Time To Get Out The Rubber Boats - Alarming Sea Level Rise Expected

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Global warming could push sea levels nearly 50 per cent higher than current models predict, flooding waterfront districts of many major cities such as New York and Miami, Alexandria and much of Tokyo, according to a study that takes a new approach to the calculation. Trumping all previous estimates, one German climatologist believes global sea levels could rise as much as 140 centimetres by the end of the century. That could mean catastrophic hurricanes and floods. But other experts discount the significance of the new model. Germany's Der Spiegel news magazine says, "It may soon be time to bring out the rubber boots - and coastal barriers." The new semi-empirical study by researcher Stefan Rahmstorf, a German oceanographer at the Potsdam Institute for Climate Impact Research (PIK), predicts that global sea levels could rise anywhere from 50 to 140 centimetres this century. Current projections of future global sea levels predict a rise of anywhere from nine to 88 centimetres, but Rahmstorf argues that these figures are unreliable. Most sea level models predict changes based on what we know about how ice sheets melt and warmer waters expand. These models suggest that by 2100 sea level will be between 4 and 35 inches (9 and 88 centimetres) higher than it was in 1990. But Der Spiegel notes that the physics of how ice sheets melt and how the oceans will expand in a warmer world is still poorly understood. So Stefan Rahmstorf, an ocean physicist at Potsdam University in Germany, took a different approach: He studied actual observations of changes in sea level collected in the 20th century to make predictions for the 21st century. Current models don't jibe with actual sea level rise during recent decades, Rahmstorf says. So he crafted a formula based on a relationship between global temperature and sea level seen during the past hundred years. "The more the temperature rises, the faster the sea level rises," he was quoted by Der Spiegel as saying. In a paper published in the journal Science, Rahmstorf applied his formula to 21st-century warming scenarios from the Intergovernmental Panel on Climate Change (IPCC). His results predict that by the end of the century sea level will rise between 20 and 55 inches (50 and 140 centimetres) above 1990 levels. "We have much larger uncertainty than we previously thought about the sea level," Rahmstorf said. Rahmstorf added that the actual range of uncertainty is probably larger than his calculations suggest. The IPCC numbers are based on an older assumption that the ice sheets over Greenland and Antarctica will melt by a steady amount over time. Recent research suggests, however, that ice sheets are melting faster. "If something dramatically new happens - something we haven't foreseen - then of course the whole approach (of using observations to make predictions) breaks down," Rhamstorf said. "We may end up with more sea level rise." Der Spiegel also quoted Konrad Steffen, a professor of geography at the University of Colorado at Boulder who studies how melting ice sheets and glaciers contribute to sea level. He was quoted as saying one wild card that could impact predictions is the so-called dynamic response of the ice sheets to warming. In the last five to eight years, he noted, the speed at which Greenland's glaciers move toward the sea has sped up dramatically. Scientists think that meltwater, which pools up on the ice, funnels down to the glacier bed. There, the water acts as a lubricant, allowing the ice to slip seaward more quickly. The process may last five or 10 years or it may last decades, Steffen said. "We have hypotheses on what is happening, but we can't model it or the future," he said. "That is where (Rahmstorf) is correct." The author of the study, Rahmstorf, notes in Science that a sea level rise of 39 inches (one metre) is plausible, if the 20th-century relationship between temperature and sea level holds true in the 21st century. That much sea level rise would expose major coastal cities such as London and New York to greater storm surges, threatening life and property. By Ernest Gill, Dpa