One of the most popular exhibits in Miraikan’s exhibition “Planetary Crisis” (5^th floor) is the display “Let’s compare the CO₂ emissions by country and region”. It shows 10 different countries and regions, including Japan, Germany and sub-Saharan Africa, and consists of wooden balls in nets that represent a country or region’s carbon dioxide (CO₂) emissions. The number of balls reflects the number of people, and the size of the balls represents the amount of CO₂ emissions per 2.5 million people. But how can we actually know how much CO₂ and other greenhouse gases* a certain country or region is emitting?

*See ‘Are “CO₂” and “greenhouse gas” different?’ in the box below.

Choosing the method

If we would measure the amount of greenhouse gases in the air with satellites, we would have no idea of the origins of those gases. Plus, there is no way we could attach greenhouse gas meters to all factories, pastures, cars, etc., to directly measure emissions. So, if we want to know the emissions of a single country, rather than directly measuring emissions, we need to do our best to make a good estimate instead.

When making an estimate, the first thing to consider is what to include in the estimate. Do you want to know about all greenhouse gas emissions or just those caused by humans? Or maybe limit your estimate to only CO₂? And if a plane flies from one country to another, which country should consider the resulting emissions theirs? Depending on what exactly you want to know, the estimation method will change.

For example, the display in the exhibition “Planetary Crisis” mentioned before is limited to only CO₂ (no other greenhouse gases), emitted during cement production and burning of fossil fuels. On the other hand, a project called the “Global Carbon Project” aims to map the world-wide carbon cycle (carbon is the C in CO₂, an element that can be found in a lot of other materials as well), so they include not only CO₂ emissions caused by humans, but also those caused by nature. Another frequently used method for emissions estimates, by the United Nations Framework Convention on Climate Change (UNFCCC), was made for the purpose of helping to reduce global warming due to human activities, so it includes not only CO₂ but also various other greenhouse gases. At the same time, it excludes emissions that were not caused by human activities.

As an example of how to estimate greenhouse gas emissions, let’s have a look at the UNFCCC’s method of estimating emissions. Their older version, the “IPCC Revised 1996 Guidelines for National Greenhouse Gas Inventories” is used by many developing countries. The newer version, the “IPCC 2006 Guidelines for National Greenhouse Gas Inventories” is used mainly by developed countries. The 2006 guidelines let countries make more accurate and precise estimates than the 1996 guidelines, but the basic way of thinking is largely the same for both guidelines.

Doing the calculations

Actually, the most basic principle for calculating greenhouse gas emissions is simple. Just find out what activities cause greenhouse gas emissions in your country (the guidelines will tell you what activities to include), then multiply the amount of these activities with an “emission factor”. The emission factor is the amount of a greenhouse gas emitted per unit of activity.

For example, let’s say your country generates 5 gigajoules of energy by burning sub-bituminous coal (a type of coal used for power generation) to generate electricity. This will result in greenhouse gas emissions. To calculate the amount of greenhouse gas emissions, you would multiply the amount of power generated by the emission factor (emissions per joule) for burning sub-bituminous coal for power generation. The IPCC offers several emission factors for sub-bituminous coal in its database. In this case, I will use an emission factor for CO₂ that was determined by actual measurements in South Korea: 96,241 kg CO₂ per gigajoule. Then the calculation will be 5 gigajoules x 96,241 kg CO₂ per gigajoule = 481,205 kg CO₂, so about 481 tons of CO₂ emissions.

In this way, you can estimate yearly greenhouse gas emissions for all activities that cause greenhouse gas emissions, and for all types of greenhouse gases one by one (the 2006 guidelines include more greenhouse gases and more types of activities than the 1996 guidelines). However, this way of calculating results in rather rough estimates. That is because even if you use the emission factors IPCC provides, the situation in your country may differ from the circumstances on which the IPCC value is based. In other words, in the example above I used a value measured in South Korea, but if other countries’ power plants work differently, the actual amount of CO₂ emissions will also differ.

This is why the guidelines encourage countries to do their best to better reflect the reality of their specific situation. For example, they can measure the emissions amount per unit of fossil fuel from their own power plants, rather than using default values provided by the IPCC. That way the resulting estimates will be closer to the actual emissions.

Adding up the results

Once you have calculated all greenhouse gas emissions, you will end up with a list of activities and emissions of different greenhouse gases. Then you can add up the results for all activities per greenhouse gas. For example, to find your country’s total CO₂ emissions, you add up all activities’ CO₂ emissions; for all activities emitting methane (another greenhouse gas), you add up all your country’s methane-emitting activities’ methane emissions; etc.

However, the amount of global warming differs per greenhouse gas. So, if you want to get a general overview of your country’s emissions, you cannot simply add up the amounts of different greenhouse gases. That is why “global warming potentials” (GWPs) are used. A greenhouse gas’ global warming potential expresses the amount of global warming it causes, compared to CO₂. For example, methane can trap 27-29.8 times more heat than CO₂ over a 100-year time span. In other words, its global warming potential per 100 years is 27-29.8. So, if 1 kg of methane is emitted, it can also be written as CO₂eq 27-29.8 kg. The “eq” in “CO₂eq” is short for “equivalent”, meaning something like “equal to”.

Using global warming potentials, you can convert the amounts of different greenhouse gases to CO₂ equivalents. Then you can sum them up, and finally present a single number as your country’s greenhouse emissions.

Conclusion

From choosing your methods and what to include or exclude, to calculating emissions per activity and summing up the results, you can see that estimating greenhouse gas emissions requires quite a lot of effort and calculations. Even so, people around the world are making these efforts, to keep track of how much we still need to do to reach net zero emissions and stop global warming. These efforts and calculations support the global effort to keep greenhouse gas concentrations below dangerous levels. Ultimately, this will help protect our lives from global warming.