What is CO2e and how is it calculated?

What is CO2e and how is it calculated?

The one term you need to know to understand global warming

Olga Rabo

Nov 18, 2020



min read

What is CO2e and how is it calculated?

When we talk about climate change, we often tend to focus on carbon dioxide emissions (CO2) — the most dominant greenhouse gas that comes from burning fossil fuels, industrial production, and land use.

However, CO2 is not the only greenhouse gas that is driving climate change. There are a number of other gases that significantly contribute to global warming, all of which together are quantified in one single metric called CO2e.

In this article, we’ll take a look at what CO2e is in more detail, how exactly it is calculated, and why it is important to do so when measuring emissions.

The Speed Read:

1. CO2 and CO2e are two different things.

2. CO2e is more accurate when it comes to calculating emissions.

3. CO2e allows “bundles” of greenhouse gases to be expressed as a single number.

4. And it also allows different bundles of greenhouse gases to be easily compared in terms of their global warming potential.

Back to basics: understanding greenhouse gas emissions

To understand what CO2e is, we first need to go back to basics and really understand what greenhouse gases are (which are also known as GHS).

Now, here’s the first thing to learn about gases: they absorb heat.

A greenhouse gas, therefore, is a gas that absorbs and emits radiant energy in the atmosphere, thus causing the so-called greenhouse effect on Earth (and also some other planets, like Venus and Mars). Without the greenhouse effect caused by the gases, the temperature on Earth would be much lower, and life in general — much colder (about −19°C on average, in fact). That’s because all gases have a different heat absorption capacity, so together they create that perfect, “just warm enough” environment for our planet to give us all life as we know it. Without the gases, the heat that Earth generates would simply escape through the atmosphere.  

But what happens when the concentration of the gases is too high?

The greenhouse effect:
Warming that results when the atmosphere traps heat radiation from Earth toward space.

Not all of the gases in the atmosphere drive global warming and do so equally. Ozone, for example, is actually good for the atmosphere because it protects us from the ultraviolet radiation from the Sun.

When it comes to having direct impact on climate change, the gases to blame are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and the so-called F-gases: hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF6), and nitrogen trifluoride (NF3).

One thing to remember here is that different greenhouse gas emissions are generated from different human activities. CO2, for instance, enters the atmosphere from burning fossil fuels (i.e. coal, natural gas, and oil), solid waste, trees and other biological materials. Methane, however, is emitted during the production and transport of fossil fuels, as well as from agricultural practices and decay of organic waste. You get the idea.

All those different gases have a different range of warming effects: one tonne of methane does not, for instance, have the same heat absorption impact as one tonne of CO2. Also, methane doesn’t stay in the atmosphere for the same period of time as CO2, as every gas’ capacity for that is different.

For this reason, there’s an individual warming metric assigned to each greenhouse gas called global warming potential, or GWP. GWP indicates the amount of warming a gas causes over a given period of time, which is normally 100 years.

Because each greenhouse gas is unique, each one has its own GWP based on their duration and heat absorption.

OK, but what is CO2e?

So now, back to the original question: what is CO2e?

As officially defined by Eurostat, CO2e (also written as carbon dioxide equivalent, CO2 equivalent or CO2eq) is a metric measure that is used to compare emissions from various greenhouse gases on the basis of their GWP by converting amounts of other gases to the equivalent amount of CO2.

This way, the main difference between CO2 and CO2e is that CO2 onlyaccounts for carbon dioxide, while CO2e accounts for carbon dioxide and all the other gases as well: methane, nitrous oxide, and others. If we calculate emissions based on CO2 only, we’re then ignoring the impact of other gases and, as a result, have an inaccurate picture overall. CO2e combines everything in one and attempts to convert the warming impact of the range of different gases into one single metric.

To exemplify this in easier terms, think of CO2e as if you’re converting several different currencies into one, e.g. EUR.

Because each currency is worth a different amount, you’d get a different equivalent in EUR in return. So, if you have 10 EUR, 10 rubles, 10 dollars, and 10 złoty, after converting it all in the same currency you’d get about 20.83 EUR all together (10.00 + 0.11 + 8.47 + 2.25 = 20.83).

As you might have guessed, to calculate CO2e you need to know the individual value of each gas. This brings us to the next question:  

How is CO2e calculated?

Carbon dioxide equivalents are commonly expressed as million metric tonnes of carbon dioxide equivalents (abbreviated as MMTCDE), but often you can see them in kilos too. To quantify CO2e, you need to multiply the amount of the greenhouse gases by their GWP.

GWP is an index, with CO2 having the index value of 1, while the GWP for all other greenhouse gases is the number of times more warming they cause when compared to CO2. So, 1kg of methane causes 25 times more warming over 100 years compared to 1kg of CO2, which is why methane’s GWP score is 25.

To calculate CO2e, scientists use the GWP value of each greenhouse gas

This way, if 1kg of methane is emitted, it can be expressed as 25kg of CO2e (1kg CH4 * 25 = 25kg CO2e).

Why is it important to account for CO2e?

The first reason why it’s important to count CO2e is simple: transparency.

Seeing a real, more complete picture allows you to get a fuller understanding of where we stand in terms of emitting emissions.

Here’s a simple example.

In 2018, Germany, Europe’s biggest carbon dioxide emitter, released 760,0 million metric tonnes of CO2. But if you include other gases in the equation, the country’s total emissions are actually higher and work out at 854.4 million tonnes of CO2e. In other words, other gases (methane, etc) added an extra 94.4 million tonnes of CO2e — not something to ignore.

Is it crucial to look for CO2e for yet another reason: to know if you’re being greenwashed. You know, something that consumer brands sometimes do (both knowingly and unknowingly).

Have you recently seen a product with a  ‘Climate Neutral’ or ‘Lower CO2 level’ label, a big trend in the consumer industry now? If so, pay attention. Zoom in. Read in. Are you being told the whole story?

A ‘Climate Neutral’ label, for instance, would only mean that the company has offset their emissions instead of reducing them. A cute place to start, but definitely not enough if you’re after a serious change.

“You need to account all emissions rising directly or indirectly from the business”, shares Elina Seppälä, Impact Lead at Cooler Future. “You therefore need to analyze the entire life cycle of your product and your entire supply chain to calculate emissions accurately. Everything counts: from transportation to packaging to the type of energy used when producing the product.”

When you’re calculating emissions, everything counts: from transportation to packaging to the type of energy used when producing the product.


A Swedish plant-based milk brand Oatly offers a climate footprint label on their oat-milk product, displaying a CO2e/kg calculation clearly. In that calculation, they account for the emissions created in the production of oats, emissions from soils and soil fertilizers, use of electricity for tractors and other machines, manufacturing and transportation of packaging materials — the list goes on and on. If a brand is truly on a mission to reduce their climate footprint, they’ll make it easy for consumers to find information on how they do their CO2e calculations and estimations. Hot tip: google everything before you buy. If information is hard to find, it’s probably for a reason.

A few more words on CO2e

Consumers are demanding transparency from brands, but to appreciate that transparency — and to fully understand what goes behind the scenes — education is crucial. For all of us, it’s vital to be constantly expanding our climate change vocabulary, digging deeper and becoming more knowledgeable by asking follow-up questions to brands and companies that claim to be carbon-neutral.