Geoscience Reference
In-Depth Information
but change is not uniform. Some areas experience more
significant increases or even decreases of temperature,
while rainfall 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 late 1960s rainfall has nearly always been less
than the long-term average, with years such as 1984
being spectacularly dry. Recent modelling work supports
the role of sea surface temperature changes in affecting
the Sahelian rainfall. 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. Chad,
Sudan, Ethiopia and Somalia, to compound the problem.
Unlike the situation with temperature, most parts of the
world do not exhibit a statistically significant trend of
precipitation ( Figure 9.8 ). Annual precipitation totals
do tend to be much more variable than annual mean
temperatures, and this makes it more difficult to estab-
lish trends. Australia is a good example of this. ENSO
effects trigger droughts and wet periods in the historical
record, with 1958-68, 1982/82 and 1991-96 being very dry
and having an impact on the agricultural economy.
Conversely, occasional wet periods in dry areas can affect
the statistics. For example, Onslow on the north coast of
Western Australia is affected by tropical cyclones and has
had annual totals between 15 mm and 1,085 mm, partly
depending upon the incidence of storms. To try to
determine an overall trend from this level of variability is
very difficult.
Such examples do confirm that changes are 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.
120
100
80
60
40
20
0
-20
-40
1900
1920
1940
1960
1980
2000
Figure 9.7 Annual rainfall departures from the 1961-90
average for the African Sahel, 10°N-20°N, 1900-99 (per cent).
Smooth curve is a ten-year filter.
Source: Mike Hulme, Climatic Research Unit, University of East Anglia
140
NW
120
100
80
140
60
SE
120
100
80
60
Figure 9.8 Annual precipitation series for the north-western
and south-eastern parts of the Alps. Although there are strong
similarities differences occur due to atmospheric circulation
factors.
Source: Auer et al.(2007)
phase of warming peaked in the 1940s, followed by a slight
decline in global mean temperatures. At that time clima-
tologists were predicting the return of cooler conditions
and perhaps even another Ice Age. From the mid-1970s
the cooling trend reversed and mean temperatures rose
suddenly and rapidly through the 1980s into the present
century ( Figure 9.6 ). Concern became directed towards
the effects of global warming and the enhanced green-
house effect rather than the imminence of the next Ice
Age. The impact of a hemispheric mean temperature
change of a few tenths of a degree may seem very small
CAUSES OF CLIMATIC CHANGE
This 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.
 
 
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