Physics:Carbon dioxide equivalent

Short description: Global warming greenhouse gas unit

This page is about measuring the potency of greenhouse gases. For carbon dioxide equivalent tax, see Carbon dioxide equivalent tax.

Carbon dioxide equivalent (CDE) and equivalent carbon dioxide (CO
2e and CO
2eq) are two related but distinct measures for estimating how much global warming a given type and amount of greenhouse gas may cause, using the functionally equivalent amount or concentration of carbon dioxide (CO
2) as the reference. CO
2e calculations depend on the time-scale chosen, typically 100 years but occasionally 20 years or less. They also depend on assumptions about the amount of time a given pollutant will remain in the atmosphere, which rely on other assumptions such as the amount of carbon sequestration that will occur in the future.

Global warming potential

Carbon dioxide equivalency is a quantity that describes, for a given mixture and amount of greenhouse gas, the amount of CO
2 that would have the same global warming potential (GWP), when measured over a specified timescale (typically 100 years). Carbon dioxide equivalency thus reflects the time-integrated radiative forcing 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 CO
2e.

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×10⁹ 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 34, and for nitrous oxide, 298. This means that emissions of 1 million tonnes of methane or nitrous oxide are equivalent to emissions of 34 or 298 million tonnes of carbon dioxide, respectively.

Equivalent carbon dioxide

Equivalent CO
2 (CO
2e) is the concentration of CO
2 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. CO
2e is expressed as parts per million by volume, ppmv.

CO
2e calculation examples:

• The radiative forcing for pure CO
  2 is approximated by [math]\displaystyle{ RF = \alpha \ln(C/C_0) }[/math] where C is the present concentration, [math]\displaystyle{ \alpha }[/math] is a constant, 5.35, and [math]\displaystyle{ C_0 }[/math] is the pre-industrial concentration, 280 ppm. Hence the value of CO
  2e for an arbitrary gas mixture with a known radiative forcing is given by [math]\displaystyle{ 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: CO
  2=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 CO
  2e = 412 ppmv.
• To calculate the CO
  2e of the additional radiative forcing calculated from April 2016's updated data: ∑ RF(GHGs) = 3.3793, thus CO
  2e = 280 e^3.3793/5.35 ppmv = 526.6 ppmv.

See also

• Carbon accounting
• Carbon footprint
• Emission standard

External links

• The different meanings of CO₂e explained

Bibliography

• Gohar and Shine, Equivalent CO
  2 and its use in understanding the climate effects of increased greenhouse gas concentrations, Weather, Nov 2007, pp. 307–311.