Reducing Methane Not As Beneficial as We Thought?
Recently, attention has been given to the idea that reducing emissions of methane and black carbon (the latter is commonly found in soot) will help prevent global temperatures from rising. In 2011, the United Nations Environmental Program (UNEP) reported that curbing such emissions will keep the Earth cooler by 0.5 degrees Celsius by 2050.
This makes sense on face value. Methane and black carbon both trap significantly more heat than carbon dioxide. However, there are other factors to consider. For example, methane only stays in the atmosphere for a decade, while black carbon only stays there for a few weeks. Carbon dioxide is a much different beast. Though 65-80% of it dissolves into the ocean after around 5 to 200 years, the remaining amount can persist for thousands of years as it's gradually dissipated by slower earthly processes. Moreover, the carbon dioxide sequestered in the oceans can be re-released over time.*
With these facts in mind, a new study from researchers at the Joint Global Change Research Institute in College Park, Maryland has found that reducing methane and black carbon emissions would only have a modest effect on global warming. Even if methane emissions were reduced by the maximally feasible extent by 2030, all residential coal and wood use was phased out by 2035, and strict particulate controls on all light cars, light trucks, and heavy trucks were enforced in all regions by 2035, global mean temperatures would only likely be cooler by 0.16 degrees Celsius by 2050. That's hardly anything to cheer about.
In addition to disagreeing with the UNEP findings, the current study also contradicts research published last year in Science, which also found that curbing black carbon and methane could reduce temperatures by 0.5 degrees Celsius by 2050. The key differences in the estimates, as with any future prediction, lies with the assumptions made. The current study assumed that economically-attractive methane reductions will happen anyway, so this may affect the temperature reduction.
According to the authors, the key takeaway is not that we shouldn't try to reduce methane or black carbon, but that "reductions in greenhouse gas emissions, both long- and short- lived, need to remain the central focus of any climate-mitigation policy that aims to stabilize the climate system."
The results were published in the Proceedings of the National Academy of Sciences.
Source: Steven J. Smith and Andrew Mizrahi. Near-term climate mitigation by short-lived forcers. PNAS. 2013
*Corrected 8/15 to clarify statement, "Carbon dioxide can last for more than 1,000 years."