Carbon dioxide equivalent (CO2e)

Definition of carbon dioxide equivalent (CO2e):

Carbon dioxide equivalent (CO2e) is a unit that allows scientists to compare the relative harm of different greenhouse gases. It uses carbon dioxide (CO2) as a point of reference; other gases are placed on a scale relative to this.

What is carbon dioxide equivalent (CO2e)?

A standard unit for measuring multiple greenhouse gases.

“Carbon” is often discussed in relation to greenhouse gas (GHG) emissions, but carbon dioxide (CO2) is far from the only gas that causes global warming. In fact, other gases such as methane (CH4) and nitrous oxide (N2O) even have more potent warming effects than CO2. To effectively compare emitters (for example, companies, and countries …) it is, therefore, necessary to use a standard measure that takes into account the different levels of global warming potential (GWP) of different gases. This is where carbon dioxide equivalents (CO2e) come in.

How does CO2e work?

The carbon dioxide equivalent of a gas refers to the number of metric tons of CO2 with the same GWP potential as one metric ton of a different GHG. Carbon dioxide has a GWP of 1, meaning it is the reference gas for all other GHGs.

For example, a metric ton of methane is 25 times more effective at trapping heat than a metric ton of carbon dioxide. To calculate methane emissions in terms of CO2e, you must multiply the amount of methane it by its GWP. For instance, to calculate the carbon dioxide equivalent of ten metric tons of methane, simply multiply 10 by 25 (the GWP of methane). In this case, if ten metric tons of methane were emitted, this could be reported as 250 metric tons of CO2e.

What are the benefits of reporting in terms of CO2e?

Reporting emissions in terms of CO2e allows more effective comparisons between different countries, organizations, and sectors because it creates a “level playing field”. The profile of GHGs emitted will vary between countries and industries. Livestock farming, for instance, will emit a high percentage of methane. Reporting in terms of carbon dioxide equivalents negates the impact of these differences. Using CO2e as a comparative measure allows the overall warming effect to be reported, no matter what the profile of gases emitted.

What are the challenges of CO2e reporting?

GWP depends on how effective a GHG is at retaining energy (“radiative efficiency”) and the amount of time it naturally stays in the atmosphere (“lifetime”). Due to these facts, the GWP of greenhouse gases can vary over time. For instance, carbon dioxide stays in the atmosphere and retains heat for between 300 to 1,000 years, whereas methane leaves the atmosphere within a decade on average. This means that, over 20 years, the estimated GWP of methane is 81–83 whereas, over 100 years, it would be 27–30. For this reason, a 100-year time horizon is used as the standard.

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