We must shake off this inertia to keep sea level rises to a minimum

Björn Lomborg's claim that sea levels are not rising faster than predicted are unfounded and used by those wanting to downplay climate change

Stefan Rahmstorf
guardian.co.uk, Tuesday 3 March 2009 10.24 GMT

Global sea level is rising, and faster than expected. We need to honestly discuss this risk rather than trying to play it down.
Measurements from tide gauge stations around the world show that the global sea level has risen by almost 20cm since 1880. Since 1993, global sea level has been measured accurately from satellites; since 1993 figures have shown leves rising at a rate of 3.2cm per decade.
The two main causes of this rise are extra water entering the ocean from melting land-ice and the expansion of ocean water as it gets warmer. Both are inevitable physical consequences of global warming. Both contributions can be estimated independently from satellite and other data, and their sum is consistent with the observed rise. Depending on the time period considered, 50% to 80% of the rise is due to melting ice.
Despite knowing the causes, we cannot predict future sea level rise very well. Particularly uncertain is how ice sheets will respond to warming, as this involves complex flow processes. For example, warming ocean waters destroy the floating tongues of ice that form when glaciers meet the sea. These ice tongues are pinned to rock outcrops and hold back the glacier behind them. When the ice tongue goes, the glacier speeds up its flow. This has happened to the Jakobshavn Isbrae and other glaciers in Greenland as well as many outlet glaciers on the Antarctic Peninsula.
The latest report by the Intergovernmental Panel on Climate Change (IPCC) found that sea level has been rising 50% faster since 1961 than its computer models predict. We published a similar conclusion for 1990 to 2006 in Science in 2007.
Björn Lomborg has recently claimed in The Guardian that sea level rise is "spot on" compared with IPCC projections. That is a debating trick frequently used by those wanting to downplay climate change: Lomborg compares the observed past rise with average projections for the coming century. However, in all projections sea level rise accelerates over time, so it is of some concern that rates of rise only expected to occur in several decades are already being observed now. Measurements since 1880 confirm that the warmer it gets, the faster sea levels rise. This is likely to continue in future, so that Lomborg's assumption of a constant rate of rise until 2100 is unfounded.
Lomborg cites the IPCC projection of sea level rise (18 to 59cm by 2100) without telling his readers the full story: that the IPCC says this range "excludes future rapid dynamical changes in ice flow" of the kind mentioned above. Several studies since the IPCC report have attempted to estimate how much the total rise will be, including the part left out by IPCC. They all have arrived at substantially higher numbers.
A commission of 20 international experts, called on by the Dutch government to help plan its coastal defences, has recently given a high-end estimate of 55cm to 110cm by 2100. Equally important, this commission has highlighted the fact that sea level rise will not stop in the year 2100. By 2200, they estimate a rise of 1.5 to 3.5m unless we stop the warming. This would spell the end of many of our coastal cities.
Even after we have stopped global warming, sea level rise set in motion by our emissions of the coming decades will continue for centuries. Such is the inertia in the response of the deep ocean and the ice sheets to warming. While we can bail out banks, there is no way to turn back sea level — our only chance is to stop the warming soon enough to keep it within manageable limits. In its report The Future Oceans, the German government's Advisory Council on Global Change has proposed to limit long-term sea level rise to a maximum of one meter, as a policy goal along-side the European Union's goal to limit warming to 2C.
Lomborg's mindset becomes clear when he told us last October that "over the past two years, sea levels have not increased at all — actually, they show a slight drop. Should we not be told that this is much better than expected?". As a trained statistician, he must surely have known that he was fooling the public with the "noise" of short-term variability rather than discussing a meaningful trend. And his claim was not even up-to-date when he made it: sea level had long resumed its rise, reaching a record high in the first half of 2008.
From 10 to 12 March hundreds of climate scientists will gather in Copenhagen to discuss their latest data. Let's hope that politicians, journalists and the public will use this opportunity to listen directly to the scientists working in the field, rather than to the distortions promoted by spin-doctors like Lomborg.
• Stefan Rahmstorf is a climate scientist and oceanographer at the Potsdam Institute for Climate Impact Research. He contributed to all three reports mentioned above: IPCC, the Dutch Delta Commission and the German Advisory Council on Global Change. He will present latest data on sea level rise at the Copenhagen Climate Congress.