Environmental Engineering Reference
In-Depth Information
AREAS OF DESERTIFICATION
The area of the world affected by desertification is not known with certainty. Despite the
continuous monitoring of Earth's surface by satellite, the values quoted are still based on
intelligent estimates rather than scientific data (Table 27.3). What Table 27.3 does show
is that a large proportion of the dry lands is affected by desertification. What it does
not
mean is that large areas of the dry lands are being engulfed by sand dunes blown in from
the desert. That image may be appropriate in a few areas, but
Table 27.3 UN Environment Programme estimates
of types of dry land deemed susceptible to
desertification,
proportion affected and actual
extent
Measure
1977
1984
1992
Climatic zones susceptible to
desertification
Arid, semi-arid
and subhumid
Arid, semi-arid
and subhumid
Arid, semi-arid and
dry subhumid
Total dryland area susceptible to
desertification (m ha)
5281
4409
5172
Proportion of susceptible dry lands
affected by desertification (%)
75
79
70
Total of susceptible dry lands
affected by desertification (m ha)
3970
3475
3592
Source
: After Thomas and Middleton (1994).
ATMOSPHERIC DUST AND ITS IMPACT
human impact
Dust in the atmosphere can scatter and absorb solar radiation as well as strongly
absorbing long-wave radiation. Although this is a natural component of the atmosphere,
as a result of human activities the amount of atmospheric dust has increased with a
potential effect on the radiation budget and other aspects of the environment. Much of
this increase in dust comes from land-use changes when forests are cleared for
agriculture, exposing bare soil, and when natural vegetation declines in abundance
through overgrazing. Plumes of dust can be seen in satellite imagery, from which
estimates can be made of their impact on Earth's radiation budget; the highest values
amount to only a few watts per square metre, so the direct effects are not large. Large
amounts of dust are deposited in the oceans on the leeward side of dry lands such as the
Atlantic Ocean off the Sahara, the Indian Ocean off Arabia and the eastern Pacific off
China (Figure 1). At Nouakchott, on the coast of Mauritania, dust storms blew, on
average, ten days per year in the relatively moist 1960s. In the mid-1980s, after over
twenty years of below-average rainfall, the average had risen to eighty days per year.
Assuming each storm carried a similar amount of dust, it represents a large increase in
material transport into the Atlantic. This material has been extracted from oceanic cores
even as far west as the Caribbean.
