CO2e is shorthand for carbon dioxide equivalent. It’s a way to get one number that represents your total carbon footprint from all the different greenhouse gases.
Carbon dioxide (CO2) is the most common greenhouse gas, and it’s the main driver of global warming. But other greenhouse gases like methane (CH4) and nitrous oxide (N2O) are much more potent than carbon dioxide, although they’re also less common.
We use global warming potential (GWP) to convert greenhouse gases to a common scale so we can summarize them all with a single number. GWP measures how much warming a gas causes relative to carbon dioxide. For example, a pound of nitrous oxide causes 298 times as much warming as a pound of carbon dioxide, so it has a GWP of 298.
| Gas | Formula | GWP |
|---|---|---|
| Carbon Dioxide | CO2 | 1 |
| Methane | CH4 | 25 |
| Nitrous Oxide | N2O | 298 |
| Sulfur Hexafluoride | SF6 | 22,800 |
| Other Gases | Varies | See Table 2.14 (pdf) |
Still confused? Let’s look at an example. Suppose producing a hamburger resulted in the release of 20 pounds of CO2, 6 pounds of CH4, and half a pound of N2O. We would multiply the quantity of each gas by the GWP for that gas to convert it into CO2e, and then add them all up to get the burger’s total footprint:
| CO2 | = 20 × 1 = 20 lbs CO2e | |
| + | CH4 | = 6 × 25 = 150 lbs CO2e |
| + | N2O | = 0.5 × 298 = 149 lbs CO2e |
| = | 319 lbs CO2e | |