Carbon dioxide equivalent

Carbon dioxide equivalent (CDE) and Equivalent carbon dioxide (or CO₂e) are two related but distinct measures for describing how much global warming a given type and amount of greenhouse gas may cause, using the functionally equivalent amount or concentration of carbon dioxide (CO₂) as the reference.

Equivalent carbon dioxide

Equivalent CO₂ (CO₂e) is the concentration of CO₂ that would cause the same level of radiative forcing as a given type and concentration of greenhouse gas. Examples of such greenhouse gases are methane, perfluorocarbons and nitrous oxide. CO₂e is expressed as parts per million by volume, ppmv.

CO₂e calculation example:

• The radiative forcing for pure CO₂ is approximated by <math>RF = \alpha ln(C/C_0)</math> where C is the present concentration, <math>\alpha</math> is a constant, 5.35 and <math>C_0</math> the pre-industrial concentration, 278 ppm. Hence the value of CO₂e for an arbitrary gas mixture with a known radiative forcing is given by <math>C_0 exp(RF/\alpha)</math> in ppmv.
• To calculate the radiative forcing for a 1998 gas mixture, IPCC 2001 gives the radiative forcing (relative to 1750) of various gases as: CO₂=1.46 (corresponding to a concentration of 365 ppmv), CH₄=0.48, N₂O=0.15 and other minor gases =0.01 W/m2. The sum of these is 2.10 W/m2. Inserting this to the above formula, we obtain CO₂e = 412 ppmv.

Carbon dioxide equivalent

Carbon dioxide equivalency is a quantity that describes, for a given mixture and amount of greenhouse gas, the amount of CO₂ that would have the same global warming potential (GWP), when measured over a specified timescale (generally, 100 years). Carbon dioxide equivalency thus reflects the time-integrated radiative forcing, rather than the instantaneous value described by CO₂e.

The carbon dioxide equivalency for a gas is obtained by multiplying the mass and the GWP of the gas. The following units are commonly used:

• By the UN climate change panel IPCC: billion metric tonnes of CO₂ equivalent (GtCO₂eq).
• In industry: million metric tonnes of carbon dioxide equivalents (MMTCDE).
• For vehicles: g of carbon dioxide equivalents / km (gCDE/km).

For example, the GWP for methane is 21 and for nitrous oxide 310. This means that emissions of 1 million metric tonnes of methane and nitrous oxide respectively is equivalent to emissions of 21 and 310 million metric tonnes of carbon dioxide.

See also

• Carbon accounting
• Emission standard
• Global warming potential

References

• Gohar and Shine, Equivalent CO₂ and its use in understanding the climate effects of increased greenhouse gas concentrations, Weather, Nov 2007, p307-311.