Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 336

year − 1 to sea-level rise in 2007-9. Between the periods 2004-6 and 2007-9 the rate
of mass loss increased sharply from 31

12 Gt year − 1 . The duration of
the study was too short to establish a long-term trend, but for 2007-9 the increase
in the rate of mass loss makes the Canadian Arctic Archipelago the single largest
contributor to eustatic sea-level rise outside Greenland and Antarctica. This sea-level
rise compares with the global rate the IPCC 2007 assessment gives for the decade
1993-2003 of 3.1

± 8to92 ±

0.7 mm year − 1 .
We should not allow current slow sea-level rise to lull us, as a period of faster
rise may yet take place. Of course, while comparisons of a present interglacial Earth
warming of 2-3 ◦ C are not directly analogous to a past glacial Earth warming 2-3 ◦ C,
we might at least bear in mind the past when considering possible future surprises of
which the IPCC warn.
Second, with regards to point 2 above, Antarctic (specifically West Antarctica) and
Greenland ice may be less stable than previously thought. There has already been
much concern over the stability of Antarctic ice sheets, especially those grounded
beneath the sea along the coast (continental ice shelves). (Those ice sheets that are
already floating - marine ice shelves - will not contribute to sea-level rise when
they melt.) In 2004 concern was raised that the Greenland ice cap was melting
faster than thought. More evidence came to light in 2006 (to which we will return
shortly). Indeed, around the turn of the millennium large parts of marine ice shelves
in the Antarctic Peninsula broke free. Among the most spectacular collapses were
those of the Larsen A and B ice shelves in 1995 and 2002. In 2004 it was reported
that a survey in West Antarctica, using ice-penetrating radar, had revealed a change
in ice flows compared to that about 1500 years ago, as deduced from geological
evidence (Siegert et al., 2004). It is now thought that coastal ice was/is physically
supporting ice further inland and that with this coastal ice gone ice inland was/is
more free to flow to the coast. It is separately known that some Antarctic glaciers
more than 100 km inland are now accelerating (Kerr, 2004). If ice caps are moving
faster than thought then IPCC estimates of sea-level rise (which are based solely on
work published more than a year prior to each report) will need to be revised. At the
moment the IPCC consider that, as previously mentioned, the Antarctic contribution
to sea-level rise in the 21st century will be negligible due to ocean evaporation falling
as Antarctic snow, which will then become trapped as ice. This provides a countering
effect to Antarctic ice melt. The question is whether Antarctica is taking more water
from the oceans, via the evaporation/snowfall route, than it is contributing to the
oceans through melt. This is a current research priority.
With regards to estimates of the mass of meltwater contributing to sea-level rise,
the IPCC (2001a, 2001b) refer to 20th-century sea-level measurements to assist them
in forecasting the future. The IPCC do warn that their 21st-century sea-level forecasts
have considerable uncertainty to them (a warning not often highlighted, or even men-
tioned in much other documentation). Indeed, there has been a discrepancy between
direct estimates of 20th-century rise compared to estimates based on contributions of
mass (meltwater) and volume (thermal expansion). Research published subsequent
to the IPCC's 2001 assessment looked at temperature and salinity close to sea gauges.
Sea temperature affects the amount of thermal expansion and salinity is an indica-
tion of how the sea is being diluted by fresh water. They found that the mass (melt)

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