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Sea level rise flies high
Last year Stefan Rahmstorf of the
University of Potsdam, Germany, wrote a paper over the weekend. Later the
simple relationship he developed between sea-level rise and temperature change
appeared in Science. This year he has surpassed himself by doing the
calculations for his talk at the AAAS Meeting on the plane on the way over.
While they still need checking, the sums appear to indicate that sea level rise
will be substantially higher than predicted in the IPCC fourth assessment
report.
“What surprised me was that even
for low emissions scenarios such as B1 the best estimate for sea level rise is
one metre by 2100,” said Rahmstorf. “Sea level rise may well exceed
one metre by 2100 if emissions continue unabated.”
For the worst case scenario, the new
calculation predicts sea level rise of 1.80 metres by 2100.
Rahmstorf developed the latest version
of his semi-empirical equation after Martin Vermeer suggested he add a fast
response term; the pair have used the relationship to predict sea level rise
under the different IPCC emissions scenarios. Their paper is currently in
preparation but Rahmstorf says the calculation “seems to work quite
well”.
“These statistical approaches are a
warning for us to be cautious on what sea-level rise might be,” said John
Church of CSIRO, Australia. “We do need to account for what these extremes
might be in our planning. We don’t know enough about ice sheet physics to put
an upper bound on the amount of rise and the rate of rise.”
Rahmstorf agreed that the physics of the
ice sheets is important. “The empirical relationship might change over
time, the physics won’t,” he said. “Physical modelling is preferable
but we have to admit that we are not there yet and we don’t understand the physics
well enough.”
Keeping the IPCC on track
“You all kept saying something
wrong,” was Stephen Schneider of Stanford University’s verdict on the sea
level rise session at the AAAS Annual Meeting. “That the IPCC
underpredicted sea level.”
Schneider was involved in working group
II of the IPCC fourth assessment report, which said it had medium confidence
there was a risk of metres of sea level rise in several centuries. That’s in
contrast to working group I, which predicted 18-59 cm of sea level rise by 2100
and did not include contributions from ice sheet dynamics because of the
uncertainties in the science.
According to Schneider, the disparity
arose because the two working groups had different philosophies. While working
group I had a “fear of a false positive” and did not want to cry wolf
on sea level rise, working group II was scared of a false negative and wanted
to stress the seriousness of the situation.
It’s vital that the working groups
interact in advance of the fifth assessment report to ensure they are making
the same assumptions about science and what science means, he said. That should
avoid a repeat of the fourth assessment report where “a train-wreck was
avoided at the last minute”.
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