Global warming potential

Global warming potential (GWP) is a relative measure of how much heat a greenhouse gas traps in the atmosphere. It compares the amount of heat trapped by a certain mass of the gas in question to the amount of heat trapped by a similar mass of carbon dioxide. A GWP is calculated over a specific time interval, commonly 20, 100 or 500 years. GWP is expressed as a factor of carbon dioxide (whose GWP is standardized to 1). In the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, methane has a lifetime of 12.4 years and with climate-carbon feedbacks a global warming potential of 86 over 20 years and 34 over 100 years in response to emissions. User related choices such as the time horizon can greatly affect the numerical values obtained for carbon dioxide equivalents. For a change in time horizon from 20 to 100 years, the GWP for methane decreases by a factor of approximately 3.^[1] The substances subject to restrictions under 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.^[2]

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.^[3]

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:

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:

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. Clarifying this, while increasing CO₂ has less and less effect on radiative absorption as ppm concentrations rise, more powerful greenhouse gases like methane and nitrous oxide have different thermal absorption frequencies to CO₂ that are not filled up (saturated) as much as CO₂, so rising ppms of these gases are far more significant.

Use in Kyoto Protocol

Under the Kyoto Protocol, the Conference of the Parties decided (decision 2/CP.3) 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.^[4]

Global Temperature change Potential (GTP)

The Global Temperature change Potential is another way to quantify the ratio change from a substance relative to that of CO₂, in global mean surface temperature, used for a specific time span.^[5]

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 34 over 100 years but 86 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 be obtained from the mass-fraction-weighted average of the GWPs of the individual gases.^[6]

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).

The values given in the table assume the same mass of compound is released. This must not be confused with chemical reactions in which masses change from reactants to products. For instance, burning methane to carbon dioxide would indeed reduce the global warming impact, but by a smaller factor than the one given in the table because the mass of methane burning is less than the mass of carbon dioxide released (ratio 1:2.75).^[10] If you started with 1 tonne of methane which has a GWP of 25, after combustion you have 2.75 tonnes of CO₂, each tonne of which has a GWP of 1. The effect of this burning is to reduce the Global warming effect of the gas released in the ratio 25:2.75 or by about 9. Similarly, for each tonne of methane burned to CO₂, the CO₂ equivalent is reduced by 25 tonne – 2.75 tonne = 22.25 tonne.

The global warming potential of perfluorotributylamine (PFTBA) over a 100-year time horizon has been estimated to be approximately 7100.^[11] It has been used by the electrical industry since the mid-20th century for electronic testing and as a heat transfer agent.^[12] PFTBA has the highest radiative efficiency (relative effectiveness of greenhouse gases to restrict long-wave radiation from escaping back into space^[13]) of any molecule detected in the atmosphere to date.^[14] The researchers found an average of 0.18 parts per trillion of PFTBA in Toronto air samples, whereas carbon dioxide exists around 400 parts per million.^[15]

Water vapour

Water vapour has a profound infrared absorption spectrum with more and broader absorption bands than CO₂, and also absorbs non-zero amounts of radiation in its low absorbing spectral regions,^[16] (see greenhouse gas (GHG)), its GWP is therefore difficult to calculate. Further, its concentration in the atmosphere depends on air temperature and water availability; using a global average temperature of ~16 °C, for example, creates an average humidity of ~18,000ppm at sea level (CO₂ is ~400ppm^[17] and so concentrations of [H₂O]/[CO₂] ~ 45x). Another issue with calculating GWP is that, unlike other GHG, water vapor does not decay in the environment, so an average over some time period or some other measure consistent with "time dependent decay," q.v., above, must be used in lieu of the time dependent decay of artificial or excess CO₂, molecules. Other factors complicating its calculation are the Earth's temperature distribution, and the differing land masses in the Northern and Southern hemispheres.

See also

• Carbon dioxide equivalent
• Emission factor
• Emission standard
• Radiative forcing
• Total equivalent warming impact

References

External links

• 2007 IPCC Fourth Assessment Report (AR4) by Working Group 1 (WG1) and Chapter 2 of that report (Changes in Atmospheric Constituents and in Radiative Forcing) which contains GWP information.
• 2001 IPCC Third Assessment Report (TAR) page on Global-Warming Potentials and Direct GWP.
• List of Global Warming Potentials and Atmospheric Lifetimes from the U.S. EPA
• An overview of the role of H₂O as a greenhouse gas from RealClimate
• GWP and the different meanings of CO₂e explained