November 2013: CO2e and What It Means

This is the eighteenth article in a series about air quality in general and how it applies to Indiana. This article will discuss the term "CO2e" and what it means.

If you have been following the discussion on greenhouse gases, you have probably come across the term CO2e. Literally what this means is CO2 (carbon dioxide) equivalent.

Different greenhouse gases have different abilities to retain heat and raise the atmospheric temperature. This is expressed as global warming potential (GWP). The main greenhouse gasses are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and fluorinated or chlorinated gasses. The global warming potential of carbon dioxide is 1. For nitrous oxide (N20) it is 310. For methane (CH4) it is 21. Sulfur hexafluoride (SF6) has a GWP of 23,900. Many chlorofluorocarbons and perfluorocarbons have GWPs above 1000.

When total greenhouse gas emissions are expressed, they are listed as CO2e. In other words, the emission total is multiplied by the GWP of the substance to get an equivalent amount of CO2. The following example shows how this works. A facility emits 6,480 tons per year of carbon dioxide (CO2) from combustion. It also emits 0.3 tons per year or methane (CH4) and 0.1 ton per year of nitrous oxide (N2O). It also loses 0.2 tons per year of sulfur hexafluoride (SF6). What are the total emissions expressed as CO2e?

Total Emissions Expressed as CO2e
Emissions (Tons/Year) x GWP = Emissions (Tons/Year) CO2e
CO2 (6,480 tons/year) x (1) = 6,480 tons/year CO2e
CH4 (0.3 tons/year) x (21) = 6.3 tons/year CO2e
N2O (0.1 tons/year) x (310) = 31 tons/year CO2e
SF6 (0.2 tons/year) x (23,900) = 4,780 tons/year CO2e
Total = 11,297.3 tons/year CO2e

So in this scenario, the facility’s total greenhouse gas emissions would be 11,297.3 tons/year, expressed as CO2e. The key point here is that some compounds have a very high global warming potential. While they may be emitted in small quantities, they may still contribute significantly to the total emissions when expressed as CO2e. In this case, the SF6 emissions contribute 0.003% on a mass basis (tons). However, when you include the GWP in the calculations, SF6 makes up 42% of the global warming potential from this facility.

The United States Environmental Protection Agency provides more information about the primary greenhouse gases on its website.

Keith Baugues