Global warming potential
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Global warming potential (GWP) is a measure of how much a given mass of greenhouse gas is estimated to contribute to global warming. It is a relative scale which compares the gas in question to that of the same mass of carbon dioxide whose GWP is one.
GWP is based on a number of factors, including the radiative efficiency (heat-absorbing ability) of each gas relative to that of carbon dioxide, as well as the decay rate of each gas (the amount removed from the atmosphere over a given number of years) relative to that of carbon dioxide [1]. The Intergovernmental Panel on Climate Change (IPCC) provides the generally accepted values for GWP, which changed slightly between 1996 and 2001 (eg methane was assessed a value of 21 in 1996). An exact definition of how GWP is calculated is to be found in the IPCC's 2001 Third Assessment Report.
Note that a substances GWP depends also on the timespan over which the potential is calculated. Thus methane has a potential of 23 over 100 years but 62 over 20 years; conversely sulfur hexafluoride has a GWP of 22,000 over 100 years but 15,100 over 20 years.
The GWP for a mixture of gases can not be determined from the GWP of the consituent gases by any form of simple linear addition.
Examples:
100 year horizons:
- carbon dioxide has a GWP of exactly 1 (since it is the baseline unit to which all other greenhouse gases are compared.)
- methane has a GWP of 21.
- nitrous oxide has a GWP of 296
- the hydrofluorocarbon HFC-23 has a GWP of 12,000
- sulfur hexafluoride (SF6) has the highest charted GWP of 22,200, used as high voltage insulator.
- water vapour has a very small GWP.