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 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.

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.

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

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).

For converting the various greenhouse gas emissions into comparable CO₂ equivalents under the Kyoto protocol, the Conference of the Parties decided decision 2/CP.3 that the older GWP values from the IPCC Second Assessment Report are to be used to compute overall emissions by sources or removals by 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

GWP values for a 100 year horizon from IPCC TAR [2]:

• 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 23.
• nitrous oxide has a GWP of 296
• the hydrofluorocarbon HFC-23 has a GWP of 12,000
• sulfur hexafluoride (SF₆), used as high voltage insulator, has the highest charted GWP of 22,200.

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
• IPCC 2001 Third Assessment Report page on Global Warming Potentials