Geoscience Reference
In-Depth Information
The warming gave a boost to sea levels during the Eemian, which were at least
3 m and possibly 5-8 m higher than today. Regional crust movement in northern
North America and northern Europe, due to the weight of the glacial Laurentide
and Fennoscandian ice sheets, lowered the land in those areas and these only slowly
recovered once the ice melted: indeed, following the LGM these regions are still
slowly rising today. This vertical crust movement on top of the mantle is known as
isostasy, and the resulting apparent change in sea level is known as 'isostatic' change.
It complicates attempts to identify past sea levels. (As we discussed earlier and will
develop in Chapter 6, sea levels can also change due to terrestrial ice formation and
melting as well as thermal expansion.)
One important question is where did this extra water early in the last interglacial
come from? The reason why this question is so important is because if we are
currently warming the planet, then we might see sea levels similar to those back in
the Eemian. If we knew where the extra sea water came from then we would know
where to monitor matters today. At first the preferred theory was that the melting of
the West Antarctic Ice Sheet (or WAIS) contributed much of this rise. However, in
2000 computer modelling tied in with the ice-core records suggesting that Greenland
may have made a significant, if not a major, contribution (Cuffey and Marshall, 2000).
Isostasy and isostatic change aside, it now seems it was the warmth that contributed
the most to sea-level rise. (Remember that in an interglacial there is less terrestrial
ice so isostasy serves at the height of an interglacial for the land to rise and hence
relative sea levels to fall.) In 2009 US researchers led by Robert Kopp and Frederik
Simons compiled a database of sea-level proxies during the warmest part of the
last interglacial. They concluded that global sea levels at this time were even higher
than the year 2000 estimates, being at least 6.6 m higher than today, and gave a
two-thirds probability that they exceeded 8.0 m, and a one-third probability that they
exceeded 9.4 m. If this level was 4 m or above it is likely that Antarctica melt makes
a significant contribution to that of Greenland. So the implication of this research is
that west Antarctic and Greenland ice sheets were significantly smaller at the height
of the last interglacial. (As we shall see, more recent research on today's sea-level rise
suggests that ice on islands off Greenland and North Canada's mainland may also
have made a significant contribution.)
The researchers also looked at the rate of sea-level rise during the last interglacial
when sea level was similar to today's. They concluded that the sea level then was rising
at over 0.56 m each century but probably less than 0.92 m each century. Their results
highlight the vulnerability of ice sheets to even relatively low levels of sustained
global warming.
Previous interglacials were often considered in the 1990s to be a palaeo-analogue
of our current interglacial. However, there are notable differences between the various
glacial and interglacial cycles, both as predicted from Milankovitch theory and seen
practically from the geological record. Fortunately the latter tend to confirm the
former. Over the years this understanding has grown as analyses of the geological
record have extended further back from one interglacial cycle in the 1970s through to
eight in 2004 from the European Project for Ice Coring in Antarctica (EPICA, 2004)
and associated analyses (Siegenthaler et al., 2005; Spahni et al., 2005). Glacial-
interglacial cycles, we now know, are not exactly the same, and other evidence
corroborates this.
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