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August 2013

Soot, Methane Reduction Benefit Smaller Than Thought

Focus on greenhouse gases needed to limit climate change

Car exhaust emitting soot
Reductions in soot and methane emissions are likely to provide only a modest, near-term reductions in global temperatures. A comprehensive climate policy provides a larger impact by 2050. Enlarge Image.

Results: Cutting the amount of short-lived climate-warming emissions—such as soot and methane—in our skies won't limit global warming as much as previous studies have suggested, according to new research by Pacific Northwest National Laboratory. While the climate benefits over the next two decades of reducing these emissions are equivalent to those that would occur if a comprehensive climate policy was implemented, the new study also found that the reductions from a comprehensive climate policy would produce more climate benefits both by mid-century and longer. The research appeared in the August 12 early online edition of Proceedings of the National Academy of Sciences.

Why it Matters: Soot and methane are called short-term climate forcers because they remain in the atmosphere for only weeks to years, unlike the better-known greenhouse gas carbon dioxide that can stick around for 1,000-plus years. Soot, also known as black carbon, is made of fine, carbon-based particles that are given off by car and truck tailpipes and wood stoves. Methane, the main component of natural gas, is released from leaking pipelines, coal mines, oil wells, cattle, rice paddies and landfills.

"Focusing on soot and methane may be worth targeting for health reasons, as previous studies have identified substantial health benefits would likely result from reducing those emissions," said lead author and climate researcher Dr. Steven J. Smith at PNNL. "To stabilize the global climate, however, the focus needs to be on carbon dioxide and other greenhouse gases."

Methods: Using the Global Change Assessment Model (GCAM), an integrated model of human and earth systems developed at PNNL with support from the U.S. Department of Energy (DOE), researchers evaluated the impact of reducing soot and methane emissions on Earth's climate. The model also incorporates greenhouse gases and pollutants that can result from those activities. They created over 1,400 potential scenarios to reflect the many possibilities surrounding the emissions and climate forcing impacts of aerosols, which are tiny particles including soot that float in the atmosphere. The team then examined what the future could look like in an idealized world where soot and methane emissions were severely cut by 2035.

What's Next? Ongoing PNNL research is evaluating how aerosol particles affect the atmosphere, including temperature and precipitation.

For more information, see PNNL news release, "Climate benefit for cutting soot, methane smaller than previous estimates."


Sponsors: This research was funded by U.S. Environmental Protection Agency Climate Change division. The research used the Global Change Assessment Model at PNNL's Joint Global Change Research Institute (JGCRI) also supported by DOE's Integrated Assessment Research Program. JGCRI is a partnership between PNNL and the University of Maryland.

Research Team: Steven J. Smith and Andrew Mizrahi at PNNL.

Research Area: Climate & Earth Systems Science

Reference: Smith SJ, and A Mizrahi. 2013. "Near-Term Climate Mitigation by Short-Lived Forcers." Proceedings of the National Academy of Sciences, August 12, early online edition. DOI: 10.1073/pnas.1308470110.

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