Saturday, February 11, 2006
Influence of volcano eruptions on ocean temperature
Scientists now realize the volcano's eruption influenced oceans worldwide for 50 or more years afterward and counteracted many effects of human-induced greenhouse gas warming in the 20th century.
In a report today in the British journal Nature, climate modelers at Lawrence Livermore National Laboratory said Krakatoa spewed so much sunlight-filtering gases and ash that the eruption not only cooled the atmosphere but created a mass of cooler water in the oceans that persisted for decades.
"In the case of Krakatoa, we see it anywhere from 40 to 100 years depending on the model you look at," said Peter Gleckler, an atmospheric scientist with Livermore's Program for Climate Model Diagnosis and Intercomparison.
More recent volcanic eruptions are producing shorter oceancooling spells, scientists said, suggesting that global warming may be balancing out the cooling effect faster.
The seawater cooled by Krakatoa's eruption was more dense than the surrounding ocean and sank to depths of at least 500 meters, untouched by warming at the surface. Scientists suggest that waters cooled by Krakatoa and other volcanic eruptions from the 1960s to the early 1990s explain the near doubling in the rate of sea level rise seen in the 1990s.
Volcanic blasts, in other words, eased ocean warming and kept sea level from rising as fast as expected. Climate scientists already knew that oceans and plants absorb most of the carbon dioxide released by human burning of fossil fuels. In keeping greenhouse gases out of the atmosphere, those ecosystems provide a partial buffer against global warming for as long as forests last and currents keep carbon-laden waters deep in the sea.
Volcanos also are known to have a cooling effect. The 1991 eruption of Mount Pinatubo in the Philippines sent 30 million tons of fine ash and aerosols into the air that rounded the planet for three years and reflected sunlight back into space. This shadowy veil cooled the planet by a few degrees Fahrenheit in places for more than a year.
But until recently, scientists didn't fully grasp the long lasting effects that volcanoes have on the ocean.
In November, scientists led by John Church of CSIRO, Australia's national science agency, reported that Pinatubo's shadow of gases and dust took 3 billion trillion joules of heat energy out of the world's oceans and almost immediately dropped global sea level by 5 millimeters.
That's because much of sea level is driven by heat and the resulting expansion of seawater. Chill the ocean, the water contracts and sea level drops. Church and his colleagues found the ocean warmed up again over more than a decade and sea level rebounded.
That partly explained why measurements of sea level rise jumped from an average for most of the century of 1.8 millimeters a year to 3.2 millimeters a year by the late 1990s.
A team of Livermore scientists noticed a similar but dramatically longer lasting effect when they ran computer simulations of climate farther back to 1850, to include the multiple Krakatoa eruptions in the 1880s. The team specializes in assessing the accuracy of the world's dozen or so leading computer models of climate simulations and found half of them don't take volcanoes into account.
"This result just sort of jumped out at us," said Gleckler. "It's a very curious feature of nature."
The ocean-cooling effects of Pinatubo lasted a much shorter time than the cooling from Krakatoa, he said, because greenhouse gases are constantly rising in the atmosphere and trapping more heat to warm the ocean back again.
"These results are suggesting that an eruption such as Pinatubo temporarily slows down the warming but it is only temporary, and in the future we can't rely on eruptions as a way of slowing the warming because we have no idea when or where they're going to occur," Gleckler said.
Gerald Meehl, a veteran climate scientist at the National Center for Atmospheric Research, said researchers are finding that human-induced greenhouse gases are competing with, or overwhelming, many natural facets of climate.
"This is the story of 20th-century climate, which is trying to understand all these competing influences," he said. "It would make sense that you would have a different mix of influences at the latter part of the century than at the beginning of the century when anthropogenic influences are a lot less. We expect it's going to continue."