CO2 Equivalents (CO2e) Explained Simply

Since global warming became impossible to ignore in the 1990s, international policymakers have looked for a common “currency” to make very different greenhouse gases comparable. The result was CO2 equivalents (CO₂e) – the metric that is now used almost as a matter of course in climate reports. But what exactly is behind it, why does the Kyoto Protocol play a key role, and which gases must be included in every modern carbon footprint?

1. From the Kyoto Protocol to CO2e logic

When the Kyoto Protocol was negotiated in 1997, it quickly became clear that a pure CO2 budget falls short: methane warms the atmosphere about thirty times more than carbon dioxide, nitrous oxide many times more still.
Kyoto therefore created the concept of the “Kyoto basket” – a basket of six regulated gases whose effect is projected onto a common scale. These include carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O) and three groups of fluorinated gases: hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulphur hexafluoride (SF₆).

Each of these gases was assigned a Global Warming Potential (GWP) by the Intergovernmental Panel on Climate Change (IPCC) – a factor describing how much energy one kilogram of the gas traps in the atmosphere over 100 years compared to one kilogram of CO2. Multiplying the mass of a gas by its GWP gives its equivalent in CO2 equivalents.
This makes it possible to add up, compare and politically cap completely different sources – from steelworks to refrigeration plants. The current reference is the IPCC’s Sixth Assessment Report (AR6), which, for example, gives fossil methane a GWP100 of 29.8.

2. Which gases must be included in a carbon footprint today?

The Kyoto Protocol obliged industrialised countries to record all six of these gases. This obligation has been adopted in almost all climate standards – from ISO 14064 to the Greenhouse Gas Protocol. In 2012, the Doha Amendment also added nitrogen trifluoride (NF₃), which is used in electronics manufacturing.

For companies, this means: a complete CO2e footprint must consider at least these seven gases, provided they occur in relevant quantities. Anything else would be incomplete – and in some jurisdictions already inadmissible. The often neglected F-gases (HFCs, PFCs, SF₆, NF₃) in particular can have a disproportionate climate effect despite their low mass; SF₆, for example, has a GWP100 of over 25,000.

* NF₃ was added to the Kyoto basket in 2012 with the Doha Amendment and is equally mandatory in ISO 14064-1:2018 and the GHG Protocol.

3. Why CO2e is more than just a calculation aid

• Policy steering variable: under Kyoto, countries committed to reducing their total emissions in CO2e; later, the EU ETS, CBAM and national CO2 taxes adopted the same logic. Without CO2e, such a cap would hardly be technically feasible.
• Corporate management: investment decisions can now be made against a clear budget: does switching to renewable process heat pay off if it saves 2,000 t CO2e and avoids emission allowances?
• Financing advantages: sustainability-linked loans tie their interest margin to verifiable CO2e reductions; green bonds must show how much CO2e is avoided per euro invested.

4. Industry example – a possible scenario for a beverage can manufacturer

To illustrate the effect of CO2 equivalents, let us take a fictitious company: the annual inventory of this example plant includes:

Although the SF₆ leakage amounts to just 600 grams, in CO2 equivalents it accounts for around 15 t CO2e – almost a third of the total balance. The Kyoto logic thus reveals a hidden hotspot that would remain concealed in a pure CO2 view.

Strategic lesson from the theoretical case:

1. Replace F-gases first – replacing the shielding gas eliminates over a third of total CO2e with comparatively little effort.
2. Close methane leaks – small investments in wastewater treatment deliver a quick climate effect.
3. Electrify process heat in the long term – after the “low-hanging fruit” of SF₆ and CH₄, the focus shifts to decarbonising the 2,000 t CO₂ from natural gas.

This example shows how only a complete CO2e footprint makes all seven Kyoto gases visible and directs investments to where they have the greatest impact.

Conclusion: looking ahead

Thanks to the Kyoto basket, we know today that climate protection is more than decarbonising fossil energy. Those who record their CO2 equivalents completely – including potent F-gases – gain not only a credible carbon footprint but also the basis for strategic investments and better sustainable finance conditions. That is the lasting legacy of the Kyoto Protocol – and the reason why every net-zero pledge is ultimately measured in CO2e.