Geoscience Reference
In-Depth Information
and make use of present-day extreme events to reconstruct past geographies in ways
that were not possible using global atmospheric circulation models. For example, the
extreme rains during the 1999 wet season in arid central Sudan filled the hollows in
the extensive dune complex immediately east of the lower White Nile and recreated
the landscape of early Holocene times (Williams and Nottage,
2006
). At this time,
Mesolithic hunter-gatherers occupied seasonal camps on top of dunes flanked by
flooded swales, and ventured forth to collect
Pila wernei
shells for food, to harpoon
hippos and to fish for Nile perch (Clark,
1989
; Williams and Nottage,
2006
).
Keeping in mind all of these caveats, we are now in a position to attempt some
cautious predictions about some of the impacts of possible future climatic changes. In
the Southern Hemisphere, the southward displacement of the Antarctic Convergence
Zone appears to be associated with a concomitant southward displacement of the
air masses that bring winter rainfall to south-west Australia, South Africa and South
America. If this trend persists, over the next few decades, we would expect a further
reduction in winter precipitation and in stream flow in all of these regions, consistent
with trends over the past forty years. In contrast, the tropical summer rainfall regions
in the Southern Hemisphere have been receiving higher amounts of precipitation,
leading to episodically widespread flooding in north-east Australia, Mozambique and
north-east Brazil. The net increase in temperature over the past hundred years has seen
an increase in spring snow-melt and river discharge in south-east Australia and the
Andes and a reduction in the size of ice caps and mountain glaciers both in the Andes
and in equatorial Africa (Ruwenzori, Kilimanjaro and Mount Kenya). The increase
in the number of very hot days, when combined with high fuel loads, drought and
strong winds, has led and will lead to extreme bushfire risk, for which communities
will need to plan. As far as the southern deserts and their margins are concerned, the
tropical northern margins will probably become less arid and the southern margins
will become more arid, with a possible increase in extreme episodes of drought and
heavy rainfall. In addition, the increase in tropical sea surface temperatures will likely
to lead an increase in the incidence of tropical cyclones and hence to higher rainfall in
those regions most prone to cyclones today, such as the arid north-west of Australia.
In the Northern Hemisphere, two dramatic changes are already very evident. One is
the steady reduction in the area of Arctic sea ice, which has reduced the albedo in this
region and has thus allowed more solar radiation to reach the surface, causing further
melting of snow and ice - an excellent example of a positive feedback process in action.
The other is the thawing of vast areas of permafrost in northern Eurasia, leading to
release of methane into the atmosphere, which will in turn accentuate high-latitude
warming. Mountain glaciers and ice caps are in steady recession across North America
and Eurasia. There is concern that as run-off from snow-melt eventually diminishes in
California and the American Southwest, there will be a reduction in river discharge and
in groundwater recharge. Higher temperatures and drier soils have already resulted in
severe bushfires in California and in the Mediterranean countries of Europe, notably
