How do we measure greenhouse gas emissions? Part 1

Before we took a little detour into the world of fossil fuels, we’d talked about what greenhouse gases were and where they came from. Now, let’s talk a little about how we measure the emission of greenhouse gases.

Often you’ll hear greenhouse gas emissions referred to as carbon emissions. That’s for a couple of reasons. One, carbon dioxide is by far the most common greenhouse gas. Secondly, if you remember back we talked about Global Warming Potential (GWP) - which compares other greenhouse gases to carbon dioxide to measure their ability to trap extra heat in the atmosphere over time, with 100 years the most common time period used.

Having GWP ratios for each greenhouse gas means that emissions can be calculated and provided as a single number, rather than breaking down measurements for each individual greenhouse gas. The standard unit used is CO2e, or carbon dioxide equivalent.

For example, let’s look at global greenhouse gas emissions. At this point, because the numbers are so big, we need to introduce one more term - the gigatonne (Gt), which is the equivalent of 1 billion tonnes. According to Our World in Data, in 2021 we released 41.06 Gt of carbon dioxide into the atmosphere (37.12 Gt from fossil fuels and 3.94 Gt from land use change). But if you include all the other greenhouse gas emissions, total emissions work out at 54.59 Gt of CO2e. In other words, those extra gases added the equivalent of 13.53 billion extra tonnes of CO2.

This article from The Guardian sets out the one caveat that should be considered when looking at CO2e numbers:

“The only wrinkle with all this is that 100 years is a fairly arbitrary time frame, and the ratios change significantly if a shorter or longer period is chosen. That's because some gases last much longer in the atmosphere than others. For instance, a tonne of CO2 emissions may warm the planet gently but over many centuries. A tonne of methane emissions, by contrast, creates a strong burst of warming over a much shorter period.

Relatively speaking, therefore, the impact of methane – and the strategic importance of tackling its sources, such as agriculture and landfill sites – depends on whether you're more interested in the next few decades or the next few centuries. Over a period of 20 years, methane's GWP rises to 72; over a period of 500 years, it falls to just 7.6.”