Present climate and biological change - Climate Change: Biological and Human Aspects - page 242

have all been marginally higher than the IPCC's SRESs. Indeed, impacts on human
well-being by extreme weather events are already becoming manifest, as the IPCC
have predicted (see Chapter 7).
Finally, notwithstanding the real possibilities of an even warmer world than the
IPCC predict, there is additional concern over sea-level rise. The IPCC have always
been cautious about sea-level estimates in each of its reports and the main IPCC
texts repeatedly refer to the considerable uncertainties in the science. This is for
a variety of reasons, including that the component factors (such as ice-melt and
thermal expansion) of existing estimates from the past two centuries of sea-level rise
(measured at various sites in Europe) have large estimated errors associated with
them. In short, we do not know for certain what has caused the annual slight 1-2 mm
changes over this time. With regards to future rise, there is considerable uncertainty
about how Antarctica and Greenland will behave with warming. Indeed, the early
21st century has seen much academic debate as to the anticipated sea-level rise by
the year 2100, and some might even say that the IPCC's 2007 estimates were too
conservative, even if they did raise the lower estimate from what it was in 2001.
By monitoring the Greenland and Antarctic ice caps in the early 21st century
(a body of work not available for consideration by those compiling the 2001 IPCC
report) we began to see far greater discharge from both these ice caps than anticipated.
Thomas et al. (2004) suggest that Antarctica, far from having a negative to 2cm

change in sea level between 1990 and 2100 (as suggested by the IPCC in 2001), had a
discharge in 2002-3 from West Antarctica that was much greater than that observed
during the 1990s. This discharge rate is of some 250 km 3 year − 1 , so by itself (without
other countering factors) it provides a rising factor to sea level of more than 0.2 mm
year − 1 . Even if the rate of discharge stops increasing and remains the same for the
rest of the 21st century, then West Antarctica's contribution to sea-level rise could -
and note that this is a speculation - be a factor of ten higher than the IPCC's 2001
forecast for all of Antarctica up to 2100.
However, even these high rates, should they come to pass, are unlikely to remain
the same in the future, as research has now revealed that most of the West Antarctica's
continental ice shelves (which are off shore but grounded) are hundreds of metres
deeper than thought. Without the near-surface underpinning rock that was until
recently assumed to be there, and with the shelves collapsing, ice will flow more
freely from further inside Antarctica (Thomas et al., 2004). For this reason alone
Antarctica's possible contribution to sea-level rise may even be found to become
greater than the 10-fold excess above the 2001 IPCC rate now being observed. This
is unless, and this caveat is important, as mentioned above, increased precipitation
(snowfall) in a warmer world freezes on East Antarctica, so having the net effect of
removing water from the Earth's oceans.
There is a second reason for the concern that Antarctica may contribute more
to sea levels in the future, and that is, if global warming is itself greater, then
Antarctica's margins (although not so much its interior, which retains considerable
thermal inertia) could become even more unstable and experience greater ablation.
Research on Antarctic ice using satellite data gathered between 1992 and 2003 reveals
a mixed picture (Davis et al., 2005). It shows that West Antarctica is losing ice but that
East Antarctica is gaining it, with its ice sheets mainly becoming thicker (Figure 5.9a).
This is because of increased snowfall, which itself is an expected feature of global

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