Everything about Global Warming Potential totally explained
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.
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. 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 (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
(External Link
).
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:
»
all divided by the same calculation for a reference compound, normally
CO2
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 (for example, 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 (for example CO
2). 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 (for example, CO
2, CH
4, and N
2O). 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 CO
2 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)
(External Link) 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 shouldn't be considered exact. For this reason when quoting a GWP it's 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.
Generally, it's by regulators (for example
CARB) the time horizon of 100 years.
Values
Carbon dioxide has a GWP of exactly 1 (since it's the baseline unit to which all other greenhouse gases are compared).
A GWP isn't usually calculated for
water vapor. 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's no possibility to directly influence atmospheric water vapour concentration, the GWP-level for water vapour isn't calculated; see
greenhouse gas.
Further Information
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