Volcanic suppression: major eruptions can reduce sea level.

Large volcanic eruptions can temporarily cool Earth's climate and, a team of scientists now suggests, lower sea level worldwide.

The tiny particles of broken rock and droplets of condensed gases that a volcano ejects high into the atmosphere reflect sunlight into space. So, after an eruption, there's less radiation reaching Earth's surface to warm it, says John A. Church, an oceanographer at the Commonwealth Scientific and Industrial Research Organization in Hobart, Tasmania. In the wake of a major eruption, this deflection of solar energy can cause global air temperatures to drop below average for months.

New analyses by Church and his colleagues suggest that these chilling effects influence the oceans as well. The water would contract as it cooled, with a concomitant drop in sea level.

To estimate the effects of volcanic eruptions on sea level, Church and his colleagues used tide data from around the world, ocean temperature and salinity data gathered by ships, and climate models that include both the atmosphere and the oceans.

After each of several major 20th-century eruptions--including those of Indonesia's Mount Agung in 1963 and the Philippines' Mount Pinatubo in 1991--the oceans cooled subtly for about 18 months, and sea level dropped, on average, several millimeters, or about three times the thickness of a penny. As natural processes scoured the volcanic material from upper levels of the atmosphere, the amount of radiation reaching Earth's surface returned to normal, the oceans warmed and expanded, and sea level recovered over the course of a decade or so.

The analysis by Church's team suggests that after the eruption of Mount Pinatubo, the most powerful one that the researchers examined, sea level dropped about 5 mm but then recovered at a rate of about 0.5 mm per year. Sea level still hadn't fully recovered as of 2000, the last year included in the scientists' analysis. The researchers report their findings in the Nov. 3 Nature.

Between 1950 and 2000, sea level rose, on average, about 1.8 mm/yr. However, scientists using satellite data gathered since 1993 estimate that the rate of sea level rise between 1993 and 2000 was about 3.2 mm/yr. Some of that apparent acceleration can be attributed to post-Pinatubo recovery, says Church.

"I've never thought about how volcanic eruptions would affect sea level, but it makes sense," says Alan Robock, an atmospheric scientist at Rutgers University in New Brunswick, N.J.

Accounting for the temporary effects on sea level of natural phenomena such as volcanic eruptions is essential to accurately predicting sea level rise in response to human-induced climate change, Anny Cazenave of the National Center for Space Studies in Toulouse, France, notes in a commentary accompanying the Nov. 3 Nature article.

BLOWING ITS TOP Ocean cooling following the June 1991 eruption of Mount Pinatubo in the Philippines caused sea level worldwide to temporarily drop about 5 millimeters.