Monday, November 07, 2005

Volcanic eruptions delay rise in sea level

Powerful volcanic eruptions over the past century have slowed the rise in sea level by releasing fine particles that deflect sunlight, cooling the oceans and thus reducing their volume, according to a study published Thursday in the journal Nature. But the effect is only temporary.

Using computer models and satellite data, researchers found that the 1991 eruption of Mount Pinatubo in the Philippines dropped sea levels by about 5 millimeters, or 1/5 inch, within a year.
But after two years, the climate began to recover from Mount Pinatubo’s effects, adding 0.5-millimeter each year for the next decade to the rate of sea level rise, said lead author John A. Church, a senior research scientist with the Australia’s Marine and Atmospheric Research Center.
The findings may explain part of the higher rate of sea level increase since 1993 -- about 3 millimeters each year. The expected long-term rise was 1.8 millimeters per year.

“The cooling effect of eruptions on the atmosphere generally lasts for only two years,” Church said. “But large bodies of water can take up a decade to warm up again because of anomalies in the way sub-surface water circulates.”

Despite the powerful effect of volcanic eruptions on world climate, they are still not powerful enough to offset the effect of global warming caused by human burning of fossil fuels -- considered by many scientists to be the primary cause of sea level rise.

Church estimated that over the past 110 years, the eruption of Mount Pinatubo along with Indonesia’s Mount Agung in 1963 and Mexico’s El Chichon in 1982 have reduced sea level rise by about 7 millimeters -- only a fraction of the overall 180 millimeter increase in sea levels in the 20th century.

“Once a volcano’s aerosols evaporate, the pace of global warming will continue to accelerate,” said climate researcher James Hansen, the director of the National Aeronautics and Space Administration’s Goddard Institute for Space Studies in New York.

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