Carbon Dioxide Equivalent (CO₂e)
Carbon dioxide equivalent (CO₂e) is the standard unit used to express the combined global warming impact of all greenhouse gases. It converts the effect of each gas into the equivalent amount of CO₂ that would produce the same warming over a specified period, typically 100 years.
What is Carbon Dioxide Equivalent (CO₂e)?
CO₂e — carbon dioxide equivalent — is the universal metric of carbon accounting. Because different greenhouse gases have different heat-trapping abilities and atmospheric lifetimes, a common unit is needed to aggregate and compare them. CO₂e achieves this by expressing every gas in terms of the amount of carbon dioxide that would cause the same cumulative radiative forcing (warming effect) over a chosen time horizon.
The conversion uses global warming potential (GWP) values published by the IPCC. The GWP of CO₂ is defined as 1. Methane (CH₄) has a GWP of 28 (AR5, 100-year horizon), meaning one tonne of methane has the same warming impact as 28 tonnes of CO₂. Nitrous oxide (N₂O) has a GWP of 265. Fluorinated gases can have GWPs in the thousands — for example, SF₆ has a GWP of 23,500 and the common refrigerant R-410A has a GWP of 2,088.
The 100-year time horizon (GWP100) is the standard used in regulatory reporting, the GHG Protocol, SECR, CDP, and the SBTi. Some climate scientists advocate for a 20-year horizon (GWP20) to better reflect the near-term warming impact of short-lived gases like methane. The GHG Protocol permits supplementary reporting using GWP20 but requires GWP100 as the primary metric.
In practice, the conversion to CO₂e is handled by emission factors. When the DEFRA dataset states that burning one litre of diesel produces 2.51 kgCO₂e, that figure already includes the CO₂, CH₄, and N₂O released during combustion, each weighted by its GWP and summed. Organisations do not typically need to perform GWP calculations manually — the emission factor does it for them.
CO₂e is expressed in various magnitudes depending on context: kgCO₂e (kilograms) for product-level or per-unit calculations, tCO₂e (tonnes) for corporate inventories and regulatory reporting, and MtCO₂e (megatonnes) or GtCO₂e (gigatonnes) for national and global-scale discussions.
Practical Examples
A company reports total annual emissions of 8,500 tCO₂e, comprising 7,200 tCO₂e from energy-related CO₂, 800 tCO₂e-equivalent from refrigerant leaks (HFCs), and 500 tCO₂e-equivalent from agricultural N₂O — all summed into a single comparable figure.
A product carbon footprint label states "1.2 kgCO₂e per unit," meaning the total warming impact of all greenhouse gases emitted across the product's lifecycle is equivalent to 1.2 kg of carbon dioxide.
An intensity metric of 45 tCO₂e per £m revenue allows a company to benchmark its emissions efficiency against industry peers, regardless of which specific gases dominate each company's footprint.
How Climatise Helps
Every calculation in Climatise outputs results in tCO₂e by default, using the latest GWP values embedded in the DEFRA and international emission factors. The platform handles all unit conversions automatically — from raw activity data to CO₂e — so your reports are consistent and audit-ready.
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