Global warming over the last ten years or so would have been more extreme if it weren't for volcanoes, says a team from the University of Colorado, Boulder.
Sulfur dioxide from the earth's surface eventually rises 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.
With sulfur dioxide levels rising, previous researchers have pointed to Asia's increasing use of coal as the cause. India and China are estimated to have increased their industrial sulfur dioxide emissions by about 60 percent from 2000 to 2010 as a result.
CU-Boulder's Ryan Neely says 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," he says.
The new study relies on long-term measurements of changes in the stratospheric aerosol layer's 'optical depth' - a measure of transparency. Since 2000, the optical depth in the stratospheric aerosol layer has increased by about four to seven percent, meaning it is slightly more opaque now than in previous years.
The team used two sophisticated computer models: the Whole Atmosphere Community Climate Model, or WACCM, Version 3, developed by NCAR, and the Community Aerosol and Radiation Model for Atmosphere, or CARMA. This allows researchers to calculate properties of specific aerosols.
The team used the Janus supercomputer on campus to conduct seven computer runs, each simulating 10 years of atmospheric activity tied to both coal-burning activities in Asia and to emissions by volcanoes around the world.
"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," says professor Brian Toon.
"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."