Tropospheric ozone and black carbon (BC) contribute to degraded air quality and global warming. While carbon dioxide is the primary driver of global warming over the long term, limiting black carbon and methane are complementary actions that would have a more immediate impact because these two pollutants circulate out of the atmosphere more quickly.

The team considered about 400 emission control measures to reduce these pollutants by using current technology and experience. In addition to the reduction in warming, the researchers estimated that implementing the 14 measures would avoid 0.7 to 4.7 million annual premature deaths from outdoor air pollution and increase annual crop yields by 30 to 135 million metric tons due to ozone reductions in 2030 and beyond.

Tropospheric ozone and black carbon (BC) are the only two agents known to cause both warming and degraded air quality. Although all emissions of BC or ozone precursors [including methane (CH₄)] degrade air quality, and studies document the climate effects of total anthropogenic BC and tropospheric ozone, published literature is inadequate to address many policy-relevant climate questions regarding these pollutants because emissions of ozone precursors have multiple cooling and warming effects, whereas BC is emitted along with other particles that cause cooling, making the net effects of real-world emissions changes obscure. Such information is needed, however, because multiple stakeholders are interested in mitigating climate change via control of non–carbon dioxide (CO₂)–forcing agents such as BC, including the G8 nations (L’Aquila Summit, 2009) and the Arctic Council (Nuuk Declaration, 2011).

Here, we show that implementing specific practical emissions reductions chosen to maximize climate benefits would have important “win-win” benefits for near-term climate, human health, agriculture, and the cryosphere, with magnitudes that vary strongly across regions.

—Shindell et al.

The team considered the 400 control measures based on technologies evaluated by the International Institute for Applied Systems Analysis in Laxenburg, Austria, and then focused on 14 measures with the greatest climate benefit. All 14 would curb the release of either black carbon or methane.

The model estimated potential worldwide emissions reductions of particulate and gaseous species on the basis of available real-world data on reduction efficiencies of these measures where they have been applied already and examined the impact of full implementation everywhere by 2030. Their potential climate impact was assessed by using published global warming potential (GWP) values for each pollutant affected. All emissions control measures are assumed to improve air quality. We then selected measures that both mitigate warming and improve air quality, ranked by climate impact.

If enhanced air quality had been paramount, the selected measures would be quite different [for example, measures primarily reducing sulfur dioxide (SO₂) emissions improve air quality but may increase warming]. The screening revealed that the top 14 measures realized nearly 90% of the maximum reduction in net GWP. Seven measures target CH₄ emissions, covering coal mining, oil and gas production, long-distance gas transmission, municipal waste and landfills, wastewater, livestock manure, and rice paddies. The others target emissions from incomplete combustion and include technical measures (set “Tech”), covering diesel vehicles, clean-burning biomass stoves, brick kilns, and coke ovens, as well as primarily regulatory measures (set “Reg”), including banning agricultural waste burning, eliminating high-emitting vehicles, and providing modern cooking and heating. We refer to these seven as “BC measures,” although in practice, we consider all co-emitted species.

—Shindell et al.

Methane and BC measures identified as mitigating climate change and improving air quality which have a large emission reduction potential. Shindell et al., Supplementary material. Click to enlarge.

The team concluded that these control measures would provide the greatest protection against global warming to Russia, Tajikistan and Kyrgyzstan—countries with large areas of snow or ice cover. Iran, Pakistan and Jordan would experience the most improvement in agricultural production. Southern Asia and the Sahel region of Africa would see the most beneficial changes to precipitation patterns. The south Asian countries of India, Bangladesh and Nepal would see the biggest reductions in premature deaths.

Resources

• Drew Shindell, Johan C. I. Kuylenstierna, Elisabetta Vignati, Rita van Dingenen, Markus Amann, Zbigniew Klimont, Susan C. Anenberg, Nicholas Muller, Greet Janssens-Maenhout, Frank Raes, Joel Schwartz, Greg Faluvegi, Luca Pozzoli, Kaarle Kupiainen, Lena Höglund-Isaksson, Lisa Emberson, David Streets, V. Ramanathan, Kevin Hicks, N. T. Kim Oanh, George Milly, Martin Williams, Volodymyr Demkine, and David Fowler (2012) Simultaneously Mitigating Near-Term Climate Change and Improving Human Health and Food Security. Science 335 (6065), 183-189. doi: 10.1126/science.1210026