Sulphur dioxide emissions from moderate volcanoes around the world can mask some of the effects of global warming by 25 per cent, a new study has found.

A team led by the University of Colorado Boulder looking for clues about why Earth did not warm as much as scientists expected between 2000 and 2010 now thinks the culprits are hiding in plain sight.

The study results essentially exonerate India and China, two countries that are estimated to have increased their industrial sulphur dioxide emissions by about 60 per cent from 2000 to 2010 through coal burning, said lead study author Ryan Neely.

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

Neely said previous observations suggest that increases in stratospheric aerosols since 2000 have counter balanced as much as 25 per cent 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 in the study published in journal Geophysical Research Letters.

The new project was undertaken in part to resolve conflicting results of two recent studies on the origins of the sulphur dioxide in the stratosphere, including a 2009 study led by the late David Hoffman of NOAA indicating aerosol increases in the stratosphere may have come from rising emissions of sulphur dioxide from India and China.

In contrast, a 2011 study led by Vernier – who also provided essential observation data for the new GRL study – showed moderate volcanic eruptions play a role in increasing particulates in the stratosphere, Neely said in a statement.

“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,” said Professors Brian Toon from CU-Boulder’s Department of Atmospheric and Oceanic Sciences.

“But 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,” Toon said.

“While small and moderate volcanoes mask some of the human-caused warming of the planet, larger volcanoes can have a much bigger effect”, said Toon.

However, the scientists said 10-year climate data sets like the one gathered for the new study are not long enough to determine climate change trends.