Geoscience Reference
In-Depth Information
to 1.8-5.2 billion by 2100 and the number living in the
direct path of storm surges will increase from 197 million
today to 399 million by 2080. The impacts of the 26
December 2004 Indian Ocean tsunami and 29 August
2005 Hurricane Katrina on New Orleans underline the
hazard. In two decades prior to Katrina, there were
250,000 deaths from tropical cyclones worldwide in
coastal regions. Highest risk is associated with densely
populated, low-lying and subsiding areas with low
adaptative capacity (for environmental as well as socio-
economic reasons) such as the large Asian and African
deltas, although the New Orleans catastrophe illustrated
the substandard levels of risk assessment and coastal
defences even in the world's greatest economic power.
Small islands are particularly vulnerable to climate
change impacts, pinched out by the convergence of sea-
level rise, increased storm surge frequency and other
extreme maritime events. There is a high risk that many
coral atolls will be completely overrun by virtue of their
very low above-water altitude, and as coral bleaching (due
to thermal stress) and storm surges destroy their natural
defensive reefs. Some islands in Tuvalu, in the western
equatorial Pacific, and elsewhere in the Pacific and Indian
Oceans have been abandoned already. In addition to
marine erosion, global warming adds to freshwater stress
on their biosphere and human populations due to very
restricted groundwater stores.
triggered suggestions of using the Canadian North West
Passage for sea transport from Europe to the Pacific.
We focus here on the geomorphic impact of a retreat-
ing cryosphere but this section can also be read in asso-
ciation with the boxes at p. 613 and p. 617. The
most expected consequence is that of glacial retreat and
resultant extension of deglaciated forelands and paraglacial
processes (see Chapter 24, p. 612 and Plate 28.6 ), but rapid
glacier advance occurs in certain situations and is covered
first (see p. 371). Advancing ice bulldozes the landscape
in its path and there are many later Little Ice Age docu-
mentary records of its devastation, particularly in human
socio-economic terms, in alpine Europe. Initially, melt-
water and sediment discharge sources advance across the
foreland and the latter increases as older moraines are
swept away. Proglacial lakes are likely to be overrun, com-
bining with spring meltwater peaks to increase the fre-
quency and power of jökulhlaupar (see p. 371). They often
caused catastrophic floods and loss of water resources
before the glacier itself delivered the coup de grâce as it
ripped up farmland, farmsteads and villages. It was not
uncommon to find advancing glaciers defying regional
retreat trends in this way until the mid to late twentieth
century but almost all areas now experience general and
often rapid glacier retreat.
Glacier retreat destabilizes the land surface in different
ways, creating new paraglacial zones exposed to subaer-
ial processes and primary vegetation succession ( Plate
28.7 ). Unweathered bedrock and raw glacigenic sedi-
ments are subjected to active weathering and high-energy
erosional processes. By definition, negative mass balance
drives high and episodic meltwater discharges augmented
by local precipitation to rework unstable ground. This
episode will create a downstream sediment pulse capable
of further disturbance beyond former glacier limits, which
will also record the climate change event. New proglacial
lakes may form temporarily behind furthest-advance or
retreat moraines. The warmer and more humid atmos-
phere potentially stimulates rapid vegetation succession,
although this has to overcome the initially unstable and
minerogenic state of the regolith and may be exposed
to increased jökulhlaup hazard. Valley sides are likely to
experience rock falls and avalanches, and be flushed
by debris flows of water-charged glacigenic and frost-
weathered debris. The geographical extent of these changes
- if not the specific land area - can be estimated from a
variety of forecast rates of alpine glacier retreat, sea-level
equivalent rise (SLE) attributed to deglaciation and glac-
ier disappearance. IPCC mean rates or forecasts of
retreat/loss are 6-15 m yr -1 , 30-60 per cent of mass, by
AD
Cryosphere
One of the more intriguing speculations about climate
change is the extent to which global warming may head
off future global cooling and the next cold stage. Our cur-
rent global temperate stage should not have long to live,
based on a continuing consistent pace and timing of tem-
perate q cold stage transformations since Termina-
tion XI and MIS 25 c. 0.9 Ma ago (see Chapter 23, p. 579
and Figure 23.9 ). It is an academic question to ask, there-
fore, whether global warming is 'a good thing' ! What mat-
ters is that the principal forecast biophysical consequences
in polar and alpine regions, for the foreseeable future, focus
on reductions in the thickness and extent of glaciers and
ice sheets, contraction of snow cover and sea ice and thin-
ning of permafrost. These are all virtually certain ( 99
per cent confidence) since ice mass reductions strongly
correlated with rising air temperature have been observed
for several decades, at increasing rates, and because global
warming continues to be strongest in the Arctic.
Temperatures north of the Arctic Circle (66.5
N) have
increased at twice the global average since AD 1965 and
are forecast to rise by 4-7
C by AD 2100. In September
2007 Arctic sea-ice area was at its lowest on record and
2100 and 0.55-0.99 mm yr -1
SLE respectively.
 
 
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