Carbon dioxide equivalents (CO₂e)

Carbon dioxide equivalents — CO₂e — are a way to measure greenhouse gas emissions. They show how much warming the release of a particular gas causes compared with carbon dioxide. To understand the term, split it in two: carbon dioxide — the greenhouse gas behind around 60% of global warming — and equivalent, meaning correspondence. So the measure expresses a given gas's equivalent in carbon dioxide.

CO₂e matters because a small amount of a potent greenhouse gas can cause the same warming as a large amount of carbon dioxide. Methane from livestock, for example, can do a lot of damage even though it is released in relatively small quantities.

Global Warming Potential (GWP)

The conversion relies on Global Warming Potential (GWP), which measures how much energy the emission of one tonne of a gas absorbs compared with one tonne of CO₂, over a set period — usually 100 years. By definition, CO₂ has a GWP of 1. Methane, by comparison, has a GWP of roughly 28–30 over 100 years, meaning the same mass of methane contributes far more to the greenhouse effect than CO₂.

To express a gas's emissions in CO₂e, you multiply the emissions by the gas's GWP. So one tonne of methane equals roughly 28–30 tonnes of CO₂ equivalents.

Greenhouse gas emissions (kg) × GWP = carbon dioxide equivalents.

Why CO₂e is useful

Because every greenhouse gas absorbs heat differently, a common unit is needed to compare them. CO₂e provides exactly that: it lets analysts add up emissions of different gases into a single figure — for example, when compiling a national greenhouse gas inventory — and lets policymakers compare emission-reduction opportunities across sectors and gases. Each year, total greenhouse gas emissions (CO₂, methane, nitrous oxide and fluorinated gases) are reported to the UN climate convention using this measure.

Source: US EPA — Understanding Global Warming Potentials