for coping with extreme
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 in
turn will lead to more frequent heat extremes.151,152
A further instance of what may be termed “maladaptation” associated with
population expansion and the associated processes of economic development and
urbanisation is the move away from traditional architecture. Traditional building
materials and architectural styles are well adapted to the extreme desert environment,
particularly in terms of temperature regulation. Historically, settlements have been
built of mud brick, an inexpensive and readily available material that is an excellent
insulator, and have incorporated convection chimneys to encourage the circulation of
cool air, and covered walkways to shelter their inhabitants from the sun. The enclosed
nature of these settlements also affords some protection from dust. However, many of
these settlements are now being abandoned in favour of generic modern towns whose
construction pays little or no attention to the particular hazards associated with a
desert environment. Streets are now open to sun and dust (partly to accommodate
vehicular traffic), and poorly insulated modern buildings fitted with air conditioners
have replaced the subtle architecture of the traditional towns. In the town of
Ghadames in western Libya, the inhabitants of a traditional town (also a UNESCO
World Heritage Site) moved to an adjacent, purpose built modern town in the
1980s. In the summer many return to the old town, where they maintain their old
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 elements
of traditional architecture with modern technology, providing comfort in an extreme
environment while ensuring access to modern amenities. Greater use of mud brick
instead of concrete and cement (the production of which is energy intensive) would
reduce both construction costs and greenhouse gas emissions, assisting both
adaptation to and mitigation of anthropogenic climate change. Fathy,153 working in
the context of development in rural Egypt, has advocated a greater emphasis on
such traditional building materials and styles in order to enhance development and
reduce poverty through the use of more affordable materials and the inclusion of
local people in the design and construction process.
The Sahara in the Earth System: Airborne Mineral Dust
Water, or rather the lack of it, is not the only ubiquitous feature of the Saharan
environment with which its inhabitants have had to contend since the onset of desiccation many millennia ago. The geomorphological processes associated with past
environmental changes in northern Africa have also resulted in the Sahara becoming
the world’s largest source of airborne mineral dust. The mobilisation, transportation
and deposition of dust has major impacts not only within the Sahara, but in many
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271
regions outside of northern Africa. Dust mobilised in the Sahara is transported large
distances and deposited not only within Africa itself but also over the Atlantic, the
Mediterranean, the Middle East, Europe and the Americas.154,155,156,157 In many of
these regions dust transport and deposition has significant implications for climate,
ecology and human health. Given the potential for human impacts on the land
surface and anthropogenically driven climate change to affect dust production, the
generation of dust represents another key interface between people and the physical
environment in North Africa, with global implications. Here we consider the role of
dust in the coupled earth-ocean-atmosphere system, before moving on to address the
nature of Saharan dust sources and their evolution over time. Finally the impacts on
human systems (principally on human health) of Saharan dust, and of changes in its
mobilisation, transport and deposition, are discussed.
Dust Impacts on Climate
Dust directly affects climate at local and regional scales by modifying the temperature
structure of the atmosphere. On the one hand dust reduces the amount of shortwave
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 the
atmosphere and causing warming in much the same way as greenhouse
gases.158,159 These two effects act in opposition to one another, and the overall
effect of dust on temperature depends on a variety of factors including the vertical
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 an
elevated layer overlying less dusty air, the overall result will be a cooling of the
Earth’s surface and the lower levels of the atmosphere during the daytime. At
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 a
humid oceanic air mass over the Atlantic Ocean and over the Sahel during the
monsoon season. Warming within the dust layer, reinforced by near-surface
cooling during the daytime, acts to reinforce the temperature inversion at the base
of the SAL and stabilise the atmosphere, inhibiting convective activity. This phenomenon has been detected over the Sahel,160 where it has been proposed as a mechanism
of drought reinforcement161. Atmospheric stabilisation by dust has also been
observed to inhibit the development of hurricanes over the Atlantic.162 The reduction
of sea surface temperatures by dust over the northern Atlantic may also play a role in
reinforcing or sustaining the dipolar temperature anomaly (cooling over the northern
hemisphere relative to the southern hemisphere and northern Indian Ocean) associated with drought conditions in the Sahel.163,164,165,166 Low altitude cooling will
also reduce evaporation over the ocean or over moist land surfaces, further reducing
the likelihood of rainfall.167 Simulations using a general circulation model168 indicate
that evaporation and precipitation are reduced globally by dust, although they suggest
that the local presence of dust increases rainfall over deserts. However, this latter
conclusion is not supported by any observational evidence.
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Dust can also affect global climate through its influence on marine and terrestrial
ecosystems. The most widely known such impact is the fertilisation of the ocean by
iron carried in aeolian dust originating in the world’s deserts. Jickells et al.169
estimate average global dust production at around 1.7 billion tonnes per year, with
almost two-thirds of this material originating from North Africa and 26 per cent of
the dust reaching the oceans. Once deposited in the ocean, iron from terrestrial
dust is believed to enhance biological productivity, which leads to the sequestering
of atmospheric carbon dioxide that is taken up by phytoplankton and exported to
deep water and ultimately to the ocean floor to be incorporated in marine sediments.170 Thus oceanic dust deposition contributes to a reduction in the greenhouse
gas content of the atmosphere, acting to modulate the rate of increase of anthropogenically driven greenhouse warming. The importance of this effect is a matter of
debate, however, and further research is necessary in order to quantify the impact
of dust on atmospheric greenhouse gas concentrations. Dust also acts to stimulate
the production of dimethyl sulphide (DMS) by marine organisms; when emitted to
the atmosphere, DMS is oxidized to sulfate aerosol, which scatters solar radiation
back to space, thereby acting to cool the Earth’s surface, further offsetting global
or regional surface warming.171
Nutrients from Saharan dust, such as phosphorous, are also important for certain
terrestrial ecosystems, and are believed to play a vital role in sustaining the Amazon
rainforest.172 Modelling studies suggest that the forests of Amazonia are vulnerable to
large-scale die-back caused by higher temperatures and reduced rainfall resulting
from anthropogenic climate change;173,174,175 any reduction in the supply of nutrients
to the forest canopy may conceivably accelerate this process. Die-back of the Amazon
forests would release large quantities of carbon dioxide into the atmosphere, accelerating global greenhouse warming.
Finally, dust may affect climate by modifying the reflectance or albedo of the land
surface. Where dust is deposited over snow fields or ice sheets it reduces the reflectance of the surface, increasing the amount of solar radiation absorbed by the surface
which results in heating and melting of the snow and ice. While such a process is
unlikely to be significant at the global scale today, this mechanism is believed to
have been important in regulating past glacial cycles, in periods when global dust
transport was much greater than at present.176 Dust deposition over mountainous
areas may accelerate the melting of glaciers and snow and ice fields, which are
already shrinking in many areas as a result of climate change. In certain mountain
regions this could impact on water resources, flood hazards and tourism, with
significant economic consequences.
Dust is clearly an important component of the atmosphere, and represents a
medium via which the Sahara influences global climate through the modulation of
global biogeochemical cycles. The Sahara, in turn, is sensitive to global climate
change, as demonstrated by its dramatic history of transitions between arid and
humid conditions. Any change in the Saharan dust cycle is likely to have significant
repercussions for other regions, and perhaps for global climate as a whole. Greater
efforts at understanding the dust cycle and the sensitivity of the Sahara to changes
in climate are thus required if we are to improve our capability to model and
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273
predict future global climate change. Any such efforts must start with an assessment
of the nature of the dust sources themselves.
The Nature of Saharan Dust Sources
Dust is not emitted from the Sahara in a uniform or random fashion; the vast majority
of aeolian material is generated from a variety of specific source regions that exhibit
particular seasonal patterns of activity.177 These regions are prescribed by a combination of geomorphological and meteorological factors, principally a supply of free
particles available for erosion, little or no vegetation cover, and wind speeds that
regularly exceed the threshold value for emissions for the surface in question.
Wind erosion may be suppressed by the presence of surface crusts (which are
common in arid regions) and the presence of obstacles such as bushes, rocks and
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) which
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 disaggregation
and the breaking up of aggregate particles within the surface. This sandblasting results
in the release of particles of dimension less than about 20 mm which are available for
uplift into the atmosphere and subsequent long-range transport.178,179
In recent years the major dust sources in northern Africa have been identified by
satellite remote sensing, principally using the Total Ozone Mapping Spectrometer
(TOMS) Aerosol Index (AI)180 and the Infra-red Difference Dust Index (IDDI)
derived from data acquired by METEOSAT.181,182 Global monitoring of aerosols
using the TOMS AI indicates that the Sahara is by far the worlds largest dust
source, and that the most active source region is the Bodele Depression, variously
described as the worlds largest single dust source, or the ‘dustiest place in the
world’.183,184 The Bodele Depression contains the exposed lake sediments of the
now desiccated Lake Megachad, and it is these sediments that provide the material
mobilised as airborne dust. However, the importance of the Bodele Depression is
also a result of its geographical situation to the southwest of the gap between the
Tibesti and Ennedi Mountains, which funnels and amplifies the prevailing northeasterly winds, resulting in a high frequency of events during which surface winds exceed
the threshold velocity for sediment mobilisation.185 In addition, field and satellite
observations reveal that the lake sediments are partially covered by mobile sand
dunes, which provide an abundant supply of coarse material with which the
palaeolake surface is sandblasted.
The western Saharan region straddling the borders of Mali, Mauritania and
Algeria also appears to be particularly active; in the IDDI imagery this region is as
prominent as the Bodele Depression, and is flanked by additional prominent
sources in south-central Algeria and the northern regions of Western Sahara
(Figure 4). Satellite imagery and field observations indicate that these source
regions are heterogeneous in nature, consisting of a number of different sources
including but not restricted to dry lake beds. Field observations by two of
the authors (Drake and Brooks) in the north of Western Sahara, coupled with
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FIGURE 4
MEAN ANNUAL IDDI VALUES OVER AFRICA, REPRESENTING AVERAGE REDUCTION IN
THE BRIGHTNESS TEMPERATURE OF THE EARTH IN KELVIN, AS MEASURED BY THE
INFRA-RED CHANNEL OF METEOSAT. HIGHER VALUES INDICATE GREATER ABSORPTION
OF INFRA-RED RADIATION BY ATMOSPHERIC AEROSOLS; OVER THE SAHARA MINERAL
DUST IS THE DOMINANT AEROSOL, WHILE HIGH VALUES IN OTHER REGIONS MAY
RESULT FROM BIOMASS BURNING. SAHARAN REGIONS WHERE IDDI VALUES ARE
CONSISTENTLY HIGH (REPRESENTED BY THE ORANGE AND RED AREAS) ARE
INTERPRETED AS REPRESENTING MAJOR DUST SOURCE REGIONS; NOTE THE BODELE
DEPRESSION IN THE CENTRAL-SOUTHERN SAHARA AND THE REGION OF HIGH VALUES
IN THE WESTERN SAHARAN REGION
observations from the Moderate Resolution Imaging Spectrometer (MODIS) have
identified clay lake beds as sources of airborne dust. An extensive gravel plain straddling the border of Western Sahara and Mauritania also appears to be a significant
source of dust; this plain is cut by dry channels and it is likely that the sediments
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 types
of data remain an enigma. The situation is complicated by disagreement between
the TOMS and IDDI datasets, which is due at least in part to the lack of sensitivity
of the TOMS AI to low-level dust.186 The main western source region is more prominent, and located further north, in the IDDI than in the TOMS dataset. In the IDDI, the
Bodele region is linked to the western source area by a zone of intermediate IDDI
values extending through central Niger and northern Mali (including the location
of the western maximum in the TOMS data); similar magnitude values are apparent
over much of south-central Mauritania near the northern limit of the monsoon rains. A
number of other regions appear as sources in the IDDI but not in the TOMS
data.187,188 These include a region extending from south to north through the
centre of northern Sudan and southern Egypt, approximately following the course
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of the Nile and extending several hundred kilometres either side of it, and a region to
the northwest of the Tibesti mountains where the borders of Libya, Niger and Chad
meet, extending northwards into the central regions of southern Libya.
Despite their disagreement, the TOMS and IDDI data clearly indicate that past
fluvial activity plays an important role in determining the distribution and nature of
the main Saharan dust sources. Sediments from palaeolake Chad provide the material
for wind erosion that makes the Bodele Depression such an active dust source, and
smaller dry lake beds are evident as dust sources in Western Sahara. However,
while topographic lows containing fluvial sediments from past humid episodes are
clearly important in the global dust cycle, a straightforward mapping of dust
sources onto such features appears overly simplistic. For example, the second
largest enclosed basin, the recently identified Lake MegaFezzan in southwestern
Libya,189 appears to generate little or no dust. This palaeolake exhibits very different
geomorphology to palaeolake Chad, as its sediments are capped by hard calcrete and
gypsum crusts and large dunes. Clearly, not all palaeolake surfaces act as dust
sources. Neither are all dust sources associated with palaeolakes, as illustrated by
the emission of dust from the gravel plains of Western Sahara. Instead, dust
sources appear to be heterogeneous in nature, with different sources being activated
at different surface wind speeds.190
Climatic Versus Human Influences on Dust Source Activity
During the 1990s it was suggested by a number of authors that apparent increases in
the mobilisation and transport of northern African dust over the latter half of the twentieth century were the result of land degradation in the Sahel, combined with climatic
desiccation.191,192,193 However, there is little or no evidence for widespread systematic land degradation resulting from human activity in this region, and a number of
studies have convincingly argued that what has often been interpreted as systematic,
regional-scale desertification (the ‘southwards march of the Sahara’) is nothing more
than the physical expression of an oscillation of the ‘desert boundary’ as the activity
and maximum northerly position of the Inter-Tropical Convergence Zone and associated monsoon air mass varies on interannual and interdecadal timescales.194,195,196,197
Historically, the controversy over the role of land degradation in dust production
comes from the paucity of measurements that would permit the identification of any
long-term trends in dust mobilisation in the Sahel. Indeed, for many years the only
location at which dust was measured on a regular basis was Barbados, some
5,000 km from the western coast of Africa.198 Available since 1965, i.e., around
the beginning of both dramatic population growth in the Sahel and the decline in rainfall in this region, this dataset would have been suitable for the search for trends in
production (rather than transport) if its ability to represent processes occurring in
the African continent were not so questionable. In contrast to this highly localised
record, satellites offer a unique opportunity to monitor atmospheric dust at the
global scale. However, suitable satellite imagery for dust monitoring has only been
available since the very end of the 1970s, and thus covers too short a period for it
to be useful in discriminating between an anthropogenic trend and the ‘natural’
impact of drought in the 1980s and 1990s. Combining satellite imagery and data
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from the Barbados record may provide a means of addressing this problem in the near
future.
However, as discussed above, assessments of the major dust source regions
indicate that they are overwhelmingly are located in Saharan regions that are unlikely
to have experienced significant human impacts.199,200,201 It appears that climatic
variations have been the principal driver of observed changes in dust production.
On the one hand variations in rainfall have affected vegetation cover and thus
influenced the susceptibility of certain surfaces to wind erosion, while on the other
changes in atmospheric processes are likely to have resulted in changes in the
balance between dust mobilisation and deposition within the Sahel and southern
Sahara.202,203 Climate change may play a key role in modulating future dust export
from the Sahara. Furthermore, Chiapello and Moulin204 have shown that the North
Atlantic Oscillation (NAO), a large-scale meteorological system that essentially
controls the strength of the Trade-Winds during the winter, explains a large part of
the year-to-year variability in dust transport from the Sahara to the tropical North
Atlantic. Its influence spreads further since a correlation of the NAO index with
dust activity in the Bodele depression suggests that the NAO influences dust
emissions as well as transport.205 It is thus likely that future changes in atmospheric
circulation will strongly affect the dust cycle, a problem that numerical models will
have to answer in the forthcoming years.
Future Dust Emissions
On long timescales, dust emissions from northern Africa are modulated by large-scale
changes in climate, as demonstrated by the abrupt increase in wind-blown dust deposited off West Africa at the end of the last Saharan humid phase around 5.5 ka.206
While dust sources do not map directly onto palaeolake surfaces, they are strongly
associated with fluvial activity, begging the question of how long they may remain
active before their reserves of erodible material are depleted once fluvial activity
has effectively ceased.
Wind action is limited to the soil surface, so emission can be sustained in the long
term only if the superficial soil layer is constantly supplied with fine material. Such a
supply can be explained in terms of the settling of dust from the atmosphere, the
disaggregation of larger particles by wind action, and the disturbance of the surface
due to the motion of sand dunes (anthropogenic disturbance, e.g. by vehicles, may
play a role, but the contribution of such processes has not been quantified and there
is no evidence for large ‘new’ dust sources, as discussed above). The occasional
action of rainfall and associated fluvial activity also represents an efficient means
of disturbing the land surface and of carrying fine material to basins via networks
of wadis. While such events are rare in the Sahara, the lack of vegetation means
that single events can disturb and mobilise large amounts of material. Furthermore,
rainfall does occur fairly regularly over some of the Saharan highlands, providing a
means of replenishing erodible material in the adjacent lowlands. Rajot et al.207
conclude that the vertical flux of emitted dust is not supply limited, even with a
fraction of silt and clay as low as 1.5 per cent near the surface. This finding is
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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 for
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 exported
from the Sahara via its impacts on the land surface. In particular, the possible ‘greening’ of the southern Sahara as a consequence of anthropogenic greenhouse warming
as discussed above, may serve to reduce the supply of dust from certain sources
through the stabilisation of the land surface. A strengthening of the African
Monsoon and resulting expansion of vegetation cover as modelled by Brovkin208
and Claussen et al.209 could conceivably cut off the dust supply from sources close
to the current monsoon belt, including the Bodele Depression. Given the role of
dust in the climatic, meteorological and biogeochemical processes discussed earlier
in this section, such a partial greening of the Sahara could have significant global
and regional implications.
Impacts of Dust on Human Systems
As a component of the Earth system which contributes to the regulation of biogeochemical cycles, ecological productivity and climate, variations in dust mobilisation,
transport and deposition have implications for human activities. Impacts on ocean
productivity may affect fish stocks, with associated economic implications, while
individual dust events affect transport and day-to-day human activities. Large-scale
influences of dust on climate and the possible role of dust in modulating rainfall
may potentially be implicated in food insecurity. The role of dust in the removal of
carbon dioxide from the atmosphere via its influence on marine and terrestrial biological productivity is likely to play a role in the sensitivity of the global climate to
anthropogenic greenhouse gas emissions. In order to model and predict future
climate changes that will require adaptive responses in human societies, dust must
be better represented in global climate simulations. The representation of dust in
regional climate models and the incorporation of its effects in regional scenarios of
future climate is necessary if certain key processes are to be represented accurately;
for example, changes in atmospheric dust content over the northern tropical Atlantic
might have profound implications for future trends in hurricane activity. Anticipation
of such trends will be crucial in the development of policies, strategies and measures
to cope with future changes in climate and their impacts.
Regardless of its impact on future changes in climate, the present-day impact of
dust on human health is sufficient grounds for further research into its characteristics,
mobilisation, transport and deposition. Mineral aerosols transported large distances
are generally less than 20 mm in dimension, and typically contain large numbers of
particles of sub-micron dimension. They therefore span the size range for particles
associated with adverse impacts the human respiratory system.210 Saharan dust
events regularly contribute to air pollution limits being exceeded in the Mediterranean
region,211 and African dust frequently reaches the Americas and north-western
Europe, where it can contribute to high atmospheric pollutant levels when combined
with aerosol particles from other sources.212 Variations in concentrations of particles
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whose size is the order of 10 mm (PM10s) in Trinidad are associated predominantly
with the transport of dust from northern Africa.213
Very little internationally published research has been carried out into the health
impacts of airborne dust within northern Africa (although see Laval, 1967214 and
Fossati, 1969.215) Nonetheless, research in other parts of the world suggests that
these impacts are likely to be great. Lung conditions in populations living in arid
or semi-arid regions have been linked with exposure to silica in the Himalayas,216
the Thar Desert of India217 and the Southwest USA,218 and the deposition of silica
in the lungs is associated with a widely recognised condition known as ‘desert lung
syndrome’.219,220 Dust events originating in the interior desert of Australia have
been associated with increased incidences of asthma in Brisbane.221 The evidence
for links between dust and asthma in the Caribbean is more equivocal. It has been
suggested that dust clouds originating in the Sahara are associated with increased
paediatric asthma accident and emergency emissions in Trinidad, where there is a
widespread belief that the passage of Saharan dust exacerbates rhinitis and
asthma.222 However, emerging findings from an ongoing study by one of the
authors (Prospero) and colleagues appear to challenge this conclusion. There is
evidence that dust from northern Africa is associated with the long-range transport
of micro-organisms; daily aerosol samples collected throughout 1996– 1997 from
the trade winds reaching Barbados yielded significant concentrations of viable (i.e.
culture-forming) bacteria and fungi only when African dust was present.223 In parts
of the Sahel, dust storms are believed to be responsible for certain potentially fatal
sicknesses.224
There is thus a strong public health case for research into the health effects of dust
in the Sahara, and into ways to ameliorate any such effects. A greater understanding of
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 well
as for activities such as climate forecasting. Such an understanding requires comprehensive campaigns of field work to investigate the geomorphological, geochemical
and mineralogical nature of the key dust sources and the surface conditions associated
with emission, as well as remote sensing studies of dust sources and transport. Such
fieldwork is currently underway in the Bodele Depression225 source region and is at
an early stage in Western Sahara, but there is little such activity in other parts of northern Africa beyond the monitoring of dust event frequencies and visibility.
Discussion and Conclusions
Research focusing on regions rather than processes is currently somewhat unfashionable. However, a focus on specific regions such as the Sahara provides an excellent
opportunity to investigate linkages between different physical and social systems
and to conduct truly interdisciplinary research. The Sahara in particular has been
neglected by the research community for a number of reasons, including (i) perceived
difficulty of access and security issues, which discourages researchers from conducting field work, (ii) the economic and political marginalisation of the region and the
associated lack of participation of Saharan countries in international research
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programmes, (iii) the difficulty faced by nationals of Saharan countries in travelling
outside of the region, (iv) the isolation of individual researchers working on Saharan
issues within countries outside of the region, and (v) the fact that what little research is
conducted in the Sahara is often seen as either rather esoteric in nature (e.g. research
into archaeology and rock art) or only of relevance to a tiny minority of specialists
(e.g. ethnographic and anthropological research).
The lack of interest in the Sahara among the wider research community is remarkable given the proximity of the region to Europe and the strong historical ties between
Europe and North Africa, not to mention the importance of a number of Saharan
nations as oil producers. Indeed, these factors have drawn the attention of the security
community, particularly in the United States, although this attention is not always
matched by detailed knowledge of the political reality on the ground.226 The
combination of drought with other economic, social and political issues to precipitate
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 in
the Sahara in Europe and North America; however, so should the broader issues
related to the Sahara’s role in the Earth system and human well-being as discussed
in this paper.
The Sahara as a Component of the Earth System
The neglect of the physical sciences in the Sahara is particularly notable. While a
number of teams are conducting archaeological and palaeoenvironmental research
in a variety of Saharan countries, there has been very little work on contemporary
environmental issues in the Sahara. This fact is thrown into sharp relief by the
recent investment of millions of US Dollars in research into the transport of
mineral dust from Chinese sources.227 Material from these sources transported over
densely populated parts of East Asia, notably the Korean peninsula, has relevance
for regional climate, human health and ecological systems, and also reaches North
America. However, there is very little funding for, and no concerted programme
of, research into the transport of dust from the Sahara, which produces more dust
than the Chinese sources, providing up to two thirds of the global atmospheric dust
burden228 and contributing to air pollution in Africa, Europe, the Caribbean and
North America. Just as in China, trends in dust storm activity and in the export of
dust over adjacent regions have been detected for the Sahara.229 Furthermore, dust
has been implicated in the suppression of rainfall and the possible exacerbation of
drought in the Sahel, and has been demonstrated to suppress tropical cyclone and hurricane formation over the Atlantic. Better understanding and monitoring of Saharan
dust events thus has the potential to improve seasonal climate forecasts for a key
marginal region of sub-Saharan Africa, and enhance hurricane forecasting capabilities. Long-range forecasts of dust mobilisation and transport based on climate
change scenarios coupled with dynamic representations of geomorphological
processes and vegetation cover may help us forecast climate variability and extremes
(specifically droughts and hurricanes), and assist in the development of policies to
deal with the potential impacts of climate change.
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On longer timescales, a better understanding of how the Saharan land surface
responds to climate change in terms of dust production and vegetation feedbacks
should enable us to reduce uncertainties in the representation of the carbon cycle
and atmospheric aerosols in global climate models. The incorporation of mineral
aerosols into such models will improve the representation of the Earth System and
should lead to more realistic climate change scenarios, with improved estimates of
the sensitivity of global climate and of ecosystems to increases in atmospheric greenhouse gas concentrations and global mean surface temperatures. This in turn will help
policy makers to assess climate change risks and design climate change mitigation
and adaptation policies. Current computer models suggest that anticipated changes
in global climate may lead to a greening of the southern Sahara and northern Sahel
in the near to medium term, a process that may reduce the supply of erodible material
for aeolian transport. Such a reduction in dust supply could exacerbate global
warming via adverse impacts on carbon-storing marine and terrestrial ecosystems,
representing a positive feedback in the climate system (although these effects
might be offset to a certain extent by increased carbon sequestration in a more
densely vegetated Sahara). Reduced dust emissions from northern Africa may also
increase the frequency and/or intensity of Atlantic hurricanes, exacerbating hurricane
risk in the Caribbean, the southeastern United States, Central America and even
northern South America. On the other hand reduced atmospheric dust content may
have beneficial impacts in terms of human health and rainfall in certain regions.
While the mechanisms associated with all these processes have been clearly identified, the processes themselves are not understood in a quantitative manner; the
importance of the associated impacts could be significant, but will remain a matter
of speculation pending further research.
Climate Change and Human Adaptation: Lessons from the Past
The past provides us with no exact analogues for the anthropogenic greenhouse
warming that is anticipated over the coming centuries. Nonetheless, studies of the
Saharan past can teach us many lessons about physical and social systems and
their interaction. The Holocene climatic optimum occurring between about 10 and
5 ka, which was associated with humid conditions in the Sahara, might provide us
with a very approximate model for future climate change, provided such comparisons
are treated with caution – the mechanisms behind the early Holocene warming and
monsoon intensification are different from those driving present day anthropogenic
climate change. Furthermore, it must be appreciated that global mean temperatures
are likely to exceed those of the Holocene climatic optimum before the end of the
twenty-first century.230
Palaeoenvironmental data can help us develop a better understanding of past
changes in climate and the connections between changes at high and low latitudes.
Such connections are exemplified by the association between cooling in the North
Atlantic, apparently originating at high latitudes, and arid crises in the Sahara and
the wider northern hemisphere subtropical and extra-tropical region during otherwise
warm humid periods (see Brooks231 for a review of these changes and their impacts
on human societies). It is plausible that such cooling episodes will result from inputs
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of freshwater from melting ice and increased river runoff in the Atlantic associated
with anthropogenic climate warming.232 The likelihood of such events being precipitated by human-induced climate change is currently a matter of great debate, and
studies of past climates can yield information about the conditions under which
such events occur. Alley argues that past events have occurred with little or no external forcing and that the difficulty current climate models have in reproducing such
events indicates that they may be more likely to occur in the future than is generally
accepted. The occurrence of such events in the warm early Holocene suggests that
anthropogenic greenhouse warming may not necessarily mean that such transient
cold, arid episodes will be absent from the foreseeable future. Studies from regions
such as the Sahara can tell us how severe such events must be in order for their
effects to be felt in low latitudes with potentially adverse consequences for human
populations. Again, such information can help policy makers develop strategies for
avoiding ‘dangerous’ climate change associated with the crossing of critical
thresholds in the climate system beyond which we are likely to experience abrupt
changes in climate.233,234,235
As well as yielding information on the workings of the climate system and links
between climate change and environmental impacts, studies of the past can tell us
much about how human societies respond to environmental change. While patterns
of vulnerability and the nature of human responses to climatic and environmental
change are context-specific, it is possible to make some very general (although not universal) observations regarding human responses to such changes.236,237 The Sahara
reinforces the lesson, evident from more recent experiences in the Sahel, that mobility
is the key to the long-term sustainability of livelihoods in highly variable environments. The long records of environmental change in northern Africa caution us to
beware of complacency during periods of abundance associated with increased rainfall, as these are invariably followed by episodes of scarcity. Social systems in such
environments must be flexible and responsive; livestock-based pastoralism is a
more appropriate strategy than large-scale rain-fed agriculture where rainfall is
scarce and unpredictable. While this observation might be a moot point in the
hyper-arid regions of the Sahara, it is of great relevance for marginal regions such
as the Sahel, where developmental pressures and aspirations of economic growth
led to agricultural expansion in the anomalously wet 1950s and early 1960s, a time
of optimism driven by independence from colonial rule and the prevailing ideology
of technological progress. This expansion only served to exacerbate the impacts of
the subsequent period of drought and desiccation.238 Although model studies (to a
certain extent supported by observations) suggest that the northern Sahel and southern
Sahara may become wetter in the near future, the real possibility of occurrence of cold
arid episodes as described above, or of a reversal of the greening trend at high atmospheric greenhouse gas concentrations, means that extreme caution should be exercised
in the exploitation of climatic amelioration along the Sahel-Sahara boundary. Modelling studies and palaeoclimatic data caution us that social systems in northern Africa
must be prepared to confront extreme environmental variability if they are to survive.
Studies of past responses to climatic and environmental change in the Sahara
paint a picture of largely reactive, ad hoc ‘adaptation’ occurring during times of
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environmental crisis. The rapid spread of cattle herding suggests migration as a last
resort to environmental deterioration, rather than a smooth and painless process in
which human populations respond to changes as they occur without suffering significant hardship.239 While this may not appear to be a very controversial conclusion, it
stands in sharp contrast to current paradigms of adaptation, which promote the development of resilience and ‘adaptive capacity’ as a means of coping with climate
change240 – the implication being that adaptation is a means of ‘neutralising’ the
impacts of climate change and thus of avoiding adverse consequences. In fact
the current view of adaptation as something wholly benign is challenged by the
archaeological record, which demonstrates that adaptation to environmental change
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, it
often has negative aspects. The archaeological record illustrates that Saharan societies
became more territorial and stratified as they responded to climatic desiccation, in
response to a reduction in the productivity of the physical environment. Where
these processes culminated in the emergence of urbanisation based on technological
solutions to adaptation, as in the Nile Valley and the Libyan Fezzan, they proved to be
vulnerable to collapse in the face of later climatic crises or simply unsustainable in the
longer term.241,242 While the factors behind the demise of the Garamantian culture in
this region are not known, it is possible that declining groundwater levels played a
role, and that the Garamantes were confronted with limits to their technological
ability to adapt to environmental change.
Future Economic Development in the Sahara
Modern technology enables settlement in previously uninhabitable regions, and
permits the rehabilitation of areas where settlement and agriculture have been in
decline, such as the Garamantian heartland in the Fezzan. This transformation (in
terms of both reality and aspiration) is evident in the local toponymy, with the
Wadi al-Ajal (usually translated as ‘Valley of Death’) having been renamed the
Wadi al-Hayat (‘Valley of Life’). Another example of technology enabling greater
access to water is the Great Manmade River, which transports fossil water from the
aquifers of the central Sahara in southern Libya to the Mediterranean coast, providing
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 water
available, and the time until it runs out, vary considerably. A better understanding of
the origins of this fossil water, of the dynamics of the aquifers, and of the conditions
necessary for their recharge, may contribute to a more sustainable approach with
water management. An appreciation of the origins and finite nature of Saharan
water resources, coupled with policies designed to encourage water conservation,
might go some way towards extending the lifetime of fossil reserves; policies
designed to foster more ‘responsible’ use of resources are more likely to succeed
when underpinned by campaigns of awareness raising, which must ultimately be
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 underpin
much of the economic development in the region. Tourism in the Sahara ultimately
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depends on the preservation of natural and cultural heritage, being predominantly
based on tourists’ interest in landscapes, history and archaeology. This heritage can
be compromised by a number of factors, including insensitive development and oil
exploration, which threaten a number of sites of scientific and archaeological interest,
and have already damaged or destroyed some.243 Of course tourism itself can cause
significant damage when it is not carefully managed, through souvenir hunting and
practices such as the wetting of rock paintings in order to temporarily enhance
their appearance for photographic purposes.244,245,246 Nonetheless, tourism represents
a potentially lucrative, and sustainable, source of income for many Saharan
communities and nations. Whereas oil reserves will eventually run out, or may
become significantly less profitable if concerns about climate change lead to a shift
to alternative energy sources, natural and cultural heritage represent an unlimited
resource if carefully managed. While it may be expedient in the short term to sacrifice
heritage for the sake of oil exploration and production, it is neither necessary nor sensible in the long-term. Oil-based development can be complemented by low-impact
tourism that exploits the great interest in natural history and the human past among
the educated populations of affluent nations, who are constantly seeking novel
destinations. Such tourism can be underpinned by research in the physical and
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 as
archaeology in order to participate more fully in heritage tourism and assist in the
management of valuable archaeological sites.247
Conclusions
In conclusion, we stress that the Sahara is not merely the ‘empty space’ of the popular
imagination. Like other regions, the Sahara is faced with the challenge of ‘sustainable
development’ in the face of a growing population and an uncertain climatic future. In
the Sahara, this challenge is compounded by a reliance on non-renewable water
resources, extreme environmental variability, and a high sensitivity to global
climate change. The challenge of coping with variability and change is particularly
acute at the southern margins of the desert, along the oscillating Sahara-Sahel boundary. Here, future development policies must be built around this variability, and
founded on a profound appreciation of it. There is much that can be learnt from
traditional resource management practices, based on flexibility and mobility, and
tried and tested livelihood models and coping strategies should not simply be
discarded in the name of progress and modernisation. Technological innovation has
a role to play in development, for example through the development of methods to
access and deploy scarce water resources, provided the consequences of such
activities are carefully considered. Economic diversification also has its part to
play, for example in the form of locally-run and sensitively managed archaeologybased tourism.
The physical and social sciences have a key role to play in the future development
of the greater Saharan region. However, in order that this role be fulfilled, there must
be more international support for Saharan research, and much more collaboration
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between members of the international scientific community and their counterparts in
the Saharan nations. Finally, the role of the Sahara in influencing global climate
requires much more investigation; the potential of Saharan dust to affect the global
carbon cycle and perhaps accelerate the process of anthropogenic climate change
means that to ignore the Sahara is to neglect a key component of the Earth system.
NOTES
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 Africa
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4. D. Jolly, S. P. Harrison, B. Damnati and R. Bonnefille, ‘Simulated climate and biomes of Africa
during the late Quaternary: Comparison with pollen and lake status data’, Quaternary Science
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 climatic
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 years
ago documented in oxygen isotope records of precipitation in Europe and Greenland’, Climate
Dynamics 14 (1998) pp. 73–81.
7. M. R. Talbot, ‘Late Pleistocene rainfall and dune building in the Sahel’, In A. A. Balkema (Ed.),
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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 Lake
Megafezzan: A Giant Palaeolake in the Fezzan Basin, Libyan Sahara’, Quaternary Science
Reviews (in press).
15. C. Causse, C. Coque, J. Ch. Fontes, F. Gasse, E. Gibert, H. Ben Ouezdou, and K. Zouari, ‘Two high
levels of continental waters in the southern Tunisian chotts at about 90 and 150 ka’, Geology 17
(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 by
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 in
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