Geoscience Reference
In-Depth Information
by Turner et al. (
1990
) covering three centuries of human impact on the earth contains
many thoughtful and perceptive contributions to this important topic, embracing both
the social and natural sciences. No comparable study has appeared since.
There is already concern that the magnitude and frequency of extreme climatic
events, such as floods, droughts and heat waves, will increase during this century as
a result of global climatic changes caused in large part by human activities (Cubas-
chietal.,
2001
; Meehl and Tebaldi,
2004
; IPCC,
2007a
; IPCC,
2007b
; Houghton,
2009
; Dessler,
2012
; Eggleton,
2013
). Indeed, if present trends continue, some work-
ers believe that those regions of the world already experiencing large variations in
precipitation from year to year will be likely to experience an even higher degree of
variability (Cubaschi et al.,
2001
), which will require adaptation and more flexible
strategies of risk management among pastoral and agricultural communities. Willi-
ams and Nottage (
2006
) noted that the areas likely to be most affected are the arid,
semi-arid and dry subhumid regions of the world that cover nearly 40 per cent of the
total land area and comprise roughly one-third of the total world population. Most at
risk will be the poorer communities in regions already vulnerable to land degradation
and desertification (Williams,
2002b
; Williams and Nottage,
2006
).
Within this context, the aim of this chapter is to consider current trends in global
and regional climate, especially precipitation and temperature, to assess some of the
possible causes of these trends and to present some possible scenarios of future climate
change and their likely impacts.
25.2 Causes of climatic change
'Let us admit at once that we do not knowwhat are the basic causes of climatic change'
(Flint,
1971
, p. 789). More than forty years have elapsed since Flint, having reviewed at
length the legacy of Quaternary glaciations on every continent and the evidence of past
environmental changes in areas that had escaped glaciation, finally pronounced that the
fundamental causes of climatic change still remained elusive. There has been a great
deal of work on this topic in the years following the publication of Flint's magisterial
volume, including the analysis of sediment cores collected from the floor of the ocean
(Shackleton,
1977
; Shackleton,
1987
; Shackleton,
2001
) and of ice cores drilled from
the Greenland ice caps (Dansgaard et al.,
1984
; Dansgaard et al.,
1985
; Dansgaard
et al.,
1993
) and the Antarctic ice caps (Jouzel et al.,
1995
; Jouzel et al.,
1997
; EPICA,
2004
; EPICA,
2006
; Jouzel et al., 2007;L¨uthi et al.,
2008
) and a number of smaller
ice caps (Thompson et al.,
1995
; Thompson et al.,
1998
; Thompson et al.,
2002
).
In addition, technical advances in methods of dating climate proxies, such as lake,
river and wind-blown sediments, as well as the development of high-precision chro-
nologies from marine and glacial archives (see
Chapter 6
) have led to major advances
in our reconstruction of the pattern and tempo of past climatic events. We now know a
great deal more about a number of the factors that control climate change at different
