Eruptions Appear to Lower Sea Levels

Powerful volcanic eruptions over the last 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 Mt. Pinatubo in the Philippines dropped sea levels by about 5 millimeters, or a fifth of an inch, within a year.

But after two years, the climate began to recover from Mt. 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 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 to a decade to warm up again because of anomalies in the way subsurface 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 -- which many scientists consider the primary cause of sea level rise.

Church estimated that over the last 110 years, the eruption of Mt. Pinatubo along with Indonesia’s Mt. Agung in 1963 and Mexico’s El Chichon in 1982 have reduced sea level rise by about 7 millimeters -- only a fraction compared with 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 NASA’s Goddard Institute for Space Studies in New York.