Recent signs point to accelerating loss of ice in both main body of the WAIS would accelerate rapidly if its
Greenland and Antarctica. It is becoming increasingly ice shelves were thinned or removed by a warming cli-
apparent that some changes, such as the break up of ice mate
36,37
. There are clues. Ice-shelf break up along the
shelves in Antarctica, are exceptional when one looks Antarctic Peninsula has resulted in massive accelera-
over periods of centuries to millennia. For the ice sheets tion of tributary glaciers and ice-shelf thinning further
of both polar regions, some of these very fast changes south, along the Amundsen Sea, also appears to have
are caused not by melting (included in the IPCC pre- caused glacier acceleration. Here, the acceleration is
dictions), but by changes in glacier dynamics (not fully more modest, but the glaciers are far bigger, so total loss-
included in the IPCC predictions). es are large. No one knows how far inland the zone of
glacier acceleration will spread, and no one knows why
The slow, measured behaviour long associated with the the ice shelves are breaking up. However, their thinning
Greenland Ice Sheet is being transformed to the rapidly is almost certainly caused by increased basal melting,
changing characteristics more typical of big glaciers in Alas- implicating the ocean. And final break up seems to be
ka and Patagonia. A zone of glacier acceleration is progres- accelerated if there is sufficient surface meltwater to fill,
sively moving northward, leaving Greenland’s southern ice and over-deepen, crevasses in the ice shelves, effectively
dome under threat from both increased summer melting wedging the ice shelf apart into fragments.
near the coasts, and increased ice discharge down glaciers
that extend their influence far inland. If this continues, it is Observations made over the past five years have made it
quite possible that the ice dome in southern Greenland will clear that existing ice-sheet models cannot simulate the
reach a tipping point, with accelerating positive feedback widespread rapid glacier thinning that is occurring, and
causing its ever more rapid decline and an associated sea- ocean models cannot simulate the changes in the ocean
level rise of about 85 cm. Moreover, continued northward that are probably causing some of the dynamic ice thin-
migration of the zone of glacier acceleration would make ning. Consequently, in its Fourth Assessment, the IPCC
the far larger northern dome also vulnerable. has taken a conservative approach by not attempting to
predict the unpredictable. As a result, these projections
35

In Antarctica, disintegration of the WAIS continues of future ice-sheet related rises in sea level should be
to be the primary threat. The key issue is whether the regarded as lower bounds.
CHAPTER 6A ICE SHEETS 111