Climate Change/Global warming potential

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 convention equal to 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.

The substances subject to restrictions in the Kyoto protocol either are rapidly increasing their concentrations in Earth's atmosphere or have a large GWP.

The GWP depends on the following factors:

• the absorption of infrared radiation by a given species
• the spectral location of its absorbing wavelengths
• the atmospheric lifetime of the species

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.

Calculating the global warming potential

Just as radiative forcing provides a simplified means of comparing the various factors that are believed to influence the climate system to one another, Global Warming Potentials (GWPs) are one type of simplified index based upon radiative properties that can be used to estimate the potential future impacts of emissions of different gases upon the climate system in a relative sense. GWP is based on a number of factors, including the radiative efficiency (infrared-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 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:

$RF=\sum _{n=1}^{100}Abs_{i}*F_{i}/(pathlength*density)$

where the subscript i represents an interval of 10 inverse centimeters. Abs_i represents the integrated infrared absorbance of the sample in that interval, and F_i 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:

$GWP\left(x\right)={\frac {\int _{0}^{TH}a_{x}\cdot \left[x(t)\right]dt}{\int _{0}^{TH}a_{r}\cdot \left[r(t)\right]dt}}$

where TH is the time horizon over which the calculation is considered; a_x 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 a_x and a_r 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.

Use 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 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).

Values

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 p212 [3] (2001 IPCC TAR [4] in parentheses)	Lifetime (years)	GWP time horizon
		20 years	100 years	500 years
Methane	12         (12)	72         (62)	25         (23)	7.6       (7)
Nitrous oxide	114       (114)	289       (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)

Although water vapour]] has a significant influence with regard to absorbing infrared radiation (which is the green house effect; see greenhouse gas), its GWP is not calculated. Its concentration in the atmosphere mainly depends on air temperature. There is no possibility to directly influence atmospheric water vapour concentration.