As a rule data in databases/tables/articles about emission of greenhouse gases is in CO2 equivalent tonnes. These are not the tonnes of emitted CO2. Actually these are not the tonnes of real gases of any kind at all. CO2 equivalent tons are theoretical quantities invented in order to express total greenhouse gas effect caused by all GHG altogether. Why do we need such theoretical quantity and why cant we just add the quantities of all greenhouse gases emissions together?
Using CO2 equivalents enables us to compare different greenhouse effects caused by emissions of different greenhouse gases and to add it together. Different gases have different ability to cause greenhouse effect; global warming potential (GWP) of different substances vary greatly. It depends of three main matters:
1) molecules ability to absorb radiation;
2) the wavelength of radiation absorbed;
3) the timeperiod of which the molecules stays in the atmosphere.
Combination of these characteristics specifies the harmfulness (GWP) of substance. This mean, that substance (gas) with very high GWP gives lot of CO2 equivalent tonnes even when its actual emission in tonnes is quite small. CO2 is taken as standard, its GWP is agreed to be 1; real CO2 emission and its emission in CO2 equivalent tonnes is the same. CO2 equivalent of other greenhouse gases is calculated by multiplying quantity of gas emission with GWP of this gas. Emission of GHG in CO2 equivalents equals the quantity of GHG multiplied by its GWP.
When emissions of all greenhouse gases are expressed through CO2 we can add them together to indicate the total greenhouse emission. Actually the real emission in tonnes of gases with high GWP (higher than that of CO2) is much smaller than their CO2 equivalents quantities.
So CO2 equivalent is defined as the amount of CO2 that would have the same global warming potential (GWP), when measured over a specified timescale (which is usually 100 years). Carbon dioxide equivalents is used to express the greenhouse gas emission (flow) into the atmosphere.