Sunday, November 06, 2005

Are volcano eruptions responsible for change in sea level?

WE CAN put away the life rafts for now. The near doubling in the rate of sea level rise during the 1990s was probably due in part to the delayed effects of the eruption of Mount Pinatubo in the Philippines - and not runaway melting of ice caps.

Volcanic eruptions have long been known to cause a global chill by filling the upper atmosphere with particles that block out some of the sun's radiation. Pinatubo's 1991 blast cooled the atmosphere for two years, but no one was sure whether similar cooling occurred in the oceans.
Now John Church of the CSIRO in Australia has combined observations of the oceans' heat content with climate simulations to calculate that ocean surface temperatures fell by up to 0.5 °C in the months after the eruption in 1991 (Nature, vol 438, p 74). Since cooling reduces the water's volume, he calculates that Pinatubo should have taken 5 millimetres off sea levels worldwide during this period.

Since then, the oceans have been regaining heat, but this warming happens more slowly that it does in the atmosphere. Church calculates that the Pinatubo rebound explains half of the surge in sea level rise measured by satellites between 1993 and 2000. During that period, sea levels rose by 3.2 millimetres a year, compared with an average of 1.8 millimetres since 1950.

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