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 by definition 1). A GWP is calculated over a specific time interval and the value of this must be stated whenever a GWP is quoted or else the value is meaningless.
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Calculation of GWP
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. An exact definition of how GWP is calculated is to be found in the IPCC's 2001 Third Assessment Report.
where: TH is the time horizon over which the calculation is considered; ax is the radiative efficiency due to a unit increase in atmospheric abundance of the substance (i.e., Wm-2 kg-1) and [x(t)] is the time-dependent decay in abundance of the substance following an instantaneous release of it at time t=0. The denominator contains the corresponding quantities for the reference gas (i.e. CO2).
GWP used in Kyoto protocol
Under the Kyoto protocol, the Conference of the Parties decided (decision 2/CP.3) [2] that the values of GWP calculated for the IPCC Second Assessment Report are to be used for converting the various greenhouse gas emissions into comparable CO2 equivalents when computing overall sources and sinks.
Importance of time horizon
Note that a substance's GWP depends on the timespan over which the potential is calculated. A gas which is quickly removed from the atmosphere may initially have a large effect but for longer time periods as it has been removed becomes less important. 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 (IPCC TAR). The GWP value depends on how the gas concentration decays over time in the atmosphere. This is often not precisely known and hence the values should not be considered exact. For this reason when quoted a GWP it is important to give a reference to the calculation.
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.
GWP Values
Carbon dioxide has a GWP of exactly 1 (since it is the baseline unit to which all other greenhouse gases are compared.)
| Gas | Lifetime (years) | GWP Time horizon | ||
|---|---|---|---|---|
| 20 years | 100 years | 500 year | ||
| Methane | 12 | 62 | 23 | 7 |
| Nitrous oxide | 114 | 275 | 296 | 156 |
| HFC-23 (hydrofluorocarbon) | 260 | 9400 | 12000 | 10000 |
| sulfur hexafluoride | 3200 | 15100 | 22200 | 32400 |
A GWP is not usually calculated for Water vapour, largely because it is not relevant; see greenhouse gas.
External links
- List of Global Warming Potentials and Atmospheric Lifetimes from the U.S. EPA
- Greenhouse Gases and Global Warming Potential Values, Excerpt from the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2000 from the U.S. EPA
- IPCC 2001 Third Assessment Report page on Global Warming Potentials
- An overview of the role of H2O as a greenhouse gas