Sunday, November 06, 2005
Volcanic eruptions affect global warning consequences
If it had not been for the eruptions, says John Church, a CSIRO climate scientist, sea levels today would be six or seven millimetres higher.
Since the mid-1950s the seas have been rising by an average of 1.8 millimetres a year. Scientists have blamed global warming for both sea water's volume expanding and ice melting.
However, mysterious variations in the rate at which sea levels have been rising have created problems for those trying to explain how the climate responds to growing greenhouse gas pollution. In some years levels have even fallen, contradicting global warming predictions.
"I was trying to understand what caused the variability," Dr Church said. "Then I realised there were major volcanic eruptions just before all the minimums in the rate of sea level rises.
"This is the first time a link between volcanic eruptions and average sea levels has been found. There have been lots of studies of the impact of volcanos on the atmosphere but hardly any on the ocean."
From 1915 to 1963 the world was relatively free of major volcanic eruptions, he said.
Then, in 1963, Indonesia's Mount Agung exploded. It was followed in 1982 by Mexico's El Chichon and, in 1991, by Mount Pinatubo in the Philippines.
The three eruptions blasted sulfate aerosols into the stratosphere, 20 to 30 kilometres up. The aerosols acted as a mirror, reflecting sunlight and cooling the world by about 0.4 degrees for a few years.
As the oceans cooled they also contracted in volume. Dr Church said Mount Pinatubo's eruption lowered sea levels by five millimetres, briefly reversing the pattern triggered by global warming. His research found that, after shrinking for 12 to 18 months in the aftermath of a major volcanic eruption, the seas start warming and rising again. "However, their recovery takes more than a decade," he said.
Dr Church said that since late 1993, 18 months after the Pinatubo eruption, sea levels had risen by "close to three millimetres a year" - almost twice the average over the past 50 years.
"About half that higher rate," Dr Church estimated, "was the sea recovering from the volcanic eruption."
He said his research, published today in the journal Nature, highlighted the complexity of climate change.
He said he hoped the discovery would allow scientists to better predict and deal with future climate change.
His team's findings will also be presented at Greenhouse 2005, an international climate-change conference to be held in Melbourne from November 13 to 17.