The Key Carbon Emission Numbers You Should Know

“Know your numbers” is a fundamental precept of business – Bill Gates

It is common knowledge that Russia is the largest country in the world. But how large?

The answer is 17.1 million km² [1]. But even with this information are you any wiser?

Without external benchmarks or other numbers to compare it against then it is difficult to quickly comprehend this solitary number.

With some extra information ’17 million km²’ can make sense. For example, Canada, China, and the US all have surface areas of between 9-10 million km². Now, armed with that extra information you are better able to appreciate that Russia is a very large country, nearly double its closest rivals.

Applying Numbers to Emissions

In many ways greenhouse gas emissions represent a similar challenge.

It is common knowledge that some governments and organisations are targeting emission reductions that achieve ‘net-zero’ by 2050 (or sooner) in order to limit global temperature rises to 1.5-2℃, but how much emissions are we producing today? How much carbon is already in the atmosphere?

Armed with these small nuggets of information you will be able to better appreciate emissions data presented in climate analysis, news, scientific papers, or media reports. And crucially judge whether they are meaningful or not.

Units of Greenhouse Gas Emissions – MtCO₂ and MtCO₂e

Emissions of CO₂ are measured in MtCO₂, which simply refers to ‘metric tonnes’ of CO₂. However, carbon dioxide is not the only greenhouse gas and if we want to take into account the effect of all these other gases then the total GHG emissions are measured in MtCO₂e, which reads as ‘metric tonnes of CO₂ equivalent’:

A metric tonne is simply 1000kg. But be aware that ‘Mt’ can also mean megatonnes or a million tonnes. The easiest way to judge if it is ‘metric tonnes’ or ‘million tonnes’ is context and scale. National or industry emissions will be in millions of tonnes, emissions on the level of an individual will usually be in metric tonnes. But if you are unsure delve a little deeper.

Another common unit is GtCO₂, which refers to gigatonnes or a billion tonnes.

‘CO₂ equivalent’ is a bit more complex, as it includes both carbon dioxide and other greenhouse gases, such as methane, nitrous oxide, and sulphur hexafluoride. Some greenhouse gases will cause more global warming than others and this can be measured relative to carbon dioxide.

For example, methane has 28 times the warming impact of carbon dioxide (when measured over 100 years [4]); so if every CO₂ molecule emitted was a methane molecule, then global warming would be approximately 28 times worse.

Therefore to take into account all the different GHGs the warming effect is calculated relative to CO_2. Then the masses of each GHG are scaled relative to their warming effect before the total warming effects of these gases (plus CO₂) are added together.

Key Numbers

Current Global Annual GHG Emissions

Let’s start with the basics, in 2018 the total global CO₂ emissions due to human activity was 37.1 GtCO₂ (gigatonnes or billion tonnes) [2]. As a historical comparison, in 1900 emissions were 1 GtCO₂, in 1950 emissions were 5 GtCO₂, and in 2000 emissions were 25 GtCO₂.

If we include all other GHG emissions (CO₂ + other GHGs) then the total global emissions increases to 51.8  GtCO₂e in 2018 [3]. From this data you can see that CO₂ easily makes up the majority of GHGs.

Annual CO₂ Emissions Per Capita

In 2017 the average CO₂ emissions per person was 4.8 tonnes [4].

However, this overall figure disguises a huge difference between countries, with emissions per person ranging 49 tonnes in Qatar to 0.06 tonnes in the Democratic Republic of Congo.

Of the larger world economies, per capita CO₂ emissions are:

• Australia, US and Canada are around 16 tonnes per person;
• Japan is at 9.5 tonnes per person;
• Western Europe is between 6-9 tonnes per person;
• China is 7 tonnes per person;
• India is just under 2 tonnes per person.

How Much CO₂ is Stored On Earth And Where?

Our planet has a complex carbon carbon cycle, whereby carbon is transferred between natural carbon stores or sinks. Land use changes, human industrial activity, and man-made carbon dioxide are upsetting this cycle and it is important to quantitatively understand how global emissions fit into the wider picture. First we must understand where on earth carbon is stored [5]:

• Atmosphere: 3,000 GtCO₂
• Oceans: 150,000 GtCO₂
• Organic Matter On Land: 15,000 GtCO₂

A Focus on Atmospheric CO₂

Although the atmosphere may appear to contain relatively less CO₂ when compared to the oceans and organic matter on land, the atmosphere contains significantly more CO₂ than before the industrial revolution and the mass burning of fossil fuels.

The pre-industrial atmosphere contained 2,100 GtCO₂. Therefore, 900 GtCO₂ out of the 3,000 GtCO₂ are from anthropogenic sources (i.e. man-made).

Since the start of the industrial revolution man-made emissions have totalled 1,500 GtCO₂, therefore 600 GtCO₂ (i.e. 1,500 GtCO₂ – 900 GtCO₂) has been captured and stored by terrestrial and ocean sinks. But the rest is accumulating in the atmosphere.

This increase is reflected in the atmospheric CO₂ concentration, which has increased 40% from 280ppm to 412ppm [7]. ‘ppm’ refers to parts per million, therefore every million molecules 410 of in the atmosphere are molecules of CO₂.

Conclusion

It’s safe to say that the majority of people are aware of climate change in one way or another. But delve any deeper and the level of understanding varies significantly.

By providing a few numerical waymarkers, emissions and climate-related data can be quickly understood. This is not because the numbers themselves are important but because it allows you to contextualise new information.

If a news report states a new carbon capture method can remove 2 GtCO₂ per year [6]. Then armed with the knowledge that human CO₂ emissions are just under 40 GtCO₂, then you can quickly assess that method. A 5% per year CO₂ emissions reduction is quite significant.

The numbers described above are one key tool in evaluating the information surrounding climate change as well as the effectiveness of different methods to reduce its impact.

See more information on the numbers surrounding carbon emissions see Our World In Data.

References

1. https://www.worldometers.info/geography/largest-countries-in-the-world/
2. https://www.carbonbrief.org/analysis-fossil-fuel-emissions-in-2018-increasing-at-fastest-rate-for-seven-years
3. https://www.pbl.nl/en/publications/trends-in-global-co2-and-totaal-greenhouse-gas-emissions-summary-of-the-2019-report
4. https://ourworldindata.org/co2-and-other-greenhouse-gas-emissions#:~:text=Summary,year%20%E2%80%93%20this%20continues%20to%20increase.
5. The Royal Society, Greenhouse Gas Removal, 2018
6. Beerling, D.J., Kantzas, E.P., Lomas, M.R. et al. Potential for large-scale CO₂ removal via enhanced rock weathering with croplands. Nature 583, 242–248 (2020). https://doi.org/10.1038/s41586-020-2448-9
7. https://climate.nasa.gov/news/2915/the-atmosphere-getting-a-handle-on-carbon-dioxide/