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  PlanetArk 9 Feb 06
Impact of Krakatoa Eruptions Lasted Decades - Study
Story by Patricia Reaney

LONDON - Sea levels would have risen higher and ocean temperatures would have been warmer in the 20th century if the Krakatoa volcano in Indonesia had not erupted in 1883, scientists said on Wednesday.

The impact of the eruption that spewed molten rock and sulphate aerosols into the atmosphere was felt for decades - much longer than previously thought. "It appears as though with a very large eruption the effect can last for many decades and possibly as long as a century," said Peter Gleckler, a climatologist at the Lawrence Livermore National Laboratory in California.

Sea levels rise when ocean temperatures are warmer and recede when they cool. Volcanoes release aerosols and dust that block sunlight and cause the ocean surface to cool which can offset, at least temporarily, sea level rises caused by increased greenhouse gases in the atmosphere.

In recent decades, the average ocean temperature has warmed by about .037 degrees Celsius, according to the scientists. Gleckler and researchers in the United States and Britain were studying models of climate simulations when they noticed the impact of volcanic eruptions. Some of the climate models included the impact of such eruptions while others did not.

"As we looked at the first picture of all these models together, we saw that just at the time of Krakatoa there was this very clean separation of those that included the eruption and those that did not," Gleckler told Reuters. "Volcanoes have a big impact. The ocean warming and sea level would have risen much more if it weren't for volcanoes," said Gleckler, who reported the findings in the journal Nature.

The study also included more recent eruptions including Pinatubo in the Philippines in 1991, which was on a similar scale to Krakatoa. But the effect of Pinatubo on ocean temperatures was much smaller because of the impact of greenhouse gases which were much higher in 1991 than in 1883.

"The Pinatubo eruption influence on sea level and heat content was dampened by this background warming," said Gleckler.

He added that scientists must think more carefully about how they include the effects of volcanic eruptions such as Krakatoa and even earlier ones, in climate modelling. "We can't rely on future volcanic eruptions slowing ocean warming and sea level rises," Gleckler added.

EurekAlert 9 Feb 06
Volcanoes helped slow ocean warming trend, researchers find

LIVERMORE, Calif. — Ocean temperatures might have risen even higher during the last century if it weren't for volcanoes that spewed ashes and aerosols into the upper atmosphere, researchers have found. The eruptions also offset a large percentage of sea level rise caused by human activity.

Using 12 new state-of-the-art climate models, the researchers found that ocean warming and sea level rise in the 20th century were substantially reduced by the 1883 eruption of the Krakatoa volcano in Indonesia. V

olcanic aerosols blocked sunlight and caused the ocean surface to cool. “That cooling penetrated into deeper layers of the ocean, where it remained for decades after the event,” said Peter Gleckler, an atmospheric scientist at Lawrence Livermore National Laboratory (LLNL).

“We found that volcanic effects on sea level can persist for many decades.” Gleckler, along with LLNL colleagues Ben Santer, Karl Taylor and Krishna AchutaRao and collaborators from the National Center for Atmospheric Research, the University of Reading and the Hadley Centre, tested the effects of volcanic eruptions on recent climate models. They examined model simulations of the climate from 1880 to 2000, comparing them with available observations. External “forcings,” such as changes in greenhouse gases, solar irradiance, sulphate and volcanic aerosols, were included in the models.

Oceans expand and contract depending on the ocean temperature. This causes sea level to increase when the water is warmer and to recede in cooler temperatures. The volume average temperature of oceans (down to 300 meters) worldwide has warmed by roughly .037 degrees Celsius in recent decades due to increasing atmospheric greenhouse gases.

While seemingly small, this corresponds to a sea level rise of several centimeters and does not include the effect of other factors such as melting glaciers.

That sea level jump, however, would have been even greater if it weren't for volcanic eruptions over the last century, Gleckler said. “The ocean warming suddenly drops,” he said.

“Volcanoes have a big impact. The ocean warming and sea level would have risen much more if it weren't for volcanoes.” Volcanic aerosols scatter sunlight and cause the ocean surface temperature to cool, an anomaly that is gradually subducted into deeper layers, where it remains for decades.

The experiments studied by Gleckler’s team also included the more recent 1991 Mt. Pinatubo eruption in the Philippines, which was comparable to Krakatoa in terms of its size and intensity.

While similar ocean surface cooling resulted from both eruptions, the heat-content recovery occurred much more quickly in the case of Pinatubo. “The heat content effects of Pinatubo and other eruptions in the late 20th century are offset by the observed warming of the upper ocean, which is primarily due to human influences,” Gleckler said.

The research appears in the Feb. 9 issue of the journal Nature.
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