1

Emission

Facts

Office of Transportation

and Air Quality

EPA420-F-05-002

February 2005

Metrics for Expressing Greenhouse

Gas Emissions: Carbon Equivalents

and Carbon Dioxide Equivalents

The U.S. Environmental Protection Agency (EPA) developed this series

of four fact sheets to facilitate consistency of assumptions and practices

in the calculation of emissions of greenhouse gases from transportation

and mobile sources. They are intended as a reference for anyone

estimating emissions benefits of mobile sources air pollution control

programs.

Global warming potentials and the use of a common

metric

Emissions of greenhouse gases are typically expressed in a common

metric, so that their impacts can be directly compared, as some gases are

more potent (have a higher global warming potential or GWP) than

others.

The international standard practice is to express greenhouse gases in

carbon dioxide (CO2) equivalents. Emissions of gases other than CO2 are

translated into CO2 equivalents using global warming potentials. The

Intergovernmental Panel on Climate Change (IPCC) recommends using

100 year potentials.
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These potentials1

 are:

 GWP

Carbon dioxide (CO2) 1

Methane (CH4

) 21

Nitrous oxide (N2O) 310

Hydrofluorocarbon (HFC)-134a

(used in mobile source air conditioning) 1,300

Carbon dioxide equivalents vs. carbon equivalents

While the international standard is to express emissions in CO2 equivalents (CO2

e), many U.S. sources have expressed emissions data in terms

of carbon equivalents (CE) in the past. In particular, EPA has used the

carbon equivalent metric in the past for budget documents. The benefits

of voluntary programs have often been expressed in million metric tons

of carbon equivalent (MMTCE). Essentially, this practice accounts for

the carbon in the CO2

 molecule, as opposed to counting the entire molecule.

EPA’s Office of Transportation and Air Quality (OTAQ) uses the CO2

equivalent metric for calculating the national inventory for greenhouse

gas emissions, and is moving towards using the CO2 metric to characterize the benefits of its voluntary programs to be consistent with international practice. Emission reductions from voluntary programs will

generally be expressed in million metric tons of CO2 equivalent. For the

purposes of national greenhouse gas inventories, emissions are expressed

as teragrams of CO2 equivalent (Tg CO2 Eq). One teragram is equal to

1012 grams, or one million metric tons.

1

 These estimates are from the IPCC’s "Second Assessment Report" (1996). These are

the values used internationally for reporting greenhouse gas (GHG) emissions to the

United Nations. (EPA also uses them for the "Inventory of U.S. Greenhouse Gas

Emissions and Sinks.") The IPCC has since published the "Third Assessment Report"

(2001) and has updated GHG potential values. These are 23 for CH4, 296 for N2O, and

1,300 for HFC-134a. These values, however, have not yet been adopted by EPA for

the purpose of developing inventories.
3

Converting between carbon dioxide equivalents and

carbon equivalents

The conversion between CO2e and CE is directly related to the ratio of

the atomic mass of a carbon dioxide molecule to the atomic mass of a

carbon atom (44:12).

To convert from CE to CO2e, multiply by 44/12

For example, 6 million metric tons of carbon equivalent (6 MMTCE) =

6 x (44/12) = 22 million metric tons of CO2

 equivalent.

1 metric ton carbon equivalent = 3.667 metric tons of CO2

 equivalent.

To convert from CO2e to CE, multiply by 12/44

For example, 11 million metric tons of CO2

 equivalent = 11 x (12/44) =

3 million metric tons of carbon equivalent (3 MMTCE).

1 metric ton of CO2

 equivalent = 0.2727 metric tons of carbon equivalent.

For More Information

You can access documents on greenhouse gas emissions on the Office of

Transportation and Air Quality Web site at:

www.epa.gov/otaq/greenhousegases.htm

For further information on calculating emissions of greenhouse gases,

please contact Ed Coe at:

U. S. Environmental Protection Agency

Office of Transportation and Air Quality

1200 Pennsylvania Ave., NW (6406J)

Washington, DC 20460

202-343-9629

E-mail: coe.edmund@epa.gov