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. Top: Increasing atmospheric CO2 levels as measured in the atmosphere and ice cores. ... Global mean surface temperatures 1856 to 2005. ... A gas is one of the four major phases of matter (after solid and liquid, and followed by plasma, that subsequently appear as a solid material is subjected to increasingly higher temperatures. ... Carbon dioxide is a chemical compound composed of one carbon and two oxygen atoms. ...

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. 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: IPCC is the science authority for the UNFCCC The Intergovernmental Panel on Climate Change (IPCC) was established in 1988 by two United Nations organizations, the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP), to assess the risk of human-induced climate change, based mainly on peer reviewed...

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). Note that since the ratio is defined in terms of a small perturbation, the issue of linearity of forcing with gas concentration does not arise.

Applications and Limitations of the GWP calculations

In the reference cited below the IPCC defines GWP as "simplified measures to estimate in an approximate fashion the relative effects of emissions of different gases".

Using the above equation to represent the relative importance of different gases is a very simple model since it implicitly assumes that the effects are linear over the range of concentrations concerned, whereas in reality this is not the case.

Beers-Lambert Law tells us that this is essentially a logrithmic relationship and therefore the absorption varies with the concentration of the gas present. As the concentration of the gas rises, less and less of the light remains to be absorbed. Simply put, doubling the gas concentration will no longer double the energy absorbed. This, unfortunatly, is what is naturally assumed to be the case in the absence of other information.

In the case of greenhouse gases, methane is still at relatively low concentrations where such a linear approximation is fairly good, however carbon dioxide is now (2007) at a level which places it well into the non-linear region, at levels at which it will absorb nearly all of the energy of the wavelengths where it is active. Increasing concentrations will have less and less additional effect.

The above equation is only valid for studying the effects of a small variation in gas concentration added to the static concentrations on which the calculations are based. In real situations where the concentrations are expected to change significantly, or for large short-term changes in the levels of gas present, the variation of absorption with concentration must be reagarded as being variable.

The formula is useful for demonstrating the importance of the different atmospheric lifetimes of the gases depending on the length of time being considered. It is design to do that and only that. It should not be regarded as a good indication of absorption effects set against a variable global gas concentration.

In a real situation where the CO₂ concentrations are rising this approximation has the effect of playing down the importance of changes in the other greenhouse gases and underlines the danger of focusing predominantly on carbon dioxide as is frequently the case in the media and public perceptions where CO₂ has become a synonym for the greenhouse effect.

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. Kyoto Protocol Opened for signature December 11, 1997 in Kyoto, Japan Entered into force February 16, 2005. ... The Second Assessment Report (SAR) of the Intergovernmental Panel on Climate Change was published in 1995. ... co2e is Carbon Dioxide Equivalent - an internationally accepted measure that encapsulates all GHGs (Greenhouse Gases) that contribute to Global Warming (e. ...

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. Sulfur hexafluoride is SF6. ...

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.

GWP values

Carbon dioxide has a GWP of exactly 1 (since it is the baseline unit to which all other greenhouse gases are compared.) Carbon dioxide is a chemical compound composed of one carbon and two oxygen atoms. ...

A GWP is not usually calculated for Water vapour, largely because it is not relevant; see greenhouse gas. Methane is a significant and plentiful fuel which is the principal component of natural gas. ... R-phrases S-phrases Supplementary data page Structure and properties n, εr, etc. ... Fluoroform (CHF3; CAS number 75-46-7), also known as trifluoromethane, is one of the haloalkanes with zero ozone depletion, as it does not contain any chlorine. ... CFC molecules CFCs (chlorofluorocarbons) are a family of artificial chemical compounds containing chlorine, fluorine and carbon. ... Sulfur hexafluoride is SF6. ... Impact of a drop of water. ... Top: Increasing atmospheric CO2 levels as measured in the atmosphere and ice cores. ...

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