Tonne of CO₂e
The usual reporting unit for greenhouse gases, equal to one tonne of CO₂ or the amount of another gas judged equivalent under a chosen metric.
Example
A footprint report may convert methane and nitrous oxide into tonnes of CO₂e before adding them to CO₂.
The catch
For methane, the CO₂e figure is much larger under GWP20 than under GWP100.
Related terms
Greenhouse gas, GWP100, GWP20, Methane, Nitrous oxide, Reporting
In more depth
Tonne of CO₂e, explained.
A tonne of CO₂e is only meaningful when the conversion behind it is known. For any gas, the figure equals its mass multiplied by a global warming potential. Under the 100-year horizon used in most reports, a tonne of methane counts as about 30 tonnes of CO₂e, because methane warms about 30 times as much as CO₂ over 100 years, mass for mass (IPCC AR6, UNEP).
Watch for unit slips. A tonne of carbon becomes far more than a tonne of CO₂ once oxygen is added, so confusing the two units misstates a figure by a large factor. The US short ton is lighter than the metric tonne. Each carbon credit normally represents a tonne of CO₂e reduced or removed, which means the GWP values a method uses affect how many credits a methane project can issue.