Geography Reference
In-Depth Information
particularly sensitive to climatic change and albedo
feedback. First-year ice thickness in the Weddell-
Enderby Basin is about 0.5 m while multi-year
ice thickness of about 1.4 m has been measured in
the Antarctic (IPCC WGII 1996). Scientific
opinion has been divided on the degree of global
warming needed to semi-permanently melt the
Arctic sea ice (Untersteiner 1984).
The total contribution of any melting of the
Greenland and Antarctic ice sheets to a rise in sea
level is thought to be close to zero at present.
Results of models simulating a 1°C rise in global
temperatures show a potential sea level rise of 0.3
mm yr
-1
, caused by the melting of ice over
Greenland and a corresponding fall for Antarctica
due to ice accumulation (IPCC WGI1996). The
difference in the model estimates between the two
ice sheets is a function of the way in which
atmospheric feedback mechanisms may operate.
A component that is not so easily modelled is
that of calving of the sea ice margins and the
production of icebergs, which eventually melt. It
is thought that in Greenland, ice loss from surface
melting and runoff is of the same order of
magnitude as loss from iceberg calving (IPCC
WGI 1996). This process is more closely related to
the frequency and tracks of storms than to global
warming directly. However, there is concern over
the possible instability of the West Antarctic Ice
Sheet, which some scientists believe could become
dislodged from its grounding 2,500 m below sea
level. If this massive volume of ice were to melt, it
would raise global sea level by 5-6 m, compared
with 8 m for the melting of the Greenland ice
sheet and 55 m for the east Antarctic ice sheet
(Untersteiner 1984). A more likely scenario of
West Antarctica ice shelf thinning for a 1°C
warming would be a sea level rise of 0.1 mm yr
-1
through to around the year 2050 (Budd
et al
.
1987).
Effects of sea level rise
The biogeophysical effects are quite diverse and
not necessarily uniform around the world. They
include the inundation of wetlands and lands close
to sea level, increased salinity of estuaries, a higher
risk of storm flooding and erosion of shorelines,
and changes to tidal ranges and the deposition of
sediments. Regional responses to sea level rise
through geomorphological and ecological systems
are proving difficult to identify or forecast (IPCC
WGII 1996).
In spite of uncertainties about the degree of
rise in sea level, any rise would pose a direct threat
to low-lying coastal zones and islands—
particularly coral atolls, reef islands and tropical
coastal wetlands, where the mangrove ecosystems
are under threat (IPCC WGII 1996). Where rising
sea level is combined with tectonic subsidence
and/or human actions that may exacerbate the
problem, the situation is potentially very serious.
Table 2.1 indicates the synthesised results from
country case studies based on a 1 m rise in sea level
by the year2100 based on the high estimate of
global warming under the 1900 business-as-usual
scenario. This may be regarded as extreme, but
down-scaling still implies substantial problems for
countries such as Bangladesh and China,
particularly where economic growth in terms of
GNP remains at a low level.
Discussion about indirect effects through
feedback mechanisms in the climate system is
fraught with uncertainties. At present, models
show rises in the mean surface air temperature,
especially over land, and the majority of models
indicate some increase in Asian monsoon rainfall.
If the occurrence of tropical cyclones and storm
surges increases, or the direction of storm tracks
changes, then either could have devastating effects
Plate 2.1
Changes in the Wardie Ice Shelf, Antarctica
(photograph: British Antarctic Survey).
