Greenhouse gases are measured as atmospheric concentrations and are expressed in parts per million (ppm). For instance, 1 metric ton of carbon equals 3.664 metric tons of CO2.

Greenhouse gases are often expressed as CO2 equivalents (CO2e), based on their respective global warming potential (GWP). GWP is based on a number of factors, including a gas’s ability to absorb heat compared to that of carbon dioxide, as well as its decay rate (the amount removed from the atmosphere over a given number of years).

Because scientists are not exactly certain how each gas concentration decays over time (some gases decay faster than others and some even grow in certain atmospheric conditions), the associated GWPs are estimates and may vary according to the purpose and parameters of a study.

The carbon dioxide equivalent for a gas is derived by multiplying the mass of the gas by the associated GWP. For example, averages of available industry-wide data give the GWP for carbon dioxide as 1, methane as 23 and nitrous oxide as 296. This means that emissions of 1 million metric ton of methane and nitrous oxide, respectively, are equivalent to emissions of 23 and 296 million metric tons of carbon dioxide.

Even though an industry may emit the same levels of annual greenhouse gas emissions, the atmospheric conditions may cause a change in the greenhouse gas concentrations.

MtCO2e = million metric tons carbon dioxide equivalent
GtCO2e = billion metric tons carbon dioxide equivalent