From Graham Faichney
In Fred Pearce’s article on controlling methane emissions, Euan Nisbet is quoted as criticising the hundred-year rule used for calculating the atmospheric warming effect of methane (16 February, p 6). This time frame is used because it is intended to account for the indirect as well as the direct contributions of methane. Although methane has a turnover time of about ten years in the atmosphere, the CO2 released as it is broken down has a turnover nearer to a hundred years.
However, as noted in the Intergovernmental Panel on Climate Change’s Supplementary Report (1992), the indirect effect of the CO2 released applies only to fossil methane emissions. The use of the same global warming potential (GWP) to convert all methane to its CO2 equivalent gives too high a value because methane from biosphere sources has a GWP of 11 whereas fossil methane has a GWP of 21. As biosphere methane emissions account for 70 to 80 per cent of anthropogenic methane emissions, the current calculations overemphasise rather than underemphasise its contribution to global warming.
Methane emissions from biosphere processes form part of the contemporary carbon cycle and do not add new carbon to the atmosphere. Surely the emphasis must be on reducing all emissions that feed new carbon into the atmosphere.
Baulkham Hills New South Wales
