Global Warming Is the Sea Level Rise Doubling Its Speed?

Trumping all previous estimates, one German climatologist believes global sea levels could rise as much as 140 centimeters by the end of the century. That could mean catastrophic hurricanes and floods. But other experts discount the significance of the new model.

By Franziska Badenschier

It may soon be time to bring out the rubber boots -- and coastal barriers. A 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 centimeters this century.

Current projections of future global sea levels predict a rise of anywhere from nine to 88 centimeters, but Rahmstorf argues that these figures are unreliable.

Cold feet: The blue-colored areas would be completely underwater if sea levels rose by one meter. Note: This add-on to Google Maps ignores local peculiarities and established flood barriers.

Cold feet: The blue-colored areas would be completely underwater if sea levels rose by one meter. Note: This add-on to Google Maps ignores local peculiarities and established flood barriers.

Previous climate models -- all created on computers -- underestimated the fact that the global warming has already precipitated a rise in the sea levels today: 20 centimeters since 1870, according to Rahmstorf. His own calculations therefore incorporate 20th-century data on air temperatures and sea level changes.

Assuming the 20th-century data bears a direct correlation to future trends, applying these new parameters to the scenarios of the Intergovernmental Panel on Climate Change (IPCC) leads to the conclusion that "sea levels in the year 2100 will be 0.5 to 1.4 meters higher than in 1990," Rahmstorf wrotes in an online advance publication of the scientific journal Science.

More ocean: 9, 43, 140 or even 1,400 centimeters

The United Nations panel IPCC assumes that the average global temperature will increase by up to 4.5 degrees Celsius by 2100, and that the sea level will rise by up to 43 centimeters as a result of the water's thermal expansion alone. Moreover, the incipient melting of Greenland's pack ice could significantly increase that number, according to the upcoming IPCC report, which is due to be released in February 2007.

Nine, 43, or 140 centimeters by the end of the century: "The fact that we get such varying results with different methods emphasizes just how uncertain our current sea level predictions still are," says Rahmstorf. But that's also why other experts consider Rahmstorf's numbers to be dubious. "His report is a valuable contribution to the discussion, but we still have to give more consideration to whether the data is good enough," says Hans von Storch, director of the Institute for Coastal Research at the GKSS Research Center in Geesthacht, Germany, told SPIEGEL ONLINE.

Nevertheless, Rahmstorf is far from being alone in making his dire predictions. A special report prepared earlier this year by the German Federal Scientific Advisory Committee on Global Climate Change (WBGU), found that "a sea level rise is part of the inescapable physical consequences of global warming." Some of the key points made in the report include:

  • At the peak of the last ice age some 20,000 years ago, the sea level was roughly 120 meters lower than it is today; the global climate was colder by four to seven degrees Celsius.
  • During the last warm period, the Eemian interglacial era of 120,000 years ago, the world was roughly one degree warmer than it is today; the sea level was approximately two to six meters higher.
  • Three million years ago in the Pliocene era, when the earth was two to three degrees warmer than it is today, sea levels were 25 to 30 meters higher.

We are still far away from anything close to a 30-meter increase in the water line. But on the flipside, there is a considerably greater danger of storms and floods in London, New York and other cities in the coastal regions of the North Atlantic. Some coastal cities could even sink completely this century, a study found early in March this year.

"The decisive factor is not the mean sea level, but rather the storm surges that are packed on top of it," German coastal climate expert Ralf Weisse told SPIEGEL ONLINE. In the worst-case scenario -- if sea levels really went up 140 centimeters by 2100 -- not even the Thames Barrier, the world's largest flood barrier, would be enough to stem the tide. However, sea levels will not increase overnight, and there is enough time to build new safeguards, Kevin Horsburgh of the Proudman Oceanographic Laboratory said.

The rising tide is unstoppable

A small extension of the map service Google Maps, "Flood," reveals what the world map would look like if Rahmstorf's projections were to come true. Developed in the spring by Britain's Andrew Tingle, Flood allows the user -- including the layman -- to inundate the world, one meter at a time, in an on-screen digital simulation.

To create Flood, Tingle used contour lines from a public-access dataset of the US National Aeronautics and Space Administration (NASA). Tingle then superimposed the contour lines, which map the geographical relief of the entire earth, over the Google maps. This simple method is not all too scientific, since it does not take into account local peculiarities or established coastal barriers; but the Google mash-up is nonetheless bound to be disquieting to many coastal residents -- especially those who habitate uncomfortably close to sea level.

Tingle did not predict a reduction in the global sea level -- and why would he? The process of rising sea levels cannot be halted, never mind reversed -- it can at best be decelerated. "We would have to cut global emissions in half by 2050 in order to avert the worst," Rahmstorf told SPIEGEL ONLINE.

The chances of that happening, however, are slim. The chances of the goals of the Kyoto Protocol -- which requires developed nations who are signatories to reduce greenhouse gases by 2012, and later developing nations -- being achieved are unlikely. Indeed, a recent UN climate change report shows in detail how the trend in greenhouse gas emissions has taken a turn for the worse. According to the report, emissions of the greenhouse gas carbon dioxide (CO2) -- the most important measurement of human impact on the global climate -- have been increasing dramatically.


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