How High Will They Rise? In 2007, the Intergovernmental Panel on Climate Change (IPCC) projected that sea levels would rise by somewhere between 18 and 59 centimeters by the last decade of this century. As the IPCC prepares to release its latest summary of climate science next week, a leaked draft forecasts a greater rise in sea level — possibly close to 1 meter by 2100. Image: Hamish Moffatt/Flickr
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The world's leading climate scientists kicked up a storm in 2007, when they issued their best estimates of how quickly the oceans would swell as the globe warms. The Intergovernmental Panel on Climate Change (IPCC) projected that sea levels would rise by somewhere between 18 and 59 centimeters by the last decade of this century — an upper limit that seemed far too low to other scientists, given the pace of melting in Greenland and other changes. “We were hugely criticized for being too conservative,” says Jerry Meehl, a climate modeler at the US National Center for Atmospheric Research in Boulder, Colorado and one of the authors of the IPCC's 2007 report.
The panel had previously projected much higher rates of sea-level rise, but its 2007 assessment admitted that it could not tackle the entire problem: the predictions did not include the possibility of rapid changes in ice cover in Greenland or the Antarctic because the authors had concluded that it was impossible to forecast such behaviour with the knowledge and models then available. Yet as early as 2009, it was clear that real sea-level rise was on pace to exceed the 2007 projections.
As the IPCC prepares to release its latest summary of climate science next week, researchers say that they now have a better grasp of the problem. Although the final report is not yet complete and the numbers could change, a leaked draft from June forecast a significantly greater rise in sea level — possibly close to 1 meter by 2100. But there is still huge uncertainty over how fast the oceans will rise, how the pattern will vary around the globe and what the ultimate high-water mark will be. Here, Nature investigates some of the big questions remaining about sea-level rise.
How fast will it rise?
Stefan Rahmstorf, a physical oceanographer at the Potsdam Institute for Climate Impact Research in Germany, is deeply unsatisfied with the standard tools for forecasting sea-level rise: 'process' models that try to represent the physics of every contributing factor. One reason for this discomfort was clear back in 2007. When researchers added up all the individual processes that contributed to rising seas, they could account for only 60% of the observed lift from 1961 to 2003 (see 'Too much water'). “The whole was bigger than the sum of its parts,” says John Church, co-lead author of the chapter on sea-level rise in the forthcoming IPCC report and an oceanographer at the Australian Commonwealth Scientific and Industrial Research Organisation in Hobart. The two biggest effects — the expansion of water as it warms, and the addition of water to the oceans from melting glaciers — each accounted for about one-quarter of the total. A little extra was added in from the melting of the Antarctic and Greenland ice sheets. That left a gaping hole.