Soot is bad stuff all around, whether you're breathing it into your lungs or it's heating the atmosphere by absorbing more of the sun's energy. But a new 4-year, 232-page assessment of soot's role in climate finds that the combustion product could be warming the world twice as much as previously thought. The study points policymakers toward the best targets for reducing climate-warming soot emissions while at the same time improving the health of billions of people.
Neither new observations nor new climate model runs were made for the study, published today in the Journal of Geophysical Research: Atmospheres. Lead author Tami C. Bond of the University of Illinois, Urbana-Champaign, says that while there was talk about reducing soot emissions for a cooler climate and better health, "there wasn't really a deep scientific basis for that." So the International Global Atmospheric Chemistry Project provided an organizational umbrella under which 31 scientists from nine countries could "look at why people were getting different answers" about the effects of soot, Bond says. "It's a deeper view."
By drawing on observations to better understand the behavior of climate models, Bond and her colleagues concluded that atmospheric soot particles 100 nanometers or so in diameter are absorbing enough solar energy to warm the atmosphere with about 1.1 watts per square meter—twice as large a driver of warming as most researchers had estimated. That makes it the second largest humanmade contributor to global warming behind the dominant driver: carbon dioxide. "If we did everything we could to reduce these emissions," said co-author Piers Forster of the University of Leeds in the United Kingdom in a statement, "we could buy ourselves up to half a degree (Celsius) less warming—or a couple of decades of respite."
The added impetus for cleaning up soot emitters is now even clearer, but the assessment also helps regulators who had been uncertain how much good reining in a particular source would do. Diesel engines can spew mostly soot, but coal burning puts out both climate-warming soot and sulfur that goes on to cool the climate by reflecting solar energy back into space. Brushfires emit soot and particles of organic matter that also tend to cool climate, but they do it by making clouds more reflective. Policymakers will be considering many nonscience factors, Bond notes, but the study points to two leading targets for cooling the planet: diesel engines and coal burning in small, poorly operated industries and in homes.
The assessment is receiving a warm welcome. "I was waiting a long time for this to come out," says atmospheric scientist Susanne Bauer of NASA's Goddard Institute for Space Studies in New York City. "It's a wonderful study. It's honest about the reality of [soot] emissions; they do justice to the complexities." Those insights will be included in the first international assessment of climate science since 2007 due out this September.