dioxideequivalent

Dioxide equivalent, often written as CO2e, is a metric used to express the climatic impact of different greenhouse gases (GHGs) in a common unit. It allows emissions of gases other than carbon dioxide to be compared and aggregated by converting them into an amount of CO2 that would have the same warming effect over a specified time horizon.

Calculation and meaning: The CO2e value for a given emission is obtained by multiplying the mass of each gas by its global warming potential (GWP) for a chosen time horizon and summing the results across all gases. CO2e is typically measured in kilograms or tonnes of CO2e. The GWP is gas- and horizon-specific, meaning the same gas can have different CO2e values depending on whether a 20-year or 100-year horizon is used. The 100-year horizon is the most commonly applied in inventories and reporting, though other horizons are used for certain analyses.

Time horizon and examples: GWP values vary widely by gas. For the 100-year horizon, methane is often cited with a GWP around a few dozen, nitrous oxide around a couple of hundred, and many fluorinated gases can have GWPs ranging from hundreds to thousands. Because GWPs are estimates that depend on atmospheric behavior and the chosen time frame, CO2e figures are tools for comparison rather than direct measurements of warming.

Applications and limitations: CO2e is widely used in national and corporate emission inventories, lifecycle assessments, product footprints, and policy contexts, including carbon pricing and regulatory reporting. It simplifies diverse emissions to a single metric, but it also abstracts away differences in timing, lifetimes, and other environmental effects. Consequently, CO2e should be interpreted as a comparative indicator rather than a precise measure of actual warming.