Global warming potential


Global warming potential is the heat absorbed by any greenhouse gas in the atmosphere, as a multiple of the heat that would be absorbed by the same mass of carbon dioxide. GWP is 1 for CO2. For other gases it depends on the gas and the time frame. Some gases, like methane, have large GWP, since a ton of methane absorbs much more heat than a ton of CO2. Some gases, again like methane, break down over time, and their heat absorption, or GWP, over the next 20 years is a bigger multiple of CO2 than their heat absorption will be over 100 or 500 years. Values of GWP are estimated and updated for each time frame as methods improve.
Carbon dioxide equivalent is calculated from GWP. It can be measured in weight or concentration. For any amount of any gas, it is the amount of CO2 which would warm the earth as much as that amount of that gas. Thus it provides a common scale for measuring the climate effects of different gases. It is calculated as GWP times amount of the other gas. For example if a gas has GWP of 100, two tons of the gas have CO2e of 200 tons, and 1 part per million of the gas in the atmosphere has CO2e of 100 parts per million.
Estimates of GWP values over 20 and 100 years are periodically compiled in reports from the Intergovernmental Panel on Climate Change.

Values

has a GWP of exactly 1. Most greenhouse gases have GWP more than, therefore more than 1. A few have values below 1, meaning they do warm the earth, but not as much as an equal amount of would. Values have been estimated on:
The values given in the table assume the same mass of compound is analyzed; different ratios will result from the conversion of one substance to another. For instance, burning methane to carbon dioxide would reduce the global warming impact, but by a smaller factor than 25:1 because the mass of methane burned is less than the mass of carbon dioxide released. If you started with 1 tonne of methane which has a GWP of 25, after combustion you would have 2.74 tonnes of CO2, each tonne of which has a GWP of 1. This is a net reduction of 22.26 tonnes of GWP, reducing the global warming effect by a ratio of 25:2.74.
The global warming potential of perfluorotributylamine over a 100-year time horizon has been estimated to be approximately 7100. It has been used by the electrical industry since the mid-20th century for electronic testing and as a heat transfer agent. PFTBA has the highest radiative efficiency of any molecule detected in the atmosphere to date. 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.

Use in Kyoto Protocol and UNFCCC

Under the Kyoto Protocol, in 1997 the Conference of the Parties standardized international reporting, by deciding that the values of GWP calculated for the IPCC Second Assessment Report were to be used for converting the various greenhouse gas emissions into comparable CO2 equivalents.
After some intermediate updates, in 2013 this standard was updated by the Warsaw meeting of the UN Framework Convention on Climate Change to require using a new set of 100-year GWP values. They published these values in Annex III, and they took them from the 4th Assessment Report of the Intergovernmental Panel on Climate Change, which had been published in 2007.
Those standards are still in effect as of 2020.

Importance of time horizon

A substance's GWP depends on the number of years 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, it 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. 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.
Commonly, a time horizon of 100 years is used by regulators.

Water vapour

is one of the primary greenhouse gases, but some issues prevent its GWP to be calculated directly. It 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. Next, 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. Unlike other GHG, water vapor does not decay in the environment, so an average over some time horizon 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 issues complicating its calculation are the Earth's temperature distribution, and the differing land masses in the Northern and Southern hemispheres.

Other metrics: Global Temperature change Potential (GTP)

The Global Temperature change Potential is another way to compare gases. While GWP estimates heat absorbed, GTP estimates the resulting rise in average surface temperature of the world, over the next 20, 50 or 100 years, caused by a greenhouse gas, relative to the temperature rise which the same mass of CO2 would cause.
Calculation of GTP requires modeling how the world, especially the oceans, will absorb heat.
GTP is published in the same IPCC tables with GWP.

Calculating the global warming potential

The GWP depends on the following factors:
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.
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.
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 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 of each gas relative to that of carbon dioxide, as well as the decay rate of each gas relative to that of carbon dioxide.
The radiative forcing capacity 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. Absi represents the integrated infrared absorbance of the sample in that interval, and Fi represents the RF for that interval.
The Intergovernmental Panel on Climate Change 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; ax is the radiative efficiency due to a unit increase in atmospheric abundance of the substance and 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. The radiative efficiencies ax and ar 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 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 as much as CO2, so rising ppms of these gases are far more significant.

Carbon dioxide equivalent

Carbon dioxide equivalent is calculated from GWP. It can be measured in weight or concentration. For any amount of any gas, it is the amount of CO2 which would warm the earth as much as that amount of that gas. Thus it provides a common scale for measuring the climate effects of different gases. It is calculated as GWP times amount of the other gas.
As weight, CO2e is the weight of CO2 which would warm the earth as much as a particular weight of some other gas;
it is calculated as GWP times weight of the other gas. For example if a gas has GWP of 100, two tons of the gas have CO2e of 200 tons, and 9 tons of the gas has CO2e of 900 tons.
As concentration, e is the concentration of CO2 which would warm the earth as much as a particular concentration of some other gas or of all gases and aerosols in the atmosphere; it is calculated as GWP times concentration of the other gas. For example CO2e of 500 parts per million would reflect a mix of atmospheric gases which warm the earth as much as 500 parts per million of CO2 would warm it.
e calculations depend on the time-scale chosen, typically 100 years or 20 years,
since gases decay in the atmophere or are absorbed naturally, at different rates.
The following units are commonly used:
For example, the table above shows GWP for methane over 20 years at 86 and nitrous oxide at 289, so emissions of 1 million tonnes of methane or nitrous oxide are equivalent to emissions of 86 or 289 million tonnes of carbon dioxide, respectively.