Volcanic eruptions may counter the effects of global warming and help keep the planet cool, according to a new study that explores why the Earth hasn't warmed as much as predicted over the past decade.
Active volcanoes spew sulphur dioxide, which rises up to the stratosphere. Scientists say the sulphur dioxide provides a protective layer that reflects sunlight back into space -- keeping the planet cooler than it would have been otherwise
Previous theories had suggested microscopic pollutants from smokestacks in China and India were responsible for the layer of sulphur dioxide, approximately 20 to 32 kilometres above the Earth. But Ryan Neely, lead author of the study, says industrial pollution had little or no effect on temperatures.
"This new study indicates it is emissions from small to moderate volcanoes that have been slowing the warming of the planet," said Neely, a scientist at the University of Colorado at Boulder.
Between 2000 and 2010, the researchers studied the atmosphere and found the average concentration of carbon dioxide 鈥� the gas that leads to global warming -- rose more than 5 per cent.
That increase, on its own, should have led to a 0.2 C rise in average global temperatures.
But because the layer of aerosol particles was thickening in the atmosphere at the same time, as much as 25 per cent of that increase was counteracted.
Aerosols are measured by their "optical depth," or transparency.
"Since 2000, the optical depth in the stratospheric aerosol layer has increased by about 4 to 7 per cent (annually), meaning it is slightly more opaque now than in previous years," says a release accompanying the study.
Scientists have been aware that aerosols are increasing in the planet's stratosphere, but they haven't agreed on the cause.
Some scientists argued that Asia was the source, largely because coal-fired generators in China and India triggered a 60 per cent increase in sulfur dioxide emissions between 2000 and 2010.
Neely and his colleagues countered this theory by comparing recorded stratospheric aerosol levels with known volcanic eruptions.
They used two sophisticated computer models in their study. One model allowed researchers to study the atmosphere's aerosol layer, while the other allowed them to track the properties of specific aerosol emissions around the planet -- and thereby isolate volcanic emissions from industrial emissions.
The two models were then combined to show the patterns of eruptions and the resulting effect in the atmosphere's aerosol levels.
Brian Toon, a professor at CU-Boulder's Department of Atmospheric and Oceanic Sciences, and a co-author of the paper, says the result clearly pointed to volcanoes as the cause of the lower-than-expected temperature increases.
"The biggest implication here is that scientists need to pay more attention to small and moderate volcanic eruptions when trying to understand changes in Earth's climate," Toon said in a news release.
"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."
The study is published online in the journal Geophysical Research Letters.
