over the past decade
14
. The thinning of the ice shelves, charge of ice from outlet glaciers into the ocean
22
. In par-
apparently from ocean currents that are on average ticular, the speeds of three of Greenland’s fastest glaciers
0.5°C warmer than freezing, is mirrored by the thinning approximately doubled since 2000
28,29
, although two of
and acceleration of their tributary glaciers
15,16
. These ac- them have partially slowed since
30
. The third glacier,
celerating glaciers drain a region widely believed to be Jakobshavn Isbrae (Figure 6A.6), increased its speed to
the most vulnerable portion of the WAIS because its bed about 14 km per year
28
after rapid thinning and break up
is so deep below sea level. Collapse of the entire region of its floating ice tongue
31
, without any signs of slowing
into the sea would raise sea level by about 1 m. down. The bed is very deep for several tens of kilome-
tres inland, allowing seaward parts of the glacier to float
Elsewhere, recent detailed high-resolution satellite im- and break up as the ice thins. By contrast, nearby gla-
agery charted the simultaneous rise and compensating ciers with shallow beds have only small thinning rates,
fall of a score of patches on the Antarctic Ice Sheet, re- indicating a strong linkage between bed topography and
flecting extensive water movement under the ice and glacier vulnerability to change.
pointing to the potentially destabilizing effect of subgla-
cial water
17–19
. Although the volumes of water are tiny In addition, marked increases in ice velocity occurring
in terms of sea-level change, the observations reveal a soon after periods of heavy surface melting suggest that
widespread, dynamic subglacial water system, which meltwater draining to the base of the ice lubricates gla-
may exert an important control on ice flow, and hence cier sliding
32
(Figure 6A.7). This indicates that increased
on the mass balance of the entire ice sheet. melting in a warmer climate could cause an almost si-
multaneous increase in ice-discharge rates.
Greenland (see Figure 6A.5)
Mass-balance estimates for Greenland show thicken- Outlook for the ice sheets
ing at high elevations since the early 1990s at rates that
increased to about 4 cm per year after 2000, consistent For many reasons, it is not possible now to predict the fu-
with expectations of increasing snowfall in a warming ture of the ice sheets, in either the short or long term, with
climate. However, this mass gain is far exceeded by loss- any confidence
33
. Modeling ice sheet dynamic behaviour
es associated with large increases in thinning of the ice is seriously hampered by a paucity of observational data
sheet near the coast. about the crucial, controlling conditions at the ice-sheet
bed
2,34
. It is because of these uncertainties that the projec-
Total loss from the ice sheet more than doubled, from tions of the IPCC 4th Assessment Report, while includ-
a few tens of billions of tonnes per year in the early ing contributions from Greenland and Antarctica at the
1990s
20
, to about 100 billion tonnes per year after 2000
27
, increased rates observed for 1993–2003, state that “larger
with perhaps a further doubling by 2005
24
. These rap- values cannot be excluded, but understanding of these
idly increasing losses result partly from more melting effects is too limited to assess their likelihood or provide
during warmer summers, and partly from increased dis- a best estimate or an upper bound for sea level rise”
35
.
CHAPTER 6A ICE SHEETS 107