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==Importance of time horizon==
==Importance of time horizon==


Note that a substances GWP depends also 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. 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. It is also important to give a reference for the calculation of GWP also.
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. 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.
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==

Revision as of 18:12, 18 January 2006

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 this must included 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.

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).

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. 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

100 year horizons: