Sir,
We are told that methane has a Global Warming Potential 25 times that of carbon dioxide. Despite my best efforts, I have been unable to discover how this figure was established. Because the concentration of atmospheric CO2 is about 400 parts per million and that of methane is only about 2 parts per million, I wonder why its GWP is 25 times higher.
It is difficult to measure accurately the atmospheric concentration of methane because errors in measurement may be greater than any changes in concentration that have been reported.
Almost all the articles I have seen on the GWP of methane refer to its emission from various sources, especially from ruminant livestock, which can be much more accurately measured than its atmospheric concentration. Are we in danger of being forced to reduce the number of livestock by politicians who are unaware that the 2 thousand parts per billion usually quoted for methane is 200 times smaller than the 400 parts per million of CO2?
Dr. Duncan Pickard, Straiton Farm, Fife
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