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What caused Global Warming to Slow Last Decade?

March 1, 2013; 12:06 PM ET

Scientists from the University of Colorado at Boulder have determined that it was volcanic emissions and not pollutants that slowed warming during the 2000 to 2010 period.

Earlier studies put the blame on the increasing amounts of pollutants being released from countries like China and India.

Instead, the researchers found that sulfur dioxide emissions from a large number of small to moderate size volcanoes across the world had a role in slowing the rate of warming last decade.

Key excerpts from the University of Colorado Boulder report.....

Ryan Neely, the lead author of the study, said previous observations suggest that increases in stratospheric aerosols since 2000 have counterbalanced as much as 25 percent of the warming scientists blame on human greenhouse gas emissions. "This new study indicates it is emissions from small to moderate volcanoes that have been slowing the warming of the planet," said Neely, a researcher at the Cooperative Institute for Research in Environmental Sciences, a joint venture of CU-Boulder and the National Oceanic and Atmospheric Administration.

The new study relies on long-term measurements of changes in the stratospheric aerosol layer's "optical depth," which is a measure of transparency, said Neely. Since 2000, the optical depth in the stratospheric aerosol layer has increased by about 4 to 7 percent, meaning it is slightly more opaque now than in previous years.

"...overall these eruptions are not going to counter the greenhouse effect. Emissions of volcanic gases go up and down, helping to cool or heat the planet, while greenhouse gas emissions from human activity just continue to go up," said co-author Brian Toon of CU-Boulder's Department of Atmospheric and Oceanic Sciences.

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