What is the difference between CO2 and CO2e?

Many of our readers and clients often ask, what is the difference between CO2 and CO2e?

Inevitably, they are in the process of calculating their carbon footprint and have realised that the emission factors that they are using are expressed as CO2e.

This article outlines the difference between CO2 and CO2e.

Difference between CO2 and CO2e

Carbon dioxide, or CO2, is a natural, colourless and odourless greenhouse gas that is emitted when fossil fuels (i.e. natural gas, oil, coal etc.) are burnt.

It is the most prevalent greenhouse gas after water vapour and has therefore become the proxy by which we measure greenhouse gas emissions.

However, carbon dioxide is only one of many greenhouse gases that are emitted when humans undertake certain activities. Other greenhouse gases are methane, nitrous oxide and ozone – all of which occur naturally in our atmosphere.

To take into account the emission of other greenhouse gases when calculating the level of greenhouse gas emissions, scientists have devised an equivalent measure – CO2e (which literally means carbon dioxide equivalent).

CO2e allows other greenhouse gas emissions to be expressed in terms of CO2 based on their relative global warming potential (GWP).

CO2 has a GWP of 1, methane has a GWP of approximately 25 (on a 100 year time horizon). In other words, for every 1 tonne of methane (CH4) emitted, an equivalent of 25 tonnes of CO2 would be emitted.

In this way we can express greenhouse gas emissions as an equivalent of CO2 using the GWP principle.

So next time you see a carbon footprint expressed in terms of CO2e, you can know for sure that all greenhouse gases have been included for each activity under scope, and therefore a fuller picture of an organisations impact has been captured.

Note: Most greenhouse gas emission factors are expressed as CO2e. This means you don’t need to make the conversion from methane GWP to carbon dioxide GWP.

Highly Recommended Reading

From a text message to a war, from a Valentine’s rose to a flight or even having a child, How Bad are Bananas? gives us the carbon answers we need and provides plenty of revelations. By talking through a hundred or so items, Mike Berners-Lee sets out to give us a carbon instinct for the footprint of literally anything we do, buy and think about. He helps us pick our battles by laying out the orders of magnitude. The book ranges from the everyday (foods, books, plastic bags, bikes, flights, baths…) and the global (deforestation, data centres, rice production, the World Cup, volcanoes, …) Be warned, some of the things you thought you knew about green living may be about to be turned on their head. Never preachy but packed full of information and always entertaining. How Bad Are Bananas?: The carbon footprint of everything

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