Carbon Dioxide Equivalent

Carbon Dioxide Equivalent (CO2e) is a term used in climate science to explain the global warming potential of the various greenhouse gases (GHGs) by comparing each gas to the most prevalent GHG, carbon dioxide (CO2). The basis for this concept is found in the comparative radiative forcing of the various gases.

The use of CO2 as a common denominator simplifies the process of measuring changes in GHG emissions, as countries attempt to meet the mitigation targets agreed under various conventions. Global warming potential (GWP) is not a static concept, but rather measures the cumulative radiative impact of GHGs over various time periods using the CO2e metric. Conventionally, the GWP of a GHG is understood to be its radiative impact over a 100-year period.

Using this principle, the Intergovernmental Panel on Climate Change (IPCC) has determined that the GWP of the various gases are as follows:

• Carbon dioxide (CO2): 1

• Methane (CH4): 21

• Nitrous oxide (N2O): 310

• Hydrofluorocarbons (HFC): 1,300 to 11,700 (there are eight different forms of HFC)

• Chlorofluorocarbons (CFC): 6,500 to 9,200 (there are four different forms of CFC)

• Sodium hexafluoride (SF6): 23,900

The final determination of GWP is based on a number of factors, including the infrared-absorbing ability of each gas as well as its natural decay rate (the amount removed from the atmosphere over a given number of years). As this suggests, the actual and relative GWP of GHGs will vary depending on the time horizon used. The current scientific consensus is that a 100-year period is sufficiently long to enable valid and reliable comparisons. Radiative forcing is defined by the IPCC as follows:

… a measure of the influence a factor has in altering the balance of incoming and outgoing energy in the Earth-atmosphere system and is an index of the importance of the factor as a potential climate change mechanism … radiative forcing values are for changes relative to preindustrial conditions defined at 1750 and are expressed in watts per square meter (W/m2).

In short, radiative forcing is a measure of changes in the way the planet absorbs and re-radiates heat energy (infrared rays), typically because of anthropogenic (human-induced) causes. Such changes could also be because of causes other than human intervention, but in the context of climate change, the term is used only for changes in the radiation balance imposed by anthropogenic factors and excludes other variables such as changes in stratospheric dynamics, changes in trophospheric motions or in the troposphere’s thermodynamic state, or dynamically induced changes in the amount and distribution of atmospheric water.

As indicated in the IPCC’s definition, the assumed start date for measuring impacts is 1750, the notional beginning of the Industrial Revolution. The idea of using CO2e as a basis for comparing GWP was introduced by the IPCC in its First Assessment Report (1990), although at the time, the convention of using the 100-year time horizon had not been clearly established, and the specific figures varied somewhat from later conventions— for example, methane is given a value of 21 in the 1990 report, but nitrous oxide has a value of 290.

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 (1995) were to be used for converting the various GHG emissions into CO2 equivalents when computing overall sources and sinks. The values used in the IPCC Third Assessment Report (2001) are slightly different; methane is given a GWP of 23. When Kyoto was signed in 1997, the 1995 Second Assessment Report was seen as the most credible source for these values, although some authorities permit more recent data to be introduced.