A study led by researchers at the University of Colorado has determined that the pace of planet warming in the first decade of this century was slowed by volcanic activity, and not by industrial activity in Asia, as was previously believed.
Previous research in 2009 had suggested that an increase in stratospheric aerosols tied to a 60 percent increase in sulfur dioxide emissions over China and India had negated about 25 percent of the global warming that scientists attribute to greenhouse gas emissions.
That cooling effect occurs when sulfur dioxide emissions rise 12 to 20 miles to the stratospheric aerosol layer of the atmosphere. There, chemical reactions create sulfuric acid and water particles that reflect sunlight back into space.
Now, research led by study author Ryan Neely, conducted as part of his doctoral thesis at CU, has shown that global warming from 2000 to 2010 was tamped down by sulfur dioxide from volcanic eruptions, not industrial emissions in China and India, where such activity has greatly increased in recent years.
"It's good to know this is coming from volcanoes; its a natural thing, and it's not something we're doing as a planet," said Neely, who is now a post-doctoral fellow in the advanced study program at the National Center for Atmospheric Research.
The 10-year window addressed by the study did not see massive activity on the scale of Mount Pinatubo in the Philippines, which erupted in 1991 in the second-largest volcanic event of the 20th century. Still, there was sufficient volcanic activity in the 2000s from the tropics to Alaska that made an impact in the stratosphere.
The new research piggybacks on a 2011 study led by Susan Solomon, a former scientist at the National Oceanic and Atmospheric Administration who is now at the Massachusetts Institute of Technology, which showed that stratospheric aerosols -- without isolating their source -- offset about one-quarter of the greenhouse-effect warming of Earth in the past 10 years.
To determine what was contributing to that, Neely said he realized, "You couldn't do it from observations alone. It's all intermingled, and you can't separate the two sources easily. It was going to take a very specialized model that no one has done before."
CU's Janus supercomputer was pressed into service to conduct seven computer runs, each of them simulating 10 years of atmospheric activity linked to both coal-burning activities in Asia and to volcanic emissions around the world.
Each run required about a week of computer time, utilizing 192 processors, enabling the team to isolate coal-burning pollution in Asia from aerosol contributions tied to volcanic eruptions.
Neely said the work would have taken a single computer about 25 years to complete.
The fact that the emissions from industrial activity on the other side of the planet is not affecting temperature fluctuations tied to particulates in the stratosphere, Neely said, does not mean that all those human-caused emissions are good for the environment.
"A lot of people would take it that way," he said, "but it's bad for other reasons -- for acid rain reasons, and just for putting pollution into the atmosphere, and not to mention all the carbon dioxide they're emitting, when they burn all the coal," he said.
In a news release, study co-author and CU professor Brian Toon said, "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.
"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."
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