Carbon dioxide (CO2) is a colorless gas that occurs naturally in earth’s atmosphere. CO2 is known as a greenhouse gas (GHG): a gas that contributes to the greenhouse effect by absorbing thermal infrared radiation and heating the planet’s lower atmosphere. CO2 is one of several gases that contributes to the greenhouse effect. Other GHGs include water vapor, methane (CH4), nitrous oxide (N2O), ozone (O3), perfluorinated compounds (PFCs), hydrofluorocarbons (HFCs), sulfur hexafluoride (SF6), and nitrogen trifluoride (NF3) the most abundant of which are water vapor, CO2, CH4, and N2O (in earth’s atmosphere). Each pollutant has been assigned a global warming potential (GWP), “a concept to compare the ability of each greenhouse gas to trap heat in the atmosphere relative to another gas” (EPA). In this case, CO2 has been assigned a GWP of one and the GWP of all other GHGs is compared to CO2.
The three largest contributors of CO2e in the United States as a whole are CO2, CH4, and N2O. Their GWPs are 1, 25, and 298, respectively.
For each GHG, a CO2 equivalence (CO2e) can be calculated by multiplying the quantity of each GHG by its corresponding global warming potential (GWP). For example, if a facility emitted 10 metric tons (MT) of CH4, the facility-wide CO2e would be 250 MT.
In 2018, the EPA published a report containing GHG emission and sink data from 1990-2016. The gross US CO2e emissions are shown below, separated by gas type.
The following chart, from the same report, indicates which economic sectors emitted GHGs in the US.
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