It’s all about me (thane)!

Well, it’s not really all about me. But methane has figured strongly in a couple of stories recently and gets an apparently-larger-than-before shout-out in Al Gore’s new book as well. Since a part of the recent discussion is based on a paper I co-authored in Science, it is probably incumbent on me to provide a little context.

First off, these latest results are being strongly misrepresented in certain quarters. It should be obvious, but still bears emphasizing, that redistributing the historic forcings between various short-lived species and CH4 is mainly an accounting exercise and doesn’t impact the absolute effect attributed to CO2 (except for a tiny impact of fossil-derived CH4 on the fossil-derived CO2). The headlines that stated that our work shows a bigger role for CH4 should have made it clear that this is at the expense of other short-lived species, not CO2. Indeed, the attribution of historical forcings to CO2 that we made back in 2006 is basically the same as it is now.

As is well known, methane (CH4) is the greenhouse gas whose anthropogenic increase comes second only to CO2 in its 20th Century effect on climate. It is often stated that methane is ‘roughly 20 times more powerful’ as a greenhouse gas than CO2 and this can refer to one of two (very different) metrics. If you calculate the instantaneous forcing for an equivalent amount of CO2 and CH4 (i.e. for a 1 ppmv increase in both), you find that the global forcing for CH4 is about 23-24 times as large (depending slightly on the background assumed). Separately, if you look up the Global Warming Potential (GWP) of CH4 in IPCC AR4 (the integrated forcing of a kg of CH4 compared to kg of CO2 over a 100 year period), you get a value of about 25. GWP is used to compare the effects of emissions today on climate in the future. The numbers are only coincidentally similar since the GWP incorporates both the weight ratio and the ratio of effective lifetimes in the atmosphere which roughly cancel for a 100 year time-horizon.

In the Second Assessment report (1995), the GWP for methane was 21, and it was increased in AR4 because of a greater appreciation for the indirect effects of methane on atmospheric chemistry, and in particular its role as a tropospheric ozone precursor (since increasing methane leads to an increase in low level ozone). There is also an indirect effect on stratospheric water vapour where methane oxidation is a significant source of water in an otherwise very dry region. Both tropospheric ozone and stratospheric water vapour are effective greenhouse gases so including these indirect effects made the net effect of methane greater.

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