Geeky point about global warming potentials
Greenhouse gases have global warming potentials that are described in terms of carbon dioxide equivalents. Methane, for example, has a warming potential of 25. This means that, over a 100 year period, one tonne of methane will warm the earth 25 times more than one tonne of carbon dioxide. Some less common gases have very high warming potentials. Sulphur hexafluoride, for example, has a warming potential of 22,800.
A central part of these factors is the length of time that the gases remain in the atmosphere. Methane, for example, remains in the atmosphere for around 12 years, while carbon dioxide remains in the atmosphere for longer – between 50 and 200 years. If warming potentials are assessed over a 20 year period, methane is 72 (not 25) times more potent than carbon dioxide. Gases with a short life look better when assessed over a long period.
Policy calculations use a time horizon of 100 years.
This is important because the timing of emissions matters. Reducing global warming now is more important than reducing it in the future. The warming potentials, however, assume that all warming caused over the 100 years following release of the gas is of equal value.
A sensible solution might be to use the atmospheric life of each gas as the time horizon, and discount the warming to reflect our preference for reducing warming closer to the present. This discount rate would also be used in the carbon capital calculations recommended in an earlier post.
For warming potentials and atmospheric lifetimes, see p212 of the IPCC’s Climate Change 2007 – The Physical Science Basis (pdf).