Environmental Engineering Reference
In-Depth Information
Figure 9.7
Annual rainfall departures (%) from the 1961-90
average for the African Sahel, 10° N-20° N, 1900-96.
Smooth curve is a ten-year filter.
Source: Mike Hulme, Climatic Research Unit, University of East Anglia.
patterns may vary too. The most publicized example of what appears to be a significant
recent change in climate has occurred in the Sahel area of Africa. We can see from the
rainfall record that at certain times there have been sequences of higher than average
rainfall followed by periods with lower than average rainfall (Figure 9.7). From the later
1960s rainfall has nearly always been less than the longterm averages, with years such as
1984 being spectacularly dry. These changes may have major human impacts. The role of
decreasing rainfall in desertification has been debated, but many of the countries affected
by this trend experienced much political and social upheaval in the 1980s and 1990s, e.g.
Niger, Sudan, Ethiopia and Somalia, to compound the problem. Such an example does
confirm that change is still taking place in our present climatic regime, though
increasingly there is debate about how much is natural and how much is the result of
human activities.
CAUSES OF CLIMATIC CHANGE
The summary of climatic history reveals that there are considerable variations of climate
at any particular area over time. Many must be the result of natural processes acting on
the Earth-atmosphere system, as they occurred well before human activity was sufficient
to have an impact on climate. Some of the more recent ones could be the results of human
impact on aspects of the system such as changing the composition of the atmosphere or
the nature of the ground surface.
What are these processes which might lead to a change in Earth's climate? Why does
the climate vary so much over time? These are questions to which we have no easy
answer. There are at least four different time scales which require explanation:
glacial/interglacial, stadial/ interstadial, postglacial oscillations and fluctuations over the
last 150 years. In looking for causes, we must be aware that the global climate is the
product of a complex system involving the atmosphere/hydrosphere/lithosphere/
cryosphere. Changes can be forced upon the system by factors which may be either
