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Warming has now been detected down to 3000m below the surface. At intermediate
depths, the Southern Ocean waters have also freshened, especially at the higher
latitudes, which is consistent with increased ice melt, but such conclusions
must be treated with care owing to the sparsity of data.
There is increasing genetic evidence to suggest that species developed on
the cool, ocean shelf around Antarctica radiated equatorwards into the deep ocean
at lower latitudes with the assistance of the meridional over-turning circulation.
Thus Antarctica may be an important cradle for evolution and biodiversity.
Sea-level rise
Sea level at the last glacial maximum, about 20 000 years ago, was about 120m
lower than today. Water from the oceans was in the form of ice over the polar regions
and higher middle latitudes. Over the intervening time, ice has melted and sea level
has risen. About 100 years ago, the average global sea-level rise was about 1.8 mm
per annum, but today the rate is four times greater. Melting of glacier ice at low
and middle latitudes and thermal expansion of the ocean have been the most
important factors, but now the rapid melting of the ice sheets in the polar regions
is becoming much more signi
cant. The Inter-Governmental Panel on Climate
Change (IPCC) in 2007 identi
ed the changes of cryosphere contribution
to sea-level rise as the most signi
cant uncertainty for future sea-level rise.
The Antarctic ice sheets contain over 10-times more ice than the rest of the Earth
and hence its future is a critical component for sea-level projections. Observations
of the changes in the ice mass have been made using radar, laser and now gravity
measurement. These have been combined with data from ground and airborne
surveys to provide a record of recent changes (~last 20 years) in the ice mass
in the Antarctic.
The Antarctic ice sheet comprises three major components, the Antarctic
Peninsula, West Antarctica and East Antarctica. The last is not changing markedly.
Figure 11.2
shows there are some areas of very modest accumulation, which are
counterbalanced with some areas of loss, especially at the exits to some glaciers,
e.g. Totten and Cook glaciers in Wilkes Land.
The Antarctic Peninsula region has shown a sharp rise in temperature in
the last half century. In summer, temperatures are now often above freezing and
hence increased melting is occurring. The vast majority (87%) of the 244 marine
glaciers are retreating, and the glaciers are
flowing faster into the sea (a 12% increase
in the decade from 1993). The Peninsula contributes about 5% of current global
sea-level rise (0.16 ± 0.06mm per annum). Some ice core records suggest that
this acceleration may have started over 100 years ago but there is considerable
uncertainly owing to the lack of instrumental records from the region.
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