Thursday, February 19, 2009

Sea Level is Rising – the question is by how much?

From a report posted by Liz Kalaugher on February 17, 2009     Permalink

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”.