Greenhouse gases: what is the "CO2 equivalent"?

April 26, 2019

The various greenhouse gases (GHGs) are distinguished by, among other things, the amount of energy they are able to absorb (1) and their "lifetime" in the atmosphere. The "CO2equivalent" (CO2eq) is a unit created by the Intergovernmental Panel on Climate Change (IPCC) (2) to compare the impacts of these different GHGs on global warming and to be able to aggregate their emissions. It is a simplified tool (3) that makes it possible to identify priority actions to fight global warming and is particularly necessary to set up "carbon markets"(4).
In practice, the CO2equivalent consists in assigning, for a given period of time, a different "global warming potential" (GWP) for each gas compared to CO2used as a standard (and for which the GWP is therefore set at) (1)(5). In other words, the GWP refers to the estimated greenhouse effect of a GHG.  
For example, the IPCC considers that one ton of methane (CH4) has a global warming potential 28 times higher on average than one ton of CO2over a 100-year period. Thus, each ton of methane is counted as 28 tons of CO2equivalent in the GHG emission balances.
However, one of the complexities of this equivalent is that different GHGs have different effects over time. Their GWP must therefore always be assessed in relation to a given time scale: a ton of methane has a GWP of 28 on a 100-year scale but 84 on a 20-year scale, taking into account its shorter estimated lifetime in the atmosphere (6) compared to CO2. When the time scale in question is not specified in the emission balances, it is set "by default" to 100 years (7).
As a reminder, global emissions of the 6 greenhouse gases covered by the Kyoto Protocol (CO2, methane, nitrous oxide, and three fluorinated gases) amounted to 54 Gt eq CO2in 2013 (over a 100-year period) (8). A reduction in these emissions by at least 40% by 2050 (compared to the 2010 level) and a nearly "carbon-neutral" economy during the second half of the 21st century would be necessary, according to the IPCC, to limit global warming to a temperature increase of 2°C by 2100 (9), as the COP21 (10) agreement aims to do.
Sources and Notes:
1.     And the "radiative power" they radiate back to the ground.
2.     Index introduced in the "IPCC First Assessment Report".
3.     The actual greenhouse effect of a gas depends on many factors, including the size of the "available" carbon sinks.
4.     The CO2equivalent is a conventional tool that is not used in prospective modelling of "future climates": the contributions of the main greenhouse gases are each subject to specific treatment. Climate Modelling Centre, IPSL.
5.     It should be noted that some organizations prefer to use "carbon equivalent" (eq C) over CO2equivalent, particularly because the latter suggests that only this gas is taken into account. Knowing that one kilogram of CO2contains nearly 273 grams of carbon, 1 eq CO2≈ 0.273 eq C and 1 eq C ≈ 3.67 eq CO2.
6.     12.4 years.
7.     Rapid atmospheric cycle greenhouse gases, which are emitted and continuously renewed, are not included in the CO2equivalent, although they can have a significant atmospheric concentration. The same applies to indirectly produced gases such as ozone.
8.     Key figures on climate in France, the EU and the world (2016 edition of the Ministry).
9.     Compared to pre-industrial temperatures.
10.  The objective of the COP21 agreement is to stabilize global warming due to human activities at the Earth's surface "significantly below" 2°C by 2100 compared to the pre-industrial era temperature (reference period 1861-1880) and to continue efforts to limit this warming to 1.5°C.

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