Volcanic aerosols, not pollutants, tamped down recent Earth warming

1 March 2013

Joint Release

WASHINGTON—Dozens of sulfur-dioxide-spewing volcanoes could be the reason that Earth warmed less than scientists expected between 2000 and 2010, a new study has found. The research indicates also that industrial sulfur dioxide emissions from India and China, which were suspected of tempering the warming, did not play a significant role, said lead study author Ryan Neely, who led the research as part of his University of Colorado Boulder doctoral thesis.

pr_2013-07-hi-res

A new study led by the University of Colorado Boulder indicates emissions from moderate volcanoes around the world like the Augustine Volcano in Alaska, shown here, can mask some of the effects of global warming. (Credit: U.S. Geological Survey

Small amounts of sulfur dioxide emissions from Earth’s surface eventually rise to 19 to 32 kilometers (12 to 20 miles) into the stratosphere, where chemical reactions create a mist, or aerosol, of sulfuric acid droplets and water droplets that reflects sunlight back to space, cooling the planet.

Neely 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 and the National Oceanic and Atmospheric Administration.

A paper on the subject has been accepted for publication in Geophysical Research Letters, a publication of the American Geophysical Union.

The new project was undertaken in part to resolve conflicting results of two recent studies on the origins of the sulfur dioxide in the stratosphere, including a 2009 study led by the late David Hoffman of NOAA, which indicated aerosol increases in the stratosphere may have come from rising emissions of sulfur dioxide from India and China. In contrast, a 2011 study led by Jean Paul Vernier of NASA’s Langley Research Center in Hampton, Va. — 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.

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

“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 Brian Toon of CU-Boulder’s Atmospheric and Oceanic Sciences Department, a co-author of the new study. “But overall theses 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.”

The key to the new results was the combined use of two sophisticated computer models, including the Whole Atmosphere Community Climate Model, or WACCM, Version 3, developed by the National Center for Atmospheric Research in Boulder and is widely used around the world by scientists to study the atmosphere.  The team coupled WACCM with a second model, the Community Aerosol and Radiation Model for Atmosphere, or CARMA, which allows researchers to calculate properties of specific aerosols and which has been under development by a team led by Toon for the past several decades.

Neely said 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. Each run took about a week of computer time using 192 processors, allowing the team to separate coal-burning pollution in Asia from aerosol contributions from moderate, global volcanic eruptions. The project would have taken a single computer processor roughly 25 years to complete, said Neely.

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. “This paper addresses a question of immediate relevance to our understanding of the human impact on climate,” said Neely. “It should interest those examining the sources of decadal climate variability, the global impact of local pollution and the role of volcanoes.”

While small and moderate volcanoes mask some of the human-caused warming of the planet, larger volcanoes can have a much bigger effect, Toon said. When Mount Pinatubo in the Philippines erupted in 1991, it emitted millions of tons of sulfur dioxide into the atmosphere that cooled the Earth slightly for the next several years.

The research for the new study was funded in part through a NOAA/ ESRL-CIRES Graduate Fellowship to Neely. The NSF and NASA also provided funding for the research project.  The Janus supercomputer is supported by NSF and CU-Boulder and is a joint effort of CU-Boulder, CU-Denver and NCAR.

Joint Release

AGU Contact:
Kate Ramsayer, +1 (202) 777-7524, kramsayer@agu.org

CU-Boulder Contact:
Jim Scott, +1 (303) 492-3114, Jim.Scott@colorado.edu


Journalists and public information officers (PIOs) of educational and scientific institutions who have registered with AGU can download a PDF copy of this accepted article by clicking on this link: http://onlinelibrary.wiley.com/doi/10.1002/grl.50263/abstract

Or, you may order a copy of the final paper by emailing your request to Kate Ramsayer at kramsayer@agu.org. Please provide your name, the name of your publication, and your phone number.

Neither the paper nor this press release are under embargo

Title

“Recent anthropogenic increases in SO2 from Asia have minimal impact on stratospheric aerosol”

R. R. Neely III
Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, Colorado, USA; NOAA, Earth System Research Laboratory, Boulder, Colorado, USA; and Cooperative Institute for Research in Environmental Sciences, Boulder, Colorado, USA;
O. B. Toon
Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, Colorado, USA; and Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado, USA;
S. Solomon
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA;
J. P. Vernier
Science Systems and Applications, Inc., Hampton, Virginia, USA; and NASA, Langley Research Center, Hampton, Virginia, USA;
C. Alvarez
NOAA, Earth System Research Laboratory, Boulder, Colorado, USA; and Cooperative Institute for Research in Environmental Sciences, Boulder, Colorado, USA;
J. M. English, M. J. Mills, and C.G. Bardeen
Earth System Laboratory, National Center for Atmospheric Research, Boulder, Colorado, USA;
K. H. Rosenlof and J. S. Daniel
NOAA, Earth System Research Laboratory, Boulder, Colorado, USA;
J. P. Thayer
Department of Aerospace Engineering Sciences, University of Colorado, Boulder, Colorado, USA.

Ryan Neely: Phone: +1 (336) 302-4244, Email: Ryan.Neely@colorado.eduBrian Toon: Phone: +1 (303) 492-1534, Email:Brian.Toon@colorado.edu