Thursday, August 13, 2009


A new study of sea level fluctuations over the last 22,000 years is the latest to predict that rising seas could reach close to one meter by the end of this century, consistent with the most recent sea level projections made by the Intergovernmental Panel on Climate Change (IPCC).

The estimate, published July 26 in the journal Nature Geoscience, is not as dire as the three-meter (nine-foot) rise forecast by some scientists. But lead author Mark Siddall, who did the research at Columbia University’s Lamont-Doherty Earth Observatory and University of Bristol, UK, says the model’s worst-case scenario still spells major problems for some countries.

“The changes with one meter of sea level rise are drastic,” said Siddall, who is now based at Bristol. “The danger to low lying areas is still there. It floods a major part of Bangladesh as well as other low lying areas and it means coastal flood events will become much more common.”

Siddall and his colleagues in the United States and Switzerland combined data from fossil corals and ice cores to reach their conclusion. Toward the end of the last ice age, about 22,000 years ago, seas began a slow rise of some 120 meters, as the planet warmed. However, the warming was not continuous, and sometimes seas retreated. Records of ancient swings in temperature were preserved in air bubbles trapped in ice cores, and corresponding sea-level swings were preserved in coral fossils. By combining the measurements, Siddall and his colleagues tracked how sea levels correlated with temperatures, and predicted how seas might react to warming projections for the 21st century.

They forecast a sea-level rise of 7 to 82 centimeters (3 to 32 inches), in response to the minimum 1.1 degree C and maximum 6.4 degree C warming projections by the IPCC. Their sea-level prediction closely mirrors the IPCC’s own 2007 estimate of 18 to 76 centimeters (7 to 30 inches). The IPCC used sophisticated climate models to carry out its analysis, while Siddall and colleagues used a far simpler one to reach a similar result.

The seas do not respond immediately to temperature changes, said Siddall, so it may take hundreds of years before we see the full effects of the heat-trapping gases that people have put into the air since the start of industrialization. “Sea levels will be rising substantially,” he said. “And what we did last century will continue to affect sea levels for centuries and centuries.”

Placing limits on expected sea-level rise over the next century is one of the most pressing challenges for climate scientists, as large uncertainties surround the different methods used. One major problem is that the melting or collapse of the vast Greenland and Antarctic ice sheets could cause catastrophic rises, but their dynamics are not well understood. Siddall’s model takes into account these ice sheets but does not allow for the catastrophic, sudden collapse of the West Antarctic Ice Sheet--an assumption that has led some climate scientists to predict far greater increases.

“We can’t say what the impact of global climate change will be in New York or Great Britain or Switzerland but we can be sure that as the climate warms, sea level will rise,” said Lamont geochemist Steven Goldstein, who was not involved in the research.

(Photo: Montage, photo of Antarctic iceberg: Louise Newman, International Project Office of Past Global Changes. Photo of Belize coral: H. Allen Curran, Smith University)

Lamont-Doherty Earth Observatory

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