The GWP depends on the following factors:
Thus, a high GWP correlates with a large infrared absorption and a long atmospheric lifetime. The dependence of GWP on the wavelength of absorption is more complicated. Even if a gas absorbs radiation efficiently at a certain wavelength, this may not affect its GWP much if the atmosphere already absorbs most radiation at that wavelength. A gas has the most effect if it absorbs in a "window" of wavelengths where the atmosphere is fairly transparent. The dependence of GWP as a function of wavelength has been found empirically and published as a graph.
Because the GWP of a greenhouse gas depends directly on its infrared spectrum, the use of infrared spectroscopy to study greenhouse gases is centrally important in the effort to understand the impact of human activities on global climate change.
The radiative forcing capacity (RF) is the amount of energy per unit area per unit time, absorbed by the greenhouse gas, that would otherwise be lost to space. It can be expressed by the formula:
where the subscript i represents an interval of 10 inverse centimeters. Absi represents the integrated infrared absorbance of the sample in that interval, and Fi represents the RF for that interval.
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 The GWP is defined as the ratio of the time-integrated radiative forcing from the instantaneous release of 1 kg of a trace substance relative to that of 1 kg of a reference gas:
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). The radiative efficiencies ax and ar are not necessarily constant over time. While the absorption of infrared radiation by many greenhouse gases varies linearly with their abundance, a few important ones display non-linear behaviour for current and likely future abundances (e.g., CO2, CH4, and N2O). For those gases, the relative radiative forcing will depend upon abundance and hence upon the future scenario adopted.
Since all GWP calculations are a comparison to CO2 which is non-linear, all GWP values are affected. Assuming otherwise as is done above will lead to lower GWPs for other gases than a more detailed approach would.
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 25 over 100 years but 72 over 20 years; conversely sulfur hexafluoride has a GWP of 22,800 over 100 years but 16,300 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 quoting 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 constituent gases by any form of simple linear addition.
Commonly, a time horizon of 100 years is used by regulators (e.g., the California Air Resources Board).
Carbon dioxide has a GWP of exactly 1 (since it is the baseline unit to which all other greenhouse gases are compared).
|GWP values and lifetimes from 2007 IPCC AR4 (2001 IPCC TAR in brackets)||Lifetime - years||GWP time horizon|
| || || |
|Methane||12 (12)||72 (62)||25 (23)||7.6 (7)|
|Nitrous oxide||114 (114)||310 (275)||298 (296)||153 (156)|
|HFC-23 (hydrofluorocarbon)||270 (260)||12,000 (9400)||14,800 (12000)||12,200 (10000)|
|HFC-134a (hydrofluorocarbon)||14 (13.8)||3830 (3300)||1430 (1300)||435 (400)|
|sulfur hexafluoride||3200 (3200)||16,300 (15100)||22,800 (22200)||32,600 (32400)|
A GWP is not usually calculated for water vapour. Water vapour has a significant influence with regard to absorbing IR radiation; however its concentration in the atmosphere mainly depends on air temperature. As there is no possibility to directly influence atmospheric water vapour concentration, the GWP-level for water vapour is not calculated; see greenhouse gas.
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