How much willsea level rise? 149removal of groundwater) can have comparable effects to the rate of sea
level rise arising from global warming. At any given place, all these
factors have to be taken into account in determining the likely value of
future sea level rise.
It is interesting and perhaps surprising that the net contribution ex-
pected from changes in the Antarctic and Greenland ice-sheets is small.
For both ice-sheets there are two competing effects.^5 In a warmer world,
there is more water vapour in the atmosphere which leads to more snow-
fall. But there is also more ablation (erosion by melting) of the ice around
the boundaries of the ice-sheets where melting of the ice andcalving of
icebergs occur during the summer months. For Antarctica, the estimates
are that accumulation is greater than ablation, leading to a small net
growth. For Greenland, ablation is greater than accumulation. For the
two taken together, under current conditions, the net effect is about zero,
although there is considerable uncertainty in that estimate.
If we look further into the future, however, larger changes in the
ice-sheets may begin to occur. The Greenland ice-sheet is the more vul-
nerable; its complete melting will cause a sea level rise of about 7 m.
Model studies of the ice-sheet show that, with a temperature rise of
more than 3◦C, ablation will significantly overtake accumulation and
meltdown of the ice cap will begin. Figure 7.3 illustrates the rate of sea
6005004003002001000Global sea level change (cm)2000 2200 2400 2600 2800 3000
Year AD+3°C+5.5°C+8°CFigure 7.3Global average sea level changes due to the response of the
Greenland ice-sheet to three climate warming scenarios during the third
millennium. The labels on the curves refer to the mean annual temperature rise
over Greenland by 3000 AD as predicted by a two-dimensional climate and
ocean model forced by greenhouse gas concentration rises until 2130 and kept
constant after that. Note that projected temperature rises over Greenland are
generally greater than those for global averages (by a factor of 1.2 to 3.1 for the
AOGCMs used in generating Figure 6.4).