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Words by Anna Pigott.
NASA scientists have discovered that the effect of methane emissions on climate forcing may be greatly underestimated.
Their research shows that gas-aerosol interactions, such as ozone precursors altering oxidant availability, aerosol formation rates and sulphate-nitrate competition for ammonium, substantially alter the relative importance of various emissions. However, current methods of calculating 100-year global warming potential (GWP) figures - such as those used to inform the strategies of the Kyoto Protocol and the Intergovernmental Panel on Climate Change (IPCC) - are based largely on the direct radiative effects of emissions but do not take into account the (often amplifying) effects of indirect gas-aerosol interactions. When these were taken into account using a Goddard Institute for Space Studies climate model, the scientists found that methane has a 100-year GWP that is ~10% greater than previously thought, rising to ~20-40% greater when aerosol-cloud interactions are included. This equates to one tonne of methane having approximately 33 times as much effect on the climate over 100 years as one tonne of carbon dioxide.
The findings have, says team leader Dr Drew Shindell, quoted in The Times, “added to the importance of measures to contain methane emissions, as well as those of carbon dioxide, which will be discussed at the Copenhagen climate summit in December.” He also stressed that “we undervalue methane. The whole point of having a scale is to relate different gases together, to enlarge the pool of mitigation options. But if you’ve got the wrong value for one, clearly you don’t have maximum efficiency.”
In addition, the study highlights the need for improving knowledge of gas-aerosol interactions in order to better understand past and future climate change, and also suggests that methane’s GWP may change with time as air quality regulations alter the background state of the trophospheric chemistry.
Shindell, D.T., G. Faluvegi, D.M. Koch, G.A. Schmidt, N. Unger, and S.E. Bauer, 2009: Improved attribution of climate forcing to emissions. Science, 326, 716-718, doi:10.1126/science.1174760.
image by Hamed Sabur