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4 A case study in geomorphic impacts of climate change: the Kalahari of southern Africa

4 A case study in geomorphic impacts of climate change: the Kalahari of southern Africa

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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,



Tropical savannas



267



FIGURE 9.12. Distributions of luminescence ages from the linear

dunes of the southwestern Kalahari (modified from Telfer and

Thomas, 2007).



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

change.



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

dunefield.

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



Tropical savannas

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

discussion.



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



269

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

variables?

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

breached.

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



Landscape element

or process



Warmer and wetter climate



Warmer and drier climate



Likelihood of change over next

100 years



Deep weathering

profiles

Groundwater

movement



Weathering accelerated



Weathering retarded



Low



Water table rise



Water table recedes



Inselbergs

Colluvium



Uncertain

Uncertain: sediment

accumulation on slopes

reduced due to increased

runoff or increases due

to higher vegetation

cover

Uncertain: stone lines or

hardpans at shallower

depth if greater runoff

accelerates sediment

removal

Higher water table,

sediment accumulation

More seasonal flooding,

sediment accumulation



Uncertain

Uncertain: lower

vegetation cover favours

sediment removal; less

runoff increases slope

sediment storage



Moderate (greater seasonality

increases amplitude of

groundwater movement with

possible secondary effects on

weathering)

Low

High



Stone lines and

duricrusts



Dambos

Pans



Gullies



Dunes



Depends on vegetation

response, accretion of

sediments possible,

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

sediment storage

Lower water table, gully

erosion

Desiccation due to

groundwater lowering,

wind deflation of surface

sediments

Depends on vegetation

response, reactivated

incision



Moderate



Lowered vegetation cover,

greater wind erosion and

dune mobility



High



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



High

Moderate



High



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



Tropical savannas

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

moral imperative to seek a deeper understanding of the nature

of savanna landscape sensitivity to global climate changes.



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10



Deserts

Nicholas Lancaster



10.1 Introduction

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

(Goudie, 2003).



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,

2007).



Geomorphology and Global Environmental Change, eds. Olav Slaymaker, Thomas Spencer and Christine Embleton-Hamann. Published by

Cambridge University Press. © Cambridge University Press 2009.



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