Global warming potential

thumb|Comparison of global warming potential of three [[greenhouse gases over a 100-year period (GWP-100) per ton: Perfluorotributylamine (PFTBA), nitrous oxide and methane, compared to carbon dioxide (the latter is the reference value, therefore it has a GWP of one).

PFTBA is here used as an example of a larger group of potent fluorinated greenhouse gases. Fluorinated hydrocarbons combined contribute about 10% to global warming.]]

Global warming potential (GWP) is a measure of how much heat a greenhouse gas traps in the atmosphere over a specific time period, relative to carbon dioxide (). 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.

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 (expressed as a number of years, denoted by a subscript) 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 (GWP100 = 25) but 86 over 20 years (GWP20 = 86); conversely sulfur hexafluoride has a GWP of 22,800 over 100 years but 16,300 over 20 years (IPCC Third Assessment Report). 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.

e calculations depend on the time-scale chosen, typically 100 years or 20 years, since gases decay in the atmosphere or are absorbed naturally, at different rates.

Carbon dioxide equivalent

Carbon dioxide equivalent mass or just carbon dioxide equivalent (symbol e or eq or -e) of a quantity of gas is calculated from its GWP. For any gas, it is the mass of which 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 multiplied by mass of the other gas. For example, if a gas has GWP of 100, two tonnes of the gas have e of 200 tonnes, and 9 tonnes of the gas has e of 900 tonnes.

On a global scale, the warming effects of one or more greenhouse gases in the atmosphere can also be expressed as a carbon dioxide equivalent concentration. It is the atmospheric concentration of which would warm the earth as much as a particular concentration of some other gas or of all gases and aerosols in the atmosphere. For example, e of 500 parts per million would reflect a mix of atmospheric gases which warm the earth as much as 500 parts per million of would warm it. Calculation of the equivalent concentration of an atmospheric greenhouse gas or aerosol is more complex and involves the atmospheric concentrations of those gases, their GWPs, and the ratios of their molar masses to the molar mass of .

The following units are commonly used:

By the UN climate change panel (IPCC): billion metric tonnes = n×109 tonnes of equivalent (Gteq)
In industry: million metric tonnes of carbon dioxide equivalents (MMTCDE) and MMT eq.

Further derived quantities include carbon dioxide equivalent mass per distance, as used for vehicle travels. It has SI units of grams per kilometer (g/km), often denoted "grams of carbon dioxide equivalent per kilometer" (ge/km) or per mile (ge/mile).

For example, the table below 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.

Calculation methods

right|thumb|upright=1.6|The [[radiative forcing (warming influence) of long-lived atmospheric greenhouse gases has accelerated, almost doubling in 40 years.]]

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 (integration period)
the atmospheric lifetime of the gas

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 (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.

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:

<math display="block">\mathit{RF} = \sum_{i=1}^{100} \text{abs}_i \cdot F_i / \left(\text{l} \cdot \text{d}\right)</math>

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 (IPCC) 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:

<math display="block">\mathit{GWP} \left(x\right) = \frac{a_x}{a_r} \frac{\int_0^{\mathit{TH [x](t)\, dt} {\int_0^{\mathit{TH [r](t)\, dt}</math>

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. ). 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 (e.g., , 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 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 (saturated) as much as , so rising ppms of these gases are far more significant.

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

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 (positive feedback). But the GWP definition excludes indirect effects. GWP definition is also based on emissions, and anthropogenic emissions of water vapour (cooling towers, irrigation) are removed via precipitation within weeks, so its GWP is negligible.

Applications

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 (HFCs), 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 (AR4). 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.

After some intermediate updates, in 2013 this standard was updated by the Warsaw meeting of the UN Framework Convention on Climate Change (UNFCCC, decision number 24/CP.19) to require using a new set of 100-year GWP values. They published these values in Annex III, and they took them from the IPCC Fourth Assessment Report, which had been published in 2007. However GWP* has been criticised both for its suitability as a metric and for inherent design features which can perpetuate injustices and inequity. Developing countries whose emissions of SLCPs are increasing are "penalized", while developed countries such as Australia or New Zealand which have steady emissions of SLCPs are not penalized in this way, though they may be penalized for their emissions of .

Calculated values

thumb|Global warming potential of five greenhouse gases over 100-year timescale.

The global warming potential (GWP) 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 (positive) 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. 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 (IPCC Sixth Assessment Report from 2021).

{| class="wikitable sortable" style="text-align: right"

|+Atmospheric lifetime and global warming potential (GWP) relative to at different time horizon for various greenhouse gases (more values provided at global warming potential)

!Gas name

!Chemical

formula

!Lifetime

(years)

!Radiative Efficiency

(Wmppb, molar basis).

| style="text-align:left;" |Carbon dioxide

| style="text-align:center;" |

|(A)

| style="text-align:left;" |Methane (fossil natural gas)

| style="text-align:center;" |

|12

|83

|30

|10

| style="text-align:left;" |Methane (pure non-fossil)

| style="text-align:center;" |

|12

|81

|27

|7.3

|- id="N2O"

| style="text-align:left;" |Nitrous oxide

| style="text-align:center;" |

|109

|273

|130

| style="text-align:left;" |CFC-11 (R-11)

| style="text-align:center;" |

|52

|8,321

|6,226

|2,093

| style="text-align:left;" |CFC-12 (R-12)

| style="text-align:center;" |

|100

|10,800

|10,200

|5,200

| style="text-align:left;" |HCFC-22 (R-22)

| style="text-align:center;" |

|12

|5,280

|1,760

|549

| style="text-align:left;" |HFC-32 (R-32)

| style="text-align:center;" |

|2,693

|771

|220

| style="text-align:left;" |HFC-134a (R-134a)

| style="text-align:center;" |

|14

|4,144

|1,526

|436

| style="text-align:left;" |Tetrafluoromethane (R-14)

| style="text-align:center;" |

|50,000

|5,301

|7,380

|10,587

| style="text-align:left;" |Hexafluoroethane

| style="text-align:center;" |

|10,000

|8,210

|11,100

|18,200

| style="text-align:left;" |Sulfur hexafluoride

| style="text-align:center;" |

|3,200

|17,500

|23,500

|32,600

| style="text-align:left;" |Nitrogen trifluoride

| style="text-align:center;" |

|500

|12,800

|16,100

|20,700

! colspan="7" |(A) No single lifetime for atmospheric can be given.

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 (Working Group I) 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 (ratio 1:2.74). 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 (approximately 9 times).

{| class="wikitable sortable"

! rowspan="2" | Greenhouse gas

! rowspan="2" data-sort-type="number" | Lifetime (years)

! colspan="3" | Global warming potential, GWP

! data-sort-type="number" | 20 years

! data-sort-type="number" | 100 years

! data-sort-type="number" | 500 years

|Hydrogen (H2)

|4–7

| data-sort-value="33" | 33 (20–44) 72 84 / 86f 80.8 (biogenic) 39 (biogenic) 40 (fossil)