Carbon dioxide equivalent

Carbon dioxide equivalent (CO₂e) is a unit that expresses emissions of any greenhouse gas as the amount of carbon dioxide that would cause the same warming. It allows emissions of different gases to be added into a single total and compared over time.

Each greenhouse gas is assigned a conversion factor, usually its Global Warming Potential over 100 years (GWP100). The Intergovernmental Panel on Climate Change (IPCC) updates these values in each assessment report, and each reporting framework specifies which report to use.

What determines the conversion factor

The conversion factor of each gas depends on the following properties:

• Radiative efficiency: how strongly the gas traps heat.
• Atmospheric lifetime: how long the gas stays in the atmosphere before it breaks down or is removed.
• Molecular weight: how heavy each molecule is, which converts the heat-trapping effect into a per-kilogram value.
• Indirect effects: extra warming or cooling caused when the gas reacts with other substances in the atmosphere, for example methane creating ozone.
• Breakdown into CO₂: some gases, like fossil methane, turn into CO₂ when they break down, which adds to their warming effect.

The conversion factor also depends on the time horizon chosen. For example, in the IPCC Sixth Assessment Report (AR6), fossil methane has a GWP of 29.8 over 100 years and 82.5 over 20 years.

Greenhouse gases

This table lists the most common greenhouse gases and their GWP100:

Comparison example

The following table shows the emissions of a hypothetical company in 2025 and 2026:

Comparing 2025 and 2026 for this company is challenging. Carbon dioxide emissions decreased by 130 tonnes, while sulfur hexafluoride increased by only 1 kilogram, yet one tonne of sulfur hexafluoride causes 24,300 times as much warming over 100 years as one tonne of carbon dioxide.

Converting each gas to CO₂e makes the two years comparable:

Expressed in CO₂e using IPCC AR6 GWP100 values, the company’s emissions decreased by 79.7 t CO₂e, or around 4.0%.

Sources

• United Nations Framework Convention on Climate Change (2018). Decision 18/CMA.1: Modalities, procedures and guidelines for the transparency framework for action and support referred to in Article 13 of the Paris Agreement.
• Forster, P., T. Storelvmo, et al. (2021). The Earth’s Energy Budget, Climate Feedbacks, and Climate Sensitivity. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, pp. 923–1054.