"CO2 equivalent" is a unit that was created by the Intergovernmental Panel on Climate Change (IPCC) to be able to compare the impact of all greenhouse gases (GHGs) on global warming. Greenhouse gases each have a different lifetime in the atmosphere as well as a different energy absorption amount. This single unit allows to aggregate the effect of various greenhouse gases and therefore to be aware of their respective relative impact and prioritize actions to fight global warming.
CO2, the gas that has the greatest global warming effect, is used as a benchmark, and we compute a “global warming potential” (GWP) for other greenhouse gases for a given period of time that represents its greenhouse effect.
This unit is calculated on a horizon based on a 100-year period, to take into account the time spent in the atmosphere by each GHG. For example, the GWP for one ton of methane is 28 times higher than one ton of CO2 over a 100-year scale. Thus, each ton of methane is equivalent to 28 tons of CO2 in the GHG emission balances.
The six greenhouse gases covered by the Kyoto Protocol (CO2, methane, nitrous oxide, and three fluorinated gases) amounted to 54 Gt eq CO2 in 2013 (over 100 years). The IPCC suggests that 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 help us limit global warming and help us stay under the 2°C goal from the COP21 agreement.