Geoscience Reference
In-Depth Information
are exacerbated during periods of drought, such
as in the Sahel in the 1970s. However, there
are problems in distinguishing natural dryland
ecosystem variability from longer term natural
fluctuations in climate. For example, Lamprey
(1975) analysed data from 1958 to 1975 and
concluded that as a function of overgrazing the
Sahara was marching northwards at 5.5 km yr
−1
.
However, new data sources and approaches (e.g.
Hellden 1988; Le Houérou et al. 1988; Tucker
et al. 1991; Hulme 1992) indicate that the de-
gradation (desertification) observed by Lamprey
was in fact attributable to natural changes in
weather conditions (i.e. drought), comparing wet
years (1958) with dry years (1975). Quaternary
records over the past 40 kyr show that the Sahara
has naturally ranged in climate from humid/
subhumid to hyper-arid/arid (Fig. 5.4). Over
the Holocene Epoch fluctuations in aridity (4.5 -
3.5 ka) have been attributed to the socio-economic
collapse of several cultures (Petit-Maire & Guo
1998). It is now accepted that dryland vegeta-
tion systems are dynamic and far from fragile,
showing adaptation and resilience to environ-
mental change. So although advancing sand dunes
destroying agricultural land may be true locally,
the issue of land degradation may be overestim-
ated and confused with natural environmental
variability (Thomas & Middleton 1994). These
concepts are particularly pertinent in the con-
text of future global climate change, which may
alter the spatial pattern and extent of drylands
and the severity of their associated problems for
human habitation. Thus there is a need to better
understand the dynamism of drylands and how
they respond to environmental change.
To assess the extent to which current trends in
dryland development are attributable to longer
term controls such as climate, there is a need
both to exploit the data stored in ancient records
and to develop higher resolution Holocene records
(Fig. 5.4). These provide valuable data stores
on the past extent of arid realms. For example,
aeolian dune sediments provide information on
past wind regimes. The identification of aeolian
dune sediments in palaeochannels may help us
identify the presence of periodically drier condi-
tions in the ancient record. Palaeolakes offer a
wealth of information on former high and low
stands reflecting changes in the hydrology of
the lake basin. Rivers and alluvial fans may yield
information on changes in climate relating to the
generation of effective runoff (Case Study 5.2).
Caves, karst, tufas and groundwater provide
information on wetter versus drier periods within
the climate record. Floral and faunal changes
may indicate landscape and climate change, such
as the pack-rat middens of the south-western
USA (Case Study 5.2). Archaeological evidence
of human behaviour may reflect changes in local
and regional climate.
Where absolute dates can be applied to the
above data then rates of change can be determined
within the arid realm. The advent of new methods
of dating enables higher resolution chronologies
to be established, allowing us to better correlate
and understand the larger scale and longer term
impacts of aridity. The responses to climate
change in arid lands can be complicated and, as
demonstrated in this chapter, wetter climates do
not necessarily mean more erosion. In contrast
drier, more seasonal climates may tend to lead to
greater erosion as a direct function of their effect
on vegetation cover and, through that, erosion.
It is also clear that an understanding of the long-
term evolution of landscapes (Case Study 5.1)
may improve understanding of both the spatial
and temporal process dynamics within an area,
and that if we are going to manage these land-
scapes to minimize the impact of human inter-
vention these need consideration.
Prediction of future events relies on an under-
standing of both the palaeoenvironmental record
and the contemporary record. As demonstrated
in the case studies in this chapter, however,
isolating cause is far from straightforward, even
in the contemporary. In addition there are the
added difficulties of the differing time-scales
on which processes operate. For example the
form of dryland valleys, which formerly has
been explained by changes in surface erosional
and depositional factors, may be more strongly
related to other factors such as groundwater
sapping and deep weathering (Nash et al. 1994)
in some arid settings, which operate on different
temporal scales.
