Geoscience Reference
In-Depth Information
extensive mantles of silt, of which the deep loess deposits in north-central China are
the thickest and best-known.
The well-preserved sedimentary and microfossil evidence from deep-sea cores
complements the more fragmentary terrestrial record from the deserts and their mar-
gins and shows that around 2.5 million years ago, the earth's climate became colder,
drier and more variable than it had been earlier. This was also the time when the first
stone tools appear in the African archaeological record, a precursor to the progress-
ively more intense interactions between prehistoric humans and their environments,
including the eventual use of fire for clearing vegetation and modifying the plant
cover.
As we come closer to the present, the desert record of past climatic changes be-
comes ever more detailed (
Table 26.1
), and with the discovery of writing, the archival
record of past floods and droughts can be compared with that from tree rings and from
precisely dated flowstone deposits in desert caves. This information reveals that cli-
matic change in deserts has operated at a variety of scales in time and space, with close
links between global sea surface temperature anomalies and the incidence of historic
floods and droughts. Volcanic eruptions have also played a role, particularly those by
explosive volcanoes located in tropical latitudes. Historic eruptions have been associ-
ated with short-term global cooling, the effect lasting only one or two years, but there
is some indication that such eruptions also helped alter the regional water balance and
led to reduced rainfall.
One question that has often generated more heat than light is the thorny problem
of desertification and, in particular, the relative importance of climatic fluctuations
and human interventions in promoting desertification processes. Part of the problem
associated with the present discourse on desertification stems from conflicting defin-
itions. At present, desertification is defined as 'land degradation in arid, semi-arid
and dry subhumid areas
resulting from various factors including climatic variations
and human activities
' (UNEP,
1992b
, italics added). Because desertification reflects
a state or condition of the land, notably, degraded land that is less able to support life
than it was previously, it might have been wiser to leave the question of causes out of
the definition and to omit that portion of the definition shown in italics. To say that
desertification is caused by various factors is stating the obvious, and it is equally
obvious that both climate and humans play a role. There are many different factors
that can lead to land degradation, both directly and indirectly (see
Chapter 24
,
Figures
24.1
and
24.2
). Ultimately, many of them are bound up with social, economic and
political factors that have for too long been ignored in the current debates.
The exponential increase in world population has led to increasing demand for
natural resources and increasing pressure on the land (Rockstrom et al.,
2009
). In the
drier parts of the world, this has meant drawing on water from deep-seated aquifers
that are no longer being recharged at a rate comparable to the rate of withdrawal - in
effect, mining the groundwater. More people and more animals on the land have led
