Global warming potential
Global warming potential is a measure of how much heat a greenhouse gas traps in the atmosphere over a specific time period, relative to carbon dioxide. It is expressed as a multiple of warming caused by the same mass of. Therefore, by definition has a GWP of 1. For other gases it depends on how strongly the gas absorbs thermal radiation, how quickly the gas leaves the atmosphere, and the time frame considered.
For example, methane has a GWP over 20 years of 81.2 meaning that, a leak of a tonne of methane is equivalent to emitting 81.2 tonnes of carbon dioxide, both measured over 20 years. As methane has a much shorter atmospheric lifetime than carbon dioxide, its GWP is much less over longer time periods, with a GWP-100 of 27.9 and a GWP-500 of 7.95.
The carbon dioxide equivalent can be calculated from the GWP. For any gas, it is the mass of that would warm the earth as much as the mass of that gas. Thus it provides a common scale for measuring the climate effects of different gases. It is calculated as GWP times mass of the other gas.
Definition
The global warming potential is defined as an "index measuring the radiative forcing following an emission of a unit mass of a given substance, accumulated over a chosen time horizon, relative to that of the reference substance, carbon dioxide. The GWP thus represents the combined effect of the differing duration these substances remain in the atmosphere and their effectiveness in causing radiative forcing."In turn, radiative forcing is a scientific concept used to quantify and compare the external drivers of change to Earth's energy balance. Radiative forcing is the change in energy flux in the atmosphere caused by natural or anthropogenic factors of climate change as measured in watts per meter squared.
Importance of time scale
A substance's GWP depends on the time scale 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 25 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.Commonly, a time scale of 100 years is used by regulators.
Use in policymaking
As governments develop policies to combat emissions from high-GWP sources, policymakers have chosen to use the 100-year GWP scale as the standard in international agreements. The Kigali Amendment to the Montreal Protocol sets the global phase-down of hydrofluorocarbons, a group of high-GWP compounds. It requires countries to use a set of GWP100 values equal to those published in the IPCC's Fourth Assessment Report. This allows policymakers to have one standard for comparison instead of changing GWP values in new assessment reports. One exception to the GWP100 standard exists: New York state’s Climate Leadership and Community Protection Act requires the use of GWP20, despite being a different standard from all other countries participating in phase downs of HFCs.Calculated values
Current values (IPCC Sixth Assessment Report from 2021)
The global warming potential depends on both the efficiency of the molecule as a greenhouse gas and its atmospheric lifetime. GWP is measured relative to the same mass of and evaluated for a specific timescale. Thus, if a gas has a high radiative forcing but also a short lifetime, it will have a large GWP on a 20-year scale but a small one on a 100-year scale. Conversely, if a molecule has a longer atmospheric lifetime than its GWP will increase when the timescale is considered. Carbon dioxide is defined to have a GWP of 1 over all time periods.Methane has an atmospheric lifetime of 12 ± 2 years. The 2021 IPCC report lists the GWP as 83 over a time scale of 20 years, 30 over 100 years and 10 over 500 years. The decrease in GWP at longer times is because methane decomposes to water and through chemical reactions in the atmosphere. Similarly the third most important GHG, nitrous oxide, is a common gas emitted through the denitrification part of the nitrogen cycle. It has a lifetime of 109 years and an even higher GWP level running at 273 over 20 and 100 years.
Examples of the atmospheric lifetime and GWP relative to for several greenhouse gases are given in the following table:
Estimates of GWP values over 20, 100 and 500 years are periodically compiled and revised in reports from the Intergovernmental Panel on Climate Change. The most recent report is the IPCC Sixth Assessment Report from 2023.
The IPCC lists many other substances not shown here. Some have high GWP but only a low concentration in the atmosphere.
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. For a starting amount of 1 tonne of methane, which has a GWP of 25, after combustion there would be 2.74 tonnes of, 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.
Earlier values from 2007
The values provided in the table below are from 2007 when they were published in the IPCC Fourth Assessment Report. These values are still used for some comparisons.Mixtures
The GWP for a mixture of gases can be obtained from the mass-fraction-weighted average of the GWPs of the individual gases.Water vapour
does contribute to anthropogenic global warming, but as the GWP is defined, it is negligible for H2O: an estimate gives a 100-year GWP between -0.001 and 0.0005.H2O can function as a greenhouse gas because it has a profound infrared absorption spectrum with more and broader absorption bands than. Its concentration in the atmosphere is limited by air temperature, so that radiative forcing by water vapour increases with global warming. But the GWP definition excludes indirect effects. GWP definition is also based on emissions, and anthropogenic emissions of water vapour are removed via precipitation within weeks, so its GWP is negligible.
Calculation methods
When calculating the GWP of a greenhouse gas, the value depends on the following factors:- the absorption of infrared radiation by the given gas
- the time horizon of interest
- the atmospheric lifetime of the gas
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 a wavenumber 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, except for methane, which had its GWP almost doubled. 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 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 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 that are not filled up as much as, so rising ppms of these gases are far more significant.