Vanillin is produced during forest fires, while sulfur is produced by acid rain from coal burning and other industrial activities.
The analysis, which appears online this week in the journal Science, was so precise that the scientists were able to distinguish not only the annual amount of soot falling on the snow from each source, but also season-to-season pollution.
The study is "blow-you-away amazing," said Richard Alley, a climate researcher from Pennsylvania State University who was not involved in the study.
"They pull up this core of ice, melt a little plug through the middle, pull out the meltwater, and say, This is the summer in 1874, or That is the winter of 1972," he said. "And they're right."
Snow's ability to reflect sunlight is almost uniquely sensitive to soot, Alley added.
"Fresh snow reflects more than nine-tenths of sunlight, so even soaking up a little more makes a difference."
Not surprisingly, McConnell's team found that prior to 1850 most of the soot was from summer forest fires.
But in the late-19th and early-20th centuries, much larger quantities of soot spewed into the air from winter coal burning.
When the United States switched to oil and gas and instituted air pollution controls, the amount of soot falling in the Arctic dropped substantially.
Still a Factor?
But that doesn't mean soot isn't still a factor in Arctic warming. The booming economies of Asia, McConnell noted, are burning a lot of coal.
(Related: "Asia Pollution Changing World's Weather, Scientists Say" [March 6, 2007].)
And while not much of that soot falls on the Arctic glaciers of Greenland, it might be having an impact on other regions, such as Alaska.
Overall, Alley of Penn State noted, soot only produces short-term warming. This year's sooty snow either melts and disappears, or gets buried by next year's snow.
"If you look out 100 or 200 years—in a business-as-usual world, if we just keep doing what we're doing—carbon dioxide comes to dominate everything," he said. (Read about the high costs of coal.)
But technology can cut down worldwide emissions and "buy some time," he said.
Lead author McConnell agrees: "There is technology to keep [soot] emissions quite low," he said. "The question is, who's using it?"
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