Why are carbon emissions measured in CO2e and what does it tell us?

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Greenhouse gases (GHGs) keep the planet warm by trapping solar energy that would otherwise radiate through the atmosphere back out to space.

In 2020 through burning fossil fuels the UK emitted 414.1 million tonnes / CO2e into the atmosphere.

As human activity continually increases the level of atmospheric greenhouse gases, we are exacerbating the warming of global average temperatures — resulting in what’s commonly known as global warming.

What isn’t common knowledge is how we measure the carbon footprint of GHGs.

To do this, climate scientists convert the amounts of individual GHGs to a CO2e equivalent.

In this article, we will explain what CO2e is and how it’s used to understand the atmospheric warming that GHGs create.

What does CO2e mean?

CO2e stands for carbon dioxide equivalent. It’s the standard measurement of GHG emissions in terms of the most common GHG, carbon dioxide (CO2).

CO2e tells us the amount of CO2 needed to produce the same amount of warming that the other GHGs create.

When referring to the total amount of global warming, CO2e allows us to quote a single figure instead of the individual warming created by each GHG.

CO2e is, therefore, a collective measurement, and is calculated using each greenhouse gas’ global warming potential.

Overall, it is a much easier metric to measure how bad an activity is, rather than having multiple metrics for the same activity which becomes confusing.

Global warming potential

The global warming potential (or GWP) of a GHG is a number given to represent the extent to which the gas warms the planet’s atmosphere over a period of 100 years.

CO2 has a GWP of 1 and nitrous oxide (N2O), another common GHG has a GWP of 298. All GWPs are relative to the GWP of CO2, so over a period of 100 years, one kilogram of N2O traps 298 times more solar energy than one kilogram of CO2.

Global Warming Potential of The Main Greenhouse Gases
GasChemical formulaGWPYears in atmosphere
Carbon dioxideCO21100
Nitrous oxide N2O298121
MethaneCH42512
Chlorofluorocarbon-12(CFC-12)CCL2F210,200100
Hydrofluorocarbon-23(HFC-12)CHF312,400220
Source: IPCC

What GWP means is that some greenhouse gasses are much worse than others! For example, while Methane will degrade much faster than CO2, it also has a warming impact 25 times greater than CO2.

When coming up with a CO2e figure, climate scientists at the Intergovernmental Panel for Climate Change (IPCC) analyse the total warming potential of all GHGs by combining their individual GWPs.

Based on the WP of CO2, they then determine how much CO2 is needed to produce the same amount of warming as all other GHGs combined.

In this article Stanford University explains how GWP is calculated.

Source: Unsplash

CO2 vs CO2e

The terms CO2 and CO2e have been thrown around a lot so it’s important to understand the difference. Put simply, CO2 measures only carbon dioxide, whereas CO2e includes all greenhouse gases. 

CO2 is the chemical formula for carbon dioxide. Making up roughly 56.4 percent of atmospheric GHGs and 80 percent of human carbon emissions, carbon dioxide is the most common GHG. Any measurement of CO2 refers to carbon dioxide alone.

CO2e is the collective measurement of the amounts of all atmospheric GHGs in terms of carbon dioxide.

By analysing global warming in terms of CO2e, scientist say:

If the only GHG in the atmosphere was carbon dioxide, there would need to be X amount of CO2 to warm the planet to the same temperature that all of the greenhouse gases together do.

What does CO2e do?

By creating a common measurement, CO2e simplifies the way in which we think about the warming effect of GHGs.

A simple example of this is two glass jars; one filled with ten units of a mixture of GHGs, and the other is empty.

Both jars are placed in the sun and the temperature inside the jar containing the mixture of GHGs is recorded.

The empty jar is then filled with the number of units of CO2 that will heat that jar to the same temperature as the other jar.

Now there are two jars with the same temperature, only one contains ten units of a mixture of GHGs and the other a number of units of just CO2.

Although the temperatures of the jars are the same, because each GHG has a unique global warming potential, the total number of units of gas inside each jar is different.

The CO2e measurement is the amount of gas inside the jar that only contains CO2.

This is exactly how climate scientists think about the atmosphere and global warming.

If they didn’t, they would need to explain the warming effect of each individual GHG, making the topic much more complex.

CO2e and SMEs

CO2e is a way of looking at carbon emissions in layman’s terms.

By digging a little deeper, you’ll find that along with absorbing solar energy at different rates, GHGs also remain in the atmosphere for different amounts of time. This makes higher atmospheric concentrations of certain GHGs extra detrimental to life on Earth.

Finding out which GHGs and the amounts emitted is an important step for SME owners to take in reducing business carbon emissions.

The most important thing to remember is that CO2e is a more complete measurement of your environmental impact as it includes all greenhouse gas emissions. When making decisions, make sure you’re using CO2e and not CO2.

Use this government guide to get started.

Alan Palazon
Author: Alan Palazon

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