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The reason why Earth did not warm as much as expected between 2000 and 2010 could be down to dozens of volcanoes spewing sulfur dioxide, U.S. scientist say.

A team led by the University of Colorado Boulder said emissions from moderate volcanoes around the world might have masked some of the effects of global warming.

Sulfur dioxide emissions from Earth's surface eventually rise 12 to 20 miles into the stratospheric aerosol layer of the atmosphere, where chemical reactions create sulfuric acid and water particles that reflect sunlight back to space, cooling the planet, the researchers said.

Scientists have been blaming increases in stratospheric aerosols since 2000 on human greenhouse gas emissions, but volcanoes may have been responsible for as much as 25 percent of it, they said.

"This new study indicates it is emissions from small to moderate volcanoes that have been slowing the warming of the planet," CU-Boulder doctoral candidate Ryan Neely said.

The study suggests scientists need to pay more attention to volcanoes when trying to understand changes in Earth's climate, atmospheric and oceanic sciences Professor Brian Toon said.

"But overall these eruptions are not going to counter the greenhouse effect," he said. "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."