Carbon dioxide may be public enemy number one in the fight against global warming. But taking aim at methane and soot has a better chance of keeping the planet cooler in the short run, a new study finds.
Cutting the amounts of these two pollutants that are poured into the sky would diminish warming by half a degree Celsius by 2050, researchers report in the Jan. 13 Science. That could buy a little time for the world — slowing sea level rise, glacial melting and other problems caused by rising temperatures. Targeting these agents of climate change would also improve air quality, potentially preventing up to 4.7 million premature deaths every year, the researchers calculate.
“These are really the low-hanging fruit both for mitigating climate change and improving air quality,” says study leader Drew Shindell, a climate scientist at the NASA Goddard Institute for Space Studies in New York City.
Shindell and his colleagues put 400 known pollution controls to test, selecting 14 interventions that had the greatest impact on warming in a computer simulation. These measures work quickly because methane and soot don’t hang around in the atmosphere for very long — about 12 years for methane and weeks for soot. Cut emissions, and levels of methane and soot drop rapidly. Carbon dioxide, on the other hand, can linger high above for centuries.
Seven of the proposed controls focus on methane, a greenhouse gas that’s about 21 times more potent at warming the atmosphere than carbon dioxide. Implementing interventions already being used in some parts of the world can prevent methane from reaching the atmosphere by filtering it out of air rising from coal mines, livestock manure, landfills and other sources.
The other recommendations target soot. Putting filters on cars that burn diesel gas, for instance, could trap these black flakes of carbon, which absorb sunlight and heat the atmosphere. Soot also darkens snow and glaciers, which hastens melting.
Piers Forster, a climate scientist at the University of Leeds in England, agrees that limiting methane emissions would affect climate. But he cautions that the case for soot may not be entirely clear cut. Some sources of this black carbon — burned agricultural waste and wood- or dung-burning cookstoves in developing countries — also give off other particles that reflect sunlight, cooling the planet.
“If you were to cut out these emissions, you might actually get a warming effect instead of a cooling effect,” says Forster.
Shindell acknowledges the uncertainties. But he says that going after soot and methane isn’t about climate change alone. Reducing soot, Shindell’s team calculates, could save more than a million lives every year in India and China, where the carbon particles damage lungs and lead to respiratory diseases. And methane reacts with other chemicals in the atmosphere to form ozone, which hurts crop production.
Ultimately, the researchers are selling a package of climate, health and economic benefits: Every ton of methane removed yields an estimated $1,100 in benefits that could offset or, in some cases exceed the costs of making the reductions.
In the long run, this package must include reductions in carbon dioxide emissions as well, says Shindell. Going after soot and methane may be a good opening strategy, but carbon dioxide’s longevity makes it the end game.