Emission factors are a crucial element in calculating the carbon footprint

Before we get started, let's briefly recall and define what an emission factor is and how we understand it in carbon footprint calculations. An emission factor is a factor used to convert data on consumption or activity (sources of greenhouse gases) into greenhouse gas emissions. These factors are usually expressed as the mass of a pollutant per unit mass, volume, distance or duration of the activity that emits the pollutant. For example, kilogram CO2/liter of diesel fuel burned or kilogram CO2/kilometer traveled by freight train. It follows that for each activity, product or energy consumption, a suitable emission factor needs to be found in order to calculate the resulting carbon footprint. This moment is critical for ensuring high-quality and defensible carbon footprint calculations. Choosing inappropriate emission factors can then lead to distorted, misleading or even false results. And no one wants that, at least not intentionally.

First of all, it is important to note that the emission factor is not just a number, often expressed in carbon dioxide equivalents (CO2e) per unit of consumption or activity. The most widely used GHG Protocol and ISO 14064 standards require the monitoring of emissions of the following six greenhouse gases – carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFC), perfluorocarbons (PFC), sulfur trifluoride (SF6) and nitrogen trifluoride (NF3). This shows that for each emission source, an emission factor needs to be obtained for each of the seven greenhouse gases. To express emissions in carbon dioxide equivalents, the emissions for each greenhouse gas need to be multiplied by its global warming potential (GWP). This is usually used for a time horizon of 100 years (GWP100). Its values ​​are defined on the basis of scientific studies by the Intergovernmental Panel on Climate Change (IPCC).

Currently, the values ​​from the 4th Assessment Report from 2007 (AR4), the 5th Assessment Report from 2014 (AR5) and the latest 6th Assessment Report from 2021 (AR6) are used. The GWP100 values ​​differ in individual reports, sometimes very significantly. Therefore, only data from one Assessment Report should be used, and if the emission factor was created using different GWPs, it needs to be recalculated. This brings us to 14 values ​​needed to obtain one emission factor expressed in carbon dioxide equivalents. They could be expanded to include biogenic emissions of carbon dioxide and methane, sinks and land use, which are, however, reported separately. And this is only the first dimension of the emission factor.

The second dimension is the distribution of the emission factor by life cycle phase. For direct emissions and emissions from purchased energy, the individual life cycle phases are reported divided into several Scopes. The direct component is almost without exception in Scope 1 and 2, while the indirect component is in Scope 3.

To illustrate, let's give an example of burning natural gas in our own boiler room. Direct emissions (Scope 1) include greenhouse gases generated by burning natural gas in our boiler room. To determine the partial carbon footprint of this phase, we need to know the emission factor. However, in order for natural gas to reach the boiler room, it must be extracted, cleaned and transported. This phase of the natural gas life cycle is reported in indirect emissions (Scope 3). And again, we need to know the emission factors for them too. When including this and the previous dimensions, we are already working with at least 28 values ​​for one emission source at once, but some of them may take on zero values. This detailed approach is recommended to be used at least for fuels and energy.

In other cases, such as purchased goods and services, a life cycle emission factor is commonly used. This makes the work easier, but it is only superficial. It is necessary to avoid double counting of some life stages. This is typically the case for upstream and downstream transport. This requires additional knowledge related to the creation of the emission factor used.

A very important dimension, especially related to reporting greenhouse gas emissions from consumed electricity, is the division into two methods, the so-called market-based and location-based. Since this is a fundamental and not entirely intuitive issue, we will discuss it in detail in one of the next articles.

And to make matters worse, for each emission factor, its uncertainty, the source from which it comes or the way it was obtained, its geographical and temporal validity and any other adjustments made to it (e.g. unit conversions) should also be monitored. For fuels, their calorific value for the needs of energy reporting should also be monitored.

From what we have described, it is clear how complex and extensive the set of necessary information is that needs to be known for the emission factor for each source of greenhouse gas emissions. Fortunately, there are a number of databases containing this information or at least a significant part of it. However, choosing the right one is also not easy and several rules should be followed. Their weight will be different for different carbon footprint calculations.

The first important parameter is the size and purpose of the database. When calculating the carbon footprint, it is recommended to avoid using a large number of databases and mixing sources in the same categories. It is necessary to evaluate the complexity and level of detail, i.e. whether the database of emission factors covers the needs of the expected carbon footprint calculations. The operator of a coal-fired power plant will have different requirements for emission factors, different steel producers and different food producers.

Each of them will very likely have to use a different database. The geographical and temporal validity of emission factors is also closely related to this. Here it is also necessary to be able to critically evaluate whether it is possible and appropriate to use emission factors for other geographical areas or old factors. For example, for diesel fuel, using the emission factor valid for Great Britain will not make a major error when calculating greenhouse gas emissions from sources in the Czech Republic. For the year 2025, the difference is only 0.5%. But for example, for electricity generation, crop cultivation or selected industrial sectors, the differences reach tens and sometimes hundreds of percent. Such an assessment of emission factors, or rather the emission factor database, is a highly valued skill.

From a practical point of view, it is necessary to consider the ease of access and use of the emission factor database. Here, the primary deciding factor is certainly whether the database is accessible for free or whether it is a paid version. In the latter case, the amounts involved are not small and the costs of acquiring annual access range from the high hundreds to tens of thousands of euros. For some uses, access via an interface such as an API is also an advantage.

The choice of database should also take into account its credibility, regular updates and availability in subsequent years. The methodology for creating emission factors should also be maintained. If this is not the case, it will be very difficult to make year-to-year comparisons of the same calculations. This may be a problem, especially in the coming years, as access to currently widely used databases such as ADEME or DEFRA will very likely be blocked.

Very extensive and widely used databases are ecoinvent, ADEME and DEFRA. They can also be used for fuels and energy, but in CI3 we prefer national emission factors, which are always more accurate. However, these are often difficult to find or compile. To obtain emission factors from the production and distribution of electricity, it is appropriate to use databases such as IEA or AIB. For purchased services and goods, emission factors for the entire life cycle are very often used. These can be obtained from countless databases of LCA (Life Cycle Assessment) and EPD (Environmental Product Declaration) reports, which are very often managed by manufacturers themselves or industry organizations and associations. In the case of a monetary approach to calculating emissions from purchased goods and services, the aforementioned ADEME or Exiobase database can be used.

It is not possible to recommend clearly when to use which emission source and from which database. As the entire text shows, there are a large number of variables and they have different importance for different calculations. The choice of appropriate emission factors and the management of our own database is very important for us at CI3 and we take it very seriously.