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4 A case study in geomorphic impacts of climate change: the Kalahari of southern Africa
266 Michael E. Meadows and David S. G. Thomas
FIGURE 9.10. The Kalahari regions. The shaded area is the
so-called Mega-Kalahari; the contemporary Kalahari ‘desert’
corresponds to that part between the Orange River in the south
and the Etosha–Okavango–Zambezi wetland zone in the north
(modiﬁed from Thomas and Shaw, 1991).
9.4.1 The Kalahari environment
Thomas and Shaw (1991) comment on the ‘unusual and
elusive’ nature of the Kalahari, which is romanticised as a
place uninhabited except by Bushmen living a Stone Age
existence surrounded by a vast landscape of gently undulating, largely vegetated, duneﬁelds on which roam the remnants of enormous herds of grazing mammals and their
associated carnivorous companions. Frequently described
as a ‘desert’, the Kalahari is really an enormous area of
savanna that covers a substantial part of southern Africa
(Fig. 9.10). It is delimited (at least in the present day for, as
described below, aeolian activity has extended way beyond
such limits in the geological past) in the north by the Etosha–
Okavango–Zambezi swamp zone and in the south by the
Orange River (Thomas and Shaw, 1991). The eastern margin
coincides with the Kalahari–Limpopo watershed while the
uplands ascending to the Great Escarpment approximate the
western boundary. The physiographic and sedimentological
uniformity characterising this zone forms but part of a much
more extensive region better termed the ‘Mega-Kalahari’
(Thomas and Shaw, 1991) consisting of a downwarped
basin in which terrestrial sediments have accumulated since
the Jurassic and over an area of some 2.5 M km2 spanning
30° of latitude. Far from being an archetypal ‘desert’, since
this is no sea of barren shifting sand, there is nevertheless a
notable lack of permanent, or even seasonal, water courses
(the Okavango, Zambezi and Chobe rivers being exceptions,
although they merely traverse the Kalahari and their
water sources are located entirely beyond the region itself).
Contemporary climate is characterised by warm to hot summers and winters with warm days and cool or cold nights,
while the mean annual precipitation, strongly summer seasonal, ranges from 150 mm in the southwest to more than
600 mm in northeast Botswana (Thomas and Shaw, 1991).
The current wind regime is one of relatively low energy, as it
is in most of the Australian arid zone, so that the duneﬁeld is
relatively stable (Knight et al., 2004). The evaporative
response to these circumstances exceeds 2000 mm annually
meaning that most parts of the Kalahari are in a permanent
state of water deﬁcit. Surface characteristics are dominated by
the quartzitic sandy parent material (Kalahari sand: see Wang
et al., 2007) which has the potential to retain moisture and
support permanent vegetation. Typically, vegetation responds
to the northeast–southwest moisture gradient, dominated by
grasses and shrubs at the arid end of this spectrum with
increasing size of trees at the wetter end, although bush
encroachment in drier areas results in an overall negative
relationship between rainfall and woody cover (Ringrose
et al., 2003).
Knowledge of the geomorphology of what early travellers interpreted as a largely featureless and homogeneous
landscape has dramatically been improved through aerial
photography and satellite imagery (Thomas and Shaw,
1991). Major drainage features include perennial rivers in
the north, the best known of which is the Cubango/Cuito/
Okavango system which culminates in an enormous inland
delta via the fault-deﬁned panhandle. Other major depressions
are occupied by pans including those of the Makgadikgadi
(Ringrose et al., 2005), Ngami (Burrough et al., 2007) and
Etosha (Brook et al., 2007), all three of which display impressive evidence of signiﬁcant and rapid hydrological change
during the recent geological past. Perhaps the most characteristic geomorphological features of the Kalahari are, however,
the extensive aeolian landforms which have provided scientists with the fascinating challenge of understanding the combination of ancient and modern environmental conditions
which led to their development. As Thomas and Shaw
(1991, p. 141) point out, the key problems relate to whether
‘these dunes are currently geomorphological active or relict
features, and, in the case of the latter, the considerable problem
of dating the time of development’. Six major dune types
are found in the region, namely parabolic dunes, blowouts,
FIGURE 9.12. Distributions of luminescence ages from the linear
dunes of the southwestern Kalahari (modiﬁed from Telfer and
FIGURE 9.11. The three major duneﬁelds of the Kalahari dominated
by linear dune forms (modiﬁed from Thomas and Shaw, 1991).
barchans, transverse ridges, linear ridges and seif dunes,
although the most widespread are the linear or longitudinal
dune forms (Fig. 9.11, Plate 24) which cover some 85% of the
Kalahari and represent 99% of all dune forms present
(Fryberger and Goudie, 1981). These dunes, with crest heights
of up to 20 m and running semi-continuously for up to
100 km, are mostly vegetated (more especially in the subhumid northern and eastern Kalahari) with woodland dominating the dune crests and grassland the interdunal depressions.
As discussed in the following section, however, this situation
may have changed frequently and dramatically at various
stages during the later part of the Quaternary and may well
be set to change again in the near future.
9.4.2 Landform sensitivity in the Kalahari:
a Quaternary perspective
Vegetated linear dunes of the Kalahari, and elsewhere, have
long attracted the attention of geomorphologists as, more
particularly since the onset of optically stimulated luminescence (OSL) dating to establish a reasonable chronology of
accumulation and activity, they offer an opportunity to
reconstruct palaeoenvironmental conditions over extended
periods (Stokes et al., 1997a). While previously interpreted
from temporal gaps in proxies of humid chronologies ﬁxed
through radiocarbon dating, the timing of evolution and
development of the extensive linear dunes is now feasible
through optical dating. By implication, it has become possible to reconstruct associated palaeoclimates in respect of,
for example, wind regime and precipitation and there are
now more robust late Quaternary chronologies indicating
dune dynamics in, for example, the northern Kalahari
(Thomas et al., 2000) and the southwestern Kalahari (Telfer
and Thomas, 2007). The most recent collation of geochronostratigraphic evidence for the southwestern part of the
region is shown in Fig. 9.12 (Telfer and Thomas, 2007)
and reveals considerable complexity in the record of aeolian
activity that needs to be interpreted with caution due to
factors such as bias induced by limitations in sampling
depth. As Telfer and Thomas (2007) note, there remains
uncertainty regarding the spatial and temporal patterns of
chronostratigraphy. Indeed, the mode of formation of linear
dunes in general is debated and it is difﬁcult to distinguish
between patterns of sediment accumulation that represent
intermittent and intense deposition from those representing
continuous low-intensity deposition. These authors suggest
that both modes of accumulation are evident in the data from
linear dunes near Witpan in South Africa. Regarding the
southwestern Kalahari in general, Stokes et al. (1997a,
1997b) document six phases of dune activity with the most
recent period focussed on the period 20–10 ka BP. Some
elements of this temporal pattern contrast with the situation
in other parts of the Kalahari which may be attributed to the
fact that different parts of the region (study sites are up to
268 Michael E. Meadows and David S. G. Thomas
1200 km apart) are subject to different environmental stimuli
(Thomas and Shaw, 2002). Figure 9.12 suggests that linear
dunes have been active to some extent throughout most of
the last glacial cycle, with a prominent peak in aeolian
activity at around 10 ka. The data set offers support for the
idea that the period around the LGM (23–19 ka BP) was one
of reduced aeolian accumulation, this being in agreement
with other proxies suggesting a relatively humid phase
around this time (e.g. Stuut et al.’s (2002) offshore sediment
record). Some 1.5 m of sand accumulates in some locations
leading Telfer and Thomas (2007) to question whether or not
these dunes are really to be considered inactive in the present
day and, may in fact, be close to a state of reactivation. It may
be concluded that the linear dune forms of this part of southern Africa are highly responsive to changes in aridity, wind
energy, wind direction and sediment supply. The following
section explores the implications of such a conclusion for the
future of the Kalahari under scenarios of global climate
9.4.3 The Kalahari in the future
FIGURE 9.13. Predicted 3-month block Kalahari duneﬁeld activity
after 2070 based on Hadcm3 runs using various emission scenarios.
Note the signiﬁcant increase in dune mobility especially during
May, June and July (southern hemisphere winter) (modiﬁed from
Thomas et al., 2005).
As is evident from the above discussion, there has been
considerable attention paid to the dynamics of the Kalahari
dune systems in the late Quaternary. Thomas et al. (2005)
note that the interplay between dune surface erodibility,
which is basically a function of vegetation cover and moisture availability, and atmospheric erosivity, characterised
by wind energy, is the prime relationship in determining
whether a dune is likely to be active or inactive. This
relationship is, of course, highly susceptible to global climate change that potentially impacts any or all of these key
parameters. In an attempt to explore the possible impact of
future climate change, Knight et al. (2004) and Thomas
et al. (2005) employ a set of general circulation model
(GCM) simulations to predict the activity of the Kalahari
In the present day, the Kalahari linear dunes, particularly
in the more arid southwestern part of the region, are prone
to mobility during droughts, as was the case in the period
1960–90 when annual precipitation totals were only in the
range of 50% of the long-term mean (Bullard et al., 1997).
If, as seems to be the case for much of the region under
IPCC (2007) climate change scenarios, droughts become
more frequent as temperatures – and correspondingly,
evaporation – are higher and soil moisture values lower,
then mobility seems set to become more frequent and
more intense. Thomas et al. (2005) attempt to assess the
extent to which such changes may impact on duneﬁeld
activity through an analysis of the outputs of several
GCMs and their effects on a standardised measure of
mobility. Mobility indices typically indicate susceptibility
to sediment movement based on parameters associated with
wind energy, for example the length of time wind exceeds
a particular velocity threshold coupled with indicators
of moisture and potential evaporation. Although there are
a number of signiﬁcant challenges to estimating the effects
of climate change on duneﬁeld mobility, not least issues
around the nature and spatial scale of GCM outputs, Knight
et al. (2004) develop novel solutions to many of these
constraints and outline the feasibility of applying climate
model outputs to an estimate of regional changes in sand
mobility. Notwithstanding the fact that the method does not
successfully model some important elements of dune
dynamics, such as the implications of elevated carbon
dioxide concentrations on vegetation productivity, Knight
et al. (2004) propose the means whereby changes in
monthly mobility indices can be computed from the outputs
of four GCMs under various emission scenarios.
The Thomas et al. (2005) method involved establishing
potential future Kalahari duneﬁeld activity by integrating
monthly determinations of surface erodibility and wind erosivity into a measure of mobility. The results (Fig. 9.13) are
striking as ‘All modelled outputs project marked increases in
dune activity during the twenty ﬁrst century in all (Kalahari)
duneﬁelds, including, after 2040 in the (currently more stable) northern duneﬁeld … and in the eastern duneﬁeld’
(Thomas et al., 2005, p. 1220). Predicted values of the
mobility index after 2070 exceed a critical threshold in
many, especially winter, months in a number of scenarios and
the ‘environmental and social consequences of these changes
will be drastic’ (p. 1221).
Across vast swathes of Angola, Botswana, South Africa,
Zambia and Zimbabwe, the large population of, mainly, poor
subsistence farmers and pastoralists reliant on the Kalahari
rangelands for their livelihoods face a mounting challenge if
the basic tenets of these predictive models hold. This case
study clearly demonstrates that, under particular atmosphere
and surface conditions, geomorphological dynamics under
rapidly changing climate can have extremely severe consequences for huge numbers of vulnerable individuals. Globally,
although duneﬁelds such as that of the Kalahari occur in
both developed and developing nations, it is those ‘with the
least capacity to adapt that could suffer the most ﬁnancial,
social and developmental consequences’ (Knight et al.,
2004, p. 198).
The predictive model presented by Thomas et al. (2005)
is not without its assumptions and simpliﬁcations and is
especially uncertain in respect of the direct and indirect
effects of the climate change scenarios on vegetation
cover which is ‘the weakest part of the equation when
trying to calculate monthly dune mobility values’ (Knight
et al., 2004, p. 210). The model also fails to take account of
any possible land use change or management response to
future changes in climate and associated environmental
characteristics. Although land cover and land use are
remarkably difﬁcult parameters to predict, being dependent
not only on physical environmental drivers but also on
socioeconomic and political factors, their importance in
gauging the geomorphological impact of climate change
is such that attempts to model these into the future are
clearly also required, as indeed is argued in the concluding
9.5 Concluding remarks
Based on the preceding discussion, it is apparent that some
elements of the savanna landscape are likely to be highly
responsive to future climate change on the timescales envisaged. Nevertheless, there are many uncertainties and
constraints to appropriately and accurately predicting the geomorphic (and other) impacts of such change because the
system is itself highly complex and will respond in a multifaceted and possibly non-linear fashion. Not only are there
uncertainties as to precisely how, and how quickly, the
geomorphologically signiﬁcant components of climate
will change (IPCC, 2007) but the integrated nature of the
processes involved renders the prediction of geomorphic
responses extremely challenging. As noted above, there are
numerous determinants of landscape in the savannas and their
interrelationships are imperfectly understood. Moreover,
the likely human response to future dynamics is cryptic, for
example, how will land use and land use management factors
be impacted and, in turn, how will this inﬂuence land cover,
biomass, grazing and ﬁre regime among many other
Notwithstanding these constraints, it is possible to assess
how the various landscape elements and processes may
respond over the coming century, especially if some assumptions about the rate and nature of climate change are made.
Examining the IPCC (2007) regional scenarios for savannas,
it is apparent that temperature increase is extremely likely
and that this will be reasonably uniform across tropical areas
at between 3 to 5 °C, whereas the precipitation predictions
are much less consistent between savanna regions. It is therefore helpful to consider the geomorphic response in relation
to climates that may become warmer and wetter (up to 25%
augmentation of mean annual precipitation) compared to a
situation in which climates become warmer and drier (up to
25% attenuation of mean annual precipitation). In line with
IPCC (2007) models, these two hypothetical climate scenarios for the savannas would both be characterised by greater
climate variability and an increased frequency of extreme
events and this also needs to be taken into account in assessing any possible geomorphic impacts. Table 9.2 represents an
attempt to summarise the projected geomorphological
response to the two kinds of scenario and indicates, for
each savanna landscape element or geomorphic process,
both the direction of response and the likelihood of that
response over the time period envisaged, namely the rest of
the twenty-ﬁrst century. Because of the many interacting
uncertainties noted above, and because of the complex network of interrelationships between geomorphology, climate,
atmospheric chemistry, vegetation structure, plant productivity, biomass, ﬁre, grazing and land use management factors,
Table 9.2 needs to be interpreted cautiously as a crude (at
best) approximation as to how the savanna landscape may
react to climate and associated environmental changes.
Moreover, it should also be recognised that increasing or
decreasing precipitation accompanied by temperature
increase along the lines suggested by the latest IPCC scenarios, may not necessarily have the same geomorphic result
across the range of savanna environments. For example,
given the existence of thresholds of sediment yield as theorised by Langbein and Schumm (1958), whether erosion is
accelerated or impeded by climate change in the savannas
could depend on whether or not – and in which direction –
the threshold of 350 to 400 mm mean annual precipitation is
Notwithstanding the limitations, there are some key pointers
to how climate change may impact savanna geomorphology
270 Michael E. Meadows and David S. G. Thomas
TABLE 9.2. Susceptibility of various landscape elements and processes to climate change in the savannas
Warmer and wetter climate
Warmer and drier climate
Likelihood of change over next
Water table rise
Water table recedes
accumulation on slopes
reduced due to increased
runoff or increases due
to higher vegetation
Uncertain: stone lines or
hardpans at shallower
depth if greater runoff
Higher water table,
More seasonal ﬂooding,
vegetation cover favours
sediment removal; less
runoff increases slope
Moderate (greater seasonality
increases amplitude of
groundwater movement with
possible secondary effects on
Stone lines and
Depends on vegetation
response, accretion of
i.e. gully inﬁlling
Greater vegetation cover,
less wind erosion and
greater dune stability
Uncertain: stone lines or
hardpans may be buried
by sediment if reduced
runoff induces slope
Lower water table, gully
Desiccation due to
wind deﬂation of surface
Depends on vegetation
Lowered vegetation cover,
greater wind erosion and
in the face of projected temperature and precipitation trends.
Clearly, some large-scale features that have evolved over long
periods of geological time, such as inselbergs and deep weathering phenomena are largely resilient to short-term climate
changes. Still other elements, for example the widespread
colluvial deposits that ﬂank many shallower savanna slopes,
may well be impacted, but the direction and nature of the
change is difﬁcult to predict even if the future precipitation
conditions are assumed. However, there are certain features
and processes that appear to be especially sensitive and may,
in a sense, be regarded as geomorphic ‘hotspots’. Dambos
(or their equivalent), widely distributed on older, deeply
weathered, land surfaces, appear to be responsive to changing
climate (and land use) parameters and they are potentially
subject to incision producing concomitant changes in seasonal
water table ﬂuctuations. Given their widespread importance
for seasonal grazing and cultivation, for example in Africa
(Roberts, 1988), there could be signiﬁcant deleterious socioeconomic effects, particularly within the subsistence economy
of affected areas.
Without doubt the most obvious and pervasive geomorphic
response to projected climate changes over the remainder of
the twenty-ﬁrst century is in the widespread reactivation of
currently metastable longitudinal dune systems. As illustrated
graphically in the Kalahari, modelled by Thomas et al. (2005)
and highlighted in the case study above, almost any combination of temperature increase, precipitation reduction,
soil moisture loss and enhanced wind ﬁeld energy can be
expected to result in loss of vegetation cover and remobilisation of sandy surface deposits across vast areas of southern
Africa. Equivalent duneﬁelds in the southern margins of the
Sahara, in India, Australia and South America may well react
similarly. The resultant scale of impact on the agricultural and
grazing economies of an enormous, vulnerable and already
stressed rural poor population is incalculable, but certainly
liable to be severe.
Indeed, the concluding message of this chapter is that,
wherever there are, as indicated, pronounced geomorphic
impacts of climate change in the savannas, the effects are
accentuated because of the fact that most of the people living
within these landscapes are directly dependent for their livelihoods, through agriculture and/or grazing, on the functioning of the ecosystem. Because the vast majority of these
people are peasant farmers, they are extremely susceptible
to landscape dynamics, despite the adoption of ﬂexible livelihood strategies. The consequences for such people, historically and currently marginalised by the combined pressures
of political and economic forces that have their roots in
colonialism and, more recently, globalisation, may well be
dire. Geomorphologists, therefore, have both a scientiﬁc and
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Deserts, deﬁned by lack of water and low density of vegetation, cover some 26.2 million km2, or about 20% of the
Earth’s land surface (Ezcurra, 2006) (Fig. 10.1). Despite
their geographic extent, there are few and very limited
discussions of the effects of future climate change on desert
regions as part of the IPCC process (e.g. IPCC, 2007a).
Most deserts are fragile environments (Plate 25), easily
affected by natural and human disturbance, and are being
impacted by a rapidly growing and increasingly urban
population, which is dependent on scarce surface and
groundwater. The importance of water to human and natural systems in deserts makes them very sensitive to those
changes in climate that affect the amount, type, timing and
effectiveness of precipitation. The rich record of past climate changes in desert regions indicates the magnitude of
their amplitude and duration as well as their effect on landscapes and ecosystems and allows the assessment of the
nature and effects of future climate changes.
Hydroclimatological observations show that changes
in seasonal and annual temperature, precipitation, snowmelt and runoff, groundwater recharge and evapotranspiration are occurring today in most deserts (e.g. Dai et al.,
2004; IPCC, 2007a) and models predict that they are
likely to continue in the future. Many areas are already
experiencing signiﬁcant increases in temperature and a
reduction in rainfall over the past two decades, manifested in extended droughts, as in the Colorado River
basin since 2000; Australia from 2001–07 (with especially severe drought in 2002–03); Southern Africa from
2001 to 2004; Iraq in 2008; and in Afghanistan from
1998 to 2005. These changes will have signiﬁcant geomorphic impacts, including changes to the ﬂuvial regime,
dust storm frequency and the mobility of sand dunes
The results of global climate models differ in their
predictions of the direction and magnitude of change in
arid regions, in large part because prediction of precipitation in global climate models is difﬁcult, but a consistent
pattern is emerging (Ezcurra, 2006; IPCC, 2007a)
(Fig. 10.2). In some areas, such as China, southeastern
Arabia and India, increased monsoon precipitation is predicted, but its effects may be offset by higher evaporation
as a result of increased temperatures. In the Sahara, there
is support in many climate model predictions for a moistening of the southern and southeastern areas (including
the Sahel), but strong drying for the northern and western
areas. Some models, however, suggest a strong drying
throughout the region (IPCC, 2007a). The differences
between model predictions show the complexity of forcing factors for this region, as well as the possible inﬂuence
of feedbacks between land surface conditions and the
atmosphere, which may affect rainfall total, effectiveness
and spatial distribution (e.g. Nicholson, 2000; Lau et al.,
2006). Most of the interior of southern Africa is also
predicted to become drier. In the southwestern USA,
higher temperatures are predicted to increase the severity
of droughts (Easterling et al., 2007). The region may
already be in transition to a new more arid state as a result
of anthropogenically inﬂuenced climate change (Seager
et al., 2007). Desert areas which receive winter rainfall
are likely to be especially vulnerable to warming (IPCC,
2007b). The effects of increased levels of carbon dioxide
on plant productivity in arid regions are uncertain, but
may favour invasive exotic species, with possible effects
on ﬁre regimes (Smith et al., 2000). Model results that
incorporate carbon dioxide fertilisation of vegetation indicate a reduction in desert areas in the next century, introducing an additional level of uncertainty (Mahowald,
Geomorphology and Global Environmental Change, eds. Olav Slaymaker, Thomas Spencer and Christine Embleton-Hamann. Published by
Cambridge University Press. © Cambridge University Press 2009.