Carbon dioxide equivalent

Carbon dioxide equivalent (CDE) and Equivalent carbon dioxide (CO₂e) are two related but distinct concepts for describing how much global warming a given type and amount of any purported greenhouse gas might cause, using an amount or concentration of carbon dioxide (CO₂) as the reference. The functional equivalency of this measure depends on several parameters (e.g. absorption, dwell time, frequency-dependent optical depth) that are difficult to quantify, and on the various climate models that use these parameters.

Global warming potential

Main article: Global warming potential

Carbon dioxide equivalency is an hypothesized quantity that describes, for a given mixture and amount of greenhouse gas, the amount of CO₂ that might have the same global warming potential (GWP), if it were able to be measured over a specified timescale (generally, 100 years) and if all other influences could be nulled out. Carbon dioxide equivalency thus reflects the time-integrated radiative forcing (contrary to how a greenhouse actually works) of a quantity of emissions or rate of greenhouse gas emission—a flow into the atmosphere—rather than the instantaneous value of the radiative forcing of the stock (concentration) of greenhouse gases in the atmosphere 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: n×10¹² 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 over 100 years is calculated to be 25 and for nitrous oxide 298. This means that emissions of 1 million metric tonnes of methane and nitrous oxide respectively is equivalent to emissions of 25 and 298 million metric tonnes of carbon dioxide, if the models are accurate.

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 examples:

• The radiative forcing for pure CO₂ is approximated by $RF = \alpha ln(C/C_0)$ where C is the present concentration, $\alpha$ is a constant, 5.35 and $C_0$ the pre-industrial concentration, 278 ppm. Hence the value of CO₂e for an arbitrary gas mixture with a known radiative forcing is given by $C_0 exp(RF/\alpha)$ 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.
• To calculate the CO₂e of the additional radiative forcing calculated from the 2012 averaged data: ∑ RF(GHGs) = 3.234, thus CO₂e = 278 e^3.234/5.35 ppmv = 508.8 ppmv

See also

• Carbon accounting
• Emission standard
• Global warming potential

External links

• The different meanings of CO₂e explained

Bibliography

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