Geoscience Reference
In-Depth Information
Figure 13.21 Time series of global mean sea level, expressed as anomalies with respect to the base
period 1980-1999, for the period before instrumental records (grey shading, designating an estimated
uncertainty in the estimated long-term rate of sea-level change) over the instrumental record and projected
through the twenty-first century. The red shading represents results from tide gauges, the green line is
based on satellite altimetry. The projections are from models participating in the IPCC Fourth Assessment
Report with the A1B emissions scenario, the blue shading is the range in model projections.
Source: IPCC (2007). Reproduced by permission of the IPCC (ch. 5, Observations: Oceanic climate change and sea level, Report
of WG1 1, IPCC, p. 409, FAQ 5.1, fig. 1).
season. It tends to be thicker than firstyear ice,
which is ice that grows in a single season. Some of
the Arctic's multiyear ice can be a decade old. Sea
ice grows and melts in response to the heat
budgets at top and bottom of the ice cover. In the
Arctic, ice is also continuously transported into
the North Atlantic via winds and ocean currents.
Most of this export is in the form of the thicker
multiyear ice. Glacier ice builds up from the net
balance of snow accumulation and summer melt
(ablation), but glacier flow transports ice towards
the terminus, where it may melt or calve into
water. In small glaciers, the ice may have a
residence time of 10s-100s of years, but in ice caps
and ice sheets this increases to 10 3 -10 6 years.
The contribution of Greenland, Antarctica and
the melt of glaciers and ice caps to recent sea-level
rise has already been addressed. However, it must
be stressed that glacier and ice cap retreat is very
much a global phenomenon ( Figure 13.22 ). This
is consistent with a warming climate, acting to
lengthen the melt season with a corresponding
rise of the snowline. In the past 15-20 years, the
freezing level in the troposphere has risen in the
inner tropics by 100-150m, contributing to rapid
ice loss on equatorial glaciers in East Africa and the
northern Andes. While even a decade ago, some
glaciers in Scandinavia were advancing due to
increased precipitation, the pattern is now one of
net mass loss. Of course one can always find some
Search WWH ::




Custom Search