CO2equivalent

CO2equivalent, commonly abbreviated as CO2e, is a metric used to express the total climate impact of different greenhouse gases (GHGs) as the amount of carbon dioxide (CO2) that would cause the same level of warming over a specified time horizon. It enables comparison and aggregation of diverse emissions into a single, comparable unit for reporting and analysis.

Calculation relies on the global warming potential (GWP) of each gas, which is a factor that measures how much warming a gas causes over a chosen time horizon relative to CO2. To obtain the CO2e of a gas, its emitted mass is multiplied by its GWP for the chosen horizon, and the CO2e of a mixture is the sum of its components. CO2 has a GWP of 1 by definition, while other gases have higher values; for example, methane (CH4) has a higher GWP over most horizons, and nitrous oxide (N2O) and many fluorinated gases can have very large GWPs. Values vary with the time horizon used, commonly 100 years, but 20- and 500-year horizons are also employed.

Time horizon choice matters because the warming impact of gases changes with different horizons. The 100-year horizon is standard for many national inventories and international reporting, but shorter horizons emphasize short-lived species and longer horizons emphasize long-lived gases.

Applications include national greenhouse gas inventories, corporate carbon footprints, product life cycle assessments, and policy analysis. CO2e is a practical tool for comparing emissions and guiding mitigation priorities, but it is a simplification: it aggregates gases with different lifetimes and mechanisms, and relies on GWPs that are themselves subject to assumptions and updates. See also global warming potential and life cycle assessment.