Science: A Focus on Methane and Black Carbon Could Hasten Control of Global Warming

Reducing methane and black carbon emissions along with carbon dioxide could help curb global warming more quickly than a carbon dioxide-only strategy could, according to a new modeling study.

The approach also could prevent millions of deaths annually caused by outdoor air pollution and improve some crop yields, providing economic benefits well above the cost of implementing these pollution-control measures, the researchers say. Their study appears in the 13 January issue of Science.

Reducing atmospheric carbon dioxide levels remains crucial to slowing human-induced climate change, but it takes decades for carbon dioxide to leave the atmosphere. Seeking shorter-term results, Drew Shindell of Columbia University and colleagues focused on methane and black carbon, which contribute to both air pollution and climate warming.

Shindell told the Science Podcast that he and his colleagues specifically chose those two factors because they play these dual roles. They asked: “Are there ways where you could reduce those and get a substantial difference in both, which would make it more palatable to deal with those problems?” Shindell said.

Methane combines with carbon monoxide to make tropospheric ozone, which resides in the lower level of the atmosphere and is different from the protective ozone layer higher in the stratosphere. Black carbon is essentially soot. Both tropospheric ozone and black carbon exit the atmosphere much more quickly than carbon dioxide does.

The authors began with roughly 2000 possible pollution-control measures and used a computer model to select those that were most effective at both mitigating warming and improving air quality. They identified 14 measures, including intermittently aerating flooded rice paddies to impede methane production by underwater bacteria and using diesel particle filters in vehicles to reduce soot emissions. Seven of the measures targeted methane emissions; the others targeted black carbon.

According to further modeling results, implementing these 14 measures could reduce projected global mean warming by about 0.5 degrees Celsius by 2050. This strategy also avoids 0.7 to 4.7 million annual premature deaths from outdoor air pollution, and increases annual crop yields by 30 to 135 million metric tons due to ozone reductions in 2030 and beyond.

Implementing these measures along with substantial carbon dioxide-reduction measures should limit global mean warming by less than 2 degrees Celsius during the next 60 years, something that neither set of emissions reductions achieves on its own, the authors say in their study.

Not all the measures had equal effects around the globe. In the United States or Europe, for example, the regulations are already fairly strict for certain sources of methane or black carbon, though emissions from landfills and livestock are still a problem. But in the developing world, changing vehicle standards and using alternatives to traditional cookstoves would have more of an impact.

“It really depends on where you’re looking in the world,” Shindell told the Science Podcast.

Putting these measures into place makes financial sense too, the researchers found.

“When you factor in also the avoided damages from climate and the avoided damages to human health, the cost-effectiveness of these measures tends to be extremely large,” Shindell said. “In most cases, the benefits from these measures actually outweigh the implementation costs.”

Kathy Wren

12 January 2012