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their uncertainty over sea-level rise: of course, there will be significant discoveries
and developments along the way.
There will be further discussion of sea-level rise when looking at possible surprises
in section 6.6.3.
To summarise, the uncertainties over the conclusions from the 2001 and 2007 IPCC
assessments do not centre on whether the Earth is getting warmer per se: the IPCC
are confident that global temperatures will rise throughout the 21st century and are
'very likely [90-99% chance] to be without precedent during at least the past 10 000
years' (IPCC, 2001b). Instead, uncertainties centre on just how much warming and
sea-level rise will take place. But, because warming and sea-level rise are projected
to continue well beyond the end of the 21st century the question really is not by how
much will the Earth warm and its seas rise, but when?
One thing we do know is that during the last interglacial temperatures briefly
peaked to within the window that the IPCC forecasts for the 21st century. We also
know that maximum sea levels during the last interglacial were about 5-6 m higher
than they are today (we will briefly return to this in section 6.6.3). However, how long
after the peak in the last interglacial's temperature the peak in sea level took place is
not known with the certainty we need to plan for the future. The computer models
(which are continually being developed) currently suggest that we will not see a 5-6
m rise in sea level during this century. Conversely, although the geological evidence
does not have the temporal resolution one might like, the evidence does suggest a
rapid rise (geologically speaking). Resolving this question (be it through improved
computer models and/or geological evidence) is a research priority. Meanwhile,
policy-makers should not be complacent and should take due heed of the IPCC's
caveats and warnings of climate-related surprises. We will return to surprises at the
end of the next chapter.
This brings us on to what will happen beyond the year 2100. In theory, in three
centuries or so, should we, as may reasonably be expected, have exploited virtually all
of our present-day economically recoverable fossil fuels, but assuming no adoption
of climate-abatement measures, the temperature rise could rise a similar number of
degrees as with the Initial Eocene Thermal Maximum (IETM) described in section
3.3.9. (And this does not take into account any warming from marine methane hydrate
[clathrate] carbon release or re-organisation of biosphere carbon pools [such as in
boreal soils], as was likely in the early Eocene.) Whereas sea-level rise over a timescale
of two or three centuries could, in a worst-case scenario, be 10 m or even more, it
would be far greater still over a timescale of many centuries, as occurred during
the Eocene event. The IPCC in 2001 only briefly considered events beyond 2100,
which then was based on science from the 1990s (which is how the IPCC works,
quite rightly). This is not to decry the IPCC: science has its limits and research
itself functions within constraints. Indeed, the first IPCC report (1990) did much to
spur investment in climate-related research. The 2007 IPCC assessment did devote
a chapter subsection to the next 1000 years under the A1B scenario (which assumes
less longer-term fossil fuel use than the A2 scenario). It suggests that temperatures
might stabilise a degree or so above that which it forecasts for the end of the 21st
century (Figure 5.7b) by the early 22nd century, and then will remain that high
through to the end of the millennium. With regards to sea-level rise, the 2007 IPCC
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