Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 337

contribution to 20th-century rise was greater than previously estimated (Miller and
Douglas, 2004). This picture has been reinforced by other research published just
prior to the IPCC's 2007 report (Overpeck et al., 2006). This compared sea-level rise
during the last interglacial with computer models and current trends. This again sug-
gests that the IPCC's 2001 estimates for sea-level rise may need to be revised upwards
and further that during the 21st century we may pass thresholds that enhance Antarctic
warming as well as - as others have thought (such as Toniazzo et al., 2004; referred to
in the previous chapter) - Greenland. For example, if the collapse of West Antarctic
Ice Sheet and other melting changes albedo sufficiently then atmospheric circula-
tion patterns may also change, so altering East Antarctic warming, not to mention
that of the whole continent. This would increase the likelihood of sea-level rise reach-
ing more quickly that of the last interglacial 127 000-130 000 years ago: 6 m above
that at the beginning of the 21st century. The big question is how quickly would this
happen and so how great a revision of the IPCC's 2001 sea-level rise estimate would
be required?
As shown in Table 5.1, in 2007 the IPCC revised their upper and median sea-level
rise estimates for the 21st century downwards but raised their lower estimates for all
scenarios. This, they said, was due to reduced uncertainty of estimates compared to
their earlier reports and because the latest report did not include a small contribution
from the thawing of permafrosts. Climate system surprises were not included but, as
the IPCC themselves say, we should be wary of them.
Again, if there is more melt, and if we have yet to experience much of the thermal
expansion anticipated from global warming, then both these factors have the possib-
ility of increasing potential 21st-century sea-level rise. This has conceivably serious
consequences for the resulting impacts on human activity. Policy-makers and planners
need to take this certain uncertainty, if you will, into account.
Finally, what of beyond the 21st century? Whereas the 6 m sea-level rise during the
last interglacial may arguably be a very (possibly unconceivable) worst-case scenario
for the 21st century, it is not so in the longer term. Conversely, it is most unlikely
(indeed, virtually impossible), for all Antarctic, Greenland and glacier ice to melt by
the end of the 21st century or even the next few centuries: so unlikely, in fact, that
it can be discounted. However, unless greenhouse emissions are curbed considerably
in the near future, and indefinitely, on the scale of many centuries if not millennia
hence, a significant proportion of such melt becomes far more probable. Indeed, if
current global warming triggered an event analogous to the IETM then such melt
would become more likely. If the Earth warmed sufficiently over the millennia sea
levels could well rise by more than 80 m (Table 6.3). This would have a profound
effect on the global civilisation. All current ports, and many capital cities, would
be under water. It would also re-shape coastlines in a manner close to (because of
plate tectonic movement) that not seen since the Pliocene. Such extremely long-
term thinking is not currently on the political agenda. It is also beyond the IPCC's
current agenda. Yet at the moment, with greenhouse gas emissions currently growing
(and not being curbed), this is the long-term future into which we are probably
heading.
Turning away from Antarctica, the climatic monitoring of the Greenland ice cap is
far better. Furthermore, computer modellers seem to have greater confidence in their