Researchers gaining a better understanding of stratospheric aerosols
By Bob Berwyn
FRISCO —A series of small to mid-sized volcanic eruptions the past 10 years were the main factor in the formation of stratospheric sulfuric acid that reflected the sun’s energy and partially offset the effects of heat-trapping greenhouse gases.
“This new study indicates it is emissions from small to moderate volcanoes that have been slowing the warming of the planet,” said Ryan Neely, a researcher at the Cooperative Institute for Research in Environmental Science and doctoral student at CU Boulder.
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.
Some studies have implicated increasing sulfur dioxide emissions from industrial sources in China and India, but Neely said those amount to only about 10 percent of the emissions from a single volcano.
“Industrial emissions have very little effect on the stratosphere …
The sulfur dioxide from industrial emissions generally get slowed down in the troposphere, where they’re affected by rain and wind, rarely rising to the realm of the stratospheric aerosol layer, some 12 to 20 miles high.
In that high layer, chemical reactions create sulfuric acid and water particles that reflect sunlight back to space, cooling the planet.
“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 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,” he said.
Sulfur dioxide emissions from industrial sources don’t climb high into the stratosphere nearly as fast as the sulfur dioxide from some volcanoes, whose eruptions can have a quick and measurable effect on the overall reflectivity of the upper atmosphere, Neely explained.
“It matters how you put it up there,” he said, adding that there’s also a difference between various types of eruptions. Some volcanoes spew mainly rocks and dust, which sinks back to Earth relatively quickly without ever reaching the upper levels of the atmosphere.
The 10-year study period will help provide good baseline data, Neely said. By starting at a point when there was a worldwide lull in volcanic activity, the researchers were able to accurately measure the effects of subsequent eruptions. Neely said he plans to study the effects of volcanic eruptions in previous decades with the same climate model, using known historic data about eruptions.
Neely said the research project was partly aimed at resolving conflicts between the results of previous studies on the origins of the sulfur dioxide in the stratosphere.
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 to 7 percent, meaning it is slightly more opaque now than in previous years.
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 added. 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 was published online in Geophysical Research Letters, a publication of the American Geophysical Union. Co-authors include Professors Brian Toon and Jeffrey Thayer from CU-Boulder; Susan Solomon, a former NOAA scientist now at the Massachusetts Institute of Technology; Jean Paul Vernier from NASA’s Langley Research Center in Hampton, Va.; Catherine Alvarez, Karen Rosenlof and John Daniel from NOAA; and Jason English, Michael Mills and Charles Bardeen from the National Center for Atmospheric Research in Boulder.
The research was funded in part through a NOAA/ ESRL-CIRES Graduate Fellowship to Neely. The National Science Foundation 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.