Monday, December 4, 2006

Krakatoa stifled sea level rise for decades

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Ocean cooling caused by the volcanic eruption of Krakatoa in 1883 kept sea level worldwide in check well into the 20th century, a new analysis suggests.

When the Indonesian volcano exploded, it hurled immense amounts of ash and other particles into the stratosphere. For up to 2 years, those aerosols blocked about 1 percent of the sunlight that had previously reached Earth, says Peter J. Gleckler, an atmospheric scientist at Lawrence Livermore (Calif.) National Laboratory. The resulting decrease in absorbed radiation caused the upper layers of the oceans to cool and contract. Worldwide, sea level dropped.

Gleckler and his colleagues used modern oceanographic data to confirm the accuracy of six computer models that consider the effects of volcanic aerosols and other factors on Earth's oceans. On average, those models suggest that, between 1955 and 1998, sea level rose about 1.7 centimeters because of the warming of ocean waters, the researchers note in the Feb. 9 Nature.

Applying those models to look farther back in time, the team detected a drop in sea level after the eruption of Krakatoa. In fact, even though the oceans were gradually warming because of changes in Earth's climate, sea level wouldn't have returned to its pre-Krakatoa height until around 1950, says Gleckler.

Thanks to rapidly rising concentrations of greenhouse gases, the heat content of Earth's oceans is increasing much faster today than it did early in the 20th century. The models suggest that the drop in sea level caused by the 1991 eruption of Mount Pinatubo, which lofted a comparable amount of aerosols as Krakatoa did, lasted only a decade or so.--S.P.

COPYRIGHT 2006 Science Service, Inc.
COPYRIGHT 2006 Gale Group

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