Tiny solid and liquid particles in the atmosphere, including volcanic ash and soot from fossil fuel burning, have kept the Earth from warming as fast as it otherwise would have in the past dozen years, according to a new study published online Thursday in the journal Science.
The findings show that both natural and human factors have slowed the rate of global warming 20 percent since 1998. Small particles, otherwise known as aerosols, help cool the Earth’s climate by blocking out sunlight.
The study is significant because although average global temperatures last decade were higher than in the 1990s and 1980s, it appears the rate of warming has slowed compared with previous decades. Now, scientists say, persistent aerosols in the stratosphere — the region of the atmosphere that contains the ozone layer — might account for why warming has not been as rapid.
John S. Daniel, who co-authored the paper and is a research scientist at the National Oceanic and Atmospheric Administration’s Earth System Research Laboratory in Boulder, Colo., said the analysis shows the impact minor volcanic eruptions and soot from coal burning is “certainly not negligible.”
By looking at both ground-based and satellite data, “you could see without a doubt volcanoes were having an impact” even though there has not been a colossal eruption since Mount Pinatubo erupted in 1991, Daniel said.
The six researchers, from France and the United States, did not determine how much of the cooling effect stemmed from natural causes and how much was from human activities such as sulfur dioxide emissions from power plants and vehicles.
Alan Robock, a professor at Rutgers University’s Department of Environmental Sciences who specializes in analyzing volcanic activity’s climatic impact, said the paper buttresses the argument that the climate change taking place is consistent with computer modeling.
“It makes it clear that our theory is consistent with observations,” he said. “It also means we have to fund satellites to observe the stratosphere.”
Congress cut NOAA’s Joint Polar Satellite System this spring by more than $500 million as part of a broader budget deal, and lawmakers are engaged in a debate right now over how much to fund the program in the future.
Other factors, including sunspot activity, also help account for why the rate of recent warming has not been faster, the study found.
Robock, who studies volcanic activity going back 1,500 years by analyzing ice core data, said that volcanic ash can persist in the stratosphere for “a year or two,” while soot from coal burning lasts in the tropospheric level of the atmosphere for “about a week.”
Humans emit about 70 million tons of sulfur dioxide into the lower atmosphere every year, compared with a major eruption such as Pinatubo, which put 20 million tons of sulfur dioxide into the stratosphere.
As a result, he said, stratospheric aerosol pollution has “a 50 times larger effect, because it lasts so much longer.”