How much willsea level rise? 147glaciers. If all glaciers outside Antarctica and Greenland were to melt, the
rise in sea level would be about 50 cm (between 40 and 60 cm). Substan-
tial glacier retreat has occurred in recent decades adding an estimated
2–4 cm to the sea level rise in the twentieth century. Modelling the effect
of climate change on the behaviour of glaciers is, however, complex.
The growth or decay of a glacier depends on the balance between the
amount of snowfall on it, especially in winter, and the amount of melting
in the summer. Both winter snowfall and average summer temperature
are important, and both must be taken into account in future projections
of the rate of glacier melting.
The average sea level rise during the twenty-first century for each of
the Special Report on Emission Scenarios (SRES) has been calculated
by adding up the various contributions. Those due to thermal expan-
sion (typically about sixty per cent of the total) and land ice changes
(typically about twenty-five per cent of the total) were calculated us-
ing a simple climate model calibrated separately for each of seven cou-
pled atmosphere–ocean general circulation models (AOGCMs) (as in the
calculations for changes in global average temperaturein Figure 6.4).
The relatively small contributions from changes in permafrost, the effect
of sediment deposition and the long-term adjustment of the ice-sheets
to past climate changewere then added. The results are shown in Fig-
ure 7.1, where it will be seen that the uncertainties in the estimates are
substantial. Apart from the uncertainties inherent in the emissions scen-
arios, there is the uncertainty in the actual temperature rise (and hence in
the contribution from thermal expansion) depending on the value cho-
sen for the climate sensitivity (Figure 6.4). Different models also give
substantially different estimates of the amount of sea level rise due to
the melt from glaciers and small ice caps. The total range of uncertainty
by 2100 is from about 10 cm to 90 cm.
The projections in Figure 7.1 apply to the next 100 years.During
that period, because of the slow mixing that occurs throughout much of
the oceans, only a small part of the ocean will have warmed significantly.
Sea level rise resulting from global warming will therefore lag behind
temperature change at the surface (Figure 7.2). During the following
centuries, as the rest of the oceans gradually warm, sea level will continue
to rise at about the same rate, even if the average temperature at the
surface were to be stabilised.
The estimates of average sea level rise in Figure 7.1 provide a general
guide as to what can be expected during the twenty-first century. Sea level
rise, however, will not be uniform over the globe. The effects of thermal
expansion in the oceans will vary considerably with location. Further,
movements of the land occurring for natural reasons due, for instance,
to tectonic movements or because of human activities (for instance, the