Global warming potential

Global warming potential (GWP) is a measure of how much heat a greenhouse gas (GHG) traps in the atmosphere over a specific time period, relative to carbon dioxide (CO2). It is a dimensionless quantity expressed as a multiple of warming caused by the same mass of CO2. Therefore, by definition CO2 has a GWP of 1. For other gases, it depends on how strongly the gas absorbs thermal radiation, how quickly it leaves the atmosphere, and the time frame considered. For example, methane (CH4) has a GWP over 20 years (GWP-20) of 81.2 meaning that, a leak of a tonne of methane is equivalent to emitting 81.2 tonnes of CO2, both measured over 20 years. As methane has a much shorter atmospheric lifetime than CO2, its GWP is much less over longer time periods, with a GWP-100 of 27.9 and a GWP-500 of 7.95. Greenhouse gas emissions (GHG emissions) can be expressed in terms of carbon dioxide equivalent mass or just carbon dioxide equivalent (symbolized CO2e or CO2eq, also denoted CO2-e or CO2-eq) can be calculated from the GWP and emitted mass. For any gas, it is the mass of CO2 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; it is typically expressed in gigatonnes (symbol Gt).

Definition

The global warming potential (GWP) 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 (CO2). 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 (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. CO2e 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 CO2e or CO2eq or CO2-e) of a quantity of gas is calculated from its GWP. For any gas, it is the mass of CO2 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 CO2e of 200 tonnes, and 9 tonnes of the gas has CO2e 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 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. 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. Calculation of the CO2 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 CO2. The following units are commonly used:

By the UN climate change panel (IPCC): billion metric tonnes = n×109 tonnes of CO2 equivalent (GtCO2eq) In industry: million metric tonnes of carbon dioxide equivalents (MMTCDE) and MMT CO2eq. 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" (gCO2e/km) or per mile (gCO2e/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

When calculating the GWP of a greenhouse gas, the value depends on the following factors:

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