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For decades, climate scientists have worked to identify and measure key substances -- notably greenhouse gases and aerosol particles -- that affect Earth's climate. And they've been aided by ever more sophisticated computer models that make estimating the relative impact of each type of pollutant more reliable.

Yet the complexity of nature -- and the models used to quantify it -- continues to serve up surprises. The most recent? Certain gases that cause warming are so closely linked with the production of aerosols that the emissions of one type of pollutant can indirectly affect the quantity of the other. And for two key gases that cause warming, these so-called "gas-aerosol interactions" can amplify their impact.

"We've known for years that methane and carbon monoxide have a warming effect," said Drew Shindell, a climate scientist at the NASA Goddard Institute for Space Studies (GISS) in New York and lead author of a study published this week in Science. "But our new findings suggest these gases have a significantly more powerful warming impact than previously thought."

When vehicles, factories, landfills, and livestock emit methane and carbon monoxide into the atmosphere, they are doing more than just increasing their atmospheric concentrations. The release of these gases also have indirect effects on a variety of other atmospheric constituents, including reducing the production of particles called aerosols that can influence both the climate and the air quality. These two gases, as well as others, are part of a complicated cascade of chemical reactions that features competition with aerosols for highly reactive molecules that cleanse the air of pollutants.

Aerosols can have either a warming or cooling effect, depending on their composition, but the two aerosol types that Shindell modeled -- sulfates and nitrates -- scatter incoming light and affect clouds in ways that cool
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