Carbon emission factors: more than just a number

In today’s world, where environmental concerns are at the forefront, understanding carbon footprints has become crucial for businesses. One key element in measuring carbon emissions is the concept of carbon emission factors. Put simply, a carbon emission factor represents the amount of greenhouse gas emissions produced per unit of activity, such as emissions per energy consumption or transportation. These factors play a pivotal role in calculating carbon footprints by providing a standardized measurement that allows us to quantify the climate impact of various activities. In this blog post, we will delve into the significance of carbon emission factors and explore their role in carbon accounting.

Laptop that shows the dashboard of the Carbon+Alt+Delete software with a report, one of its features.

Carbon emission factors encompass more than just a single number. They consist of various dimensions that take into account various aspects of greenhouse gas emissions. Understanding these dimensions is crucial for accurately assessing the environmental impact of activities and calculating carbon footprints. Let us explore the key dimensions of carbon emission factors.

Dimension 1: split per greenhouse gas

Carbon emission factors consider the different types of greenhouse gases emitted during an activity. While carbon dioxide (CO2) is the prototypical greenhouse gas, other gases such as methane (CH4) and nitrous oxide (N2O) also contribute to climate change. The Greenhouse Gas Protocol instructs to include 7 greenhouse gases and groups in a carbon footprint: CO₂, CH₄, N₂O, SF₆, NF₃, hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs), following the Kyoto Protocol convention. Note that there are also other greenhouse gases or groups , but they should be reported out of scope (see dimension 3).

Each greenhouse gas has a different impact on climate change, expressed in its Global Warming Potential (GWP). GWP is a measurement used to compare the potential of different greenhouse gases to contribute to global warming over a specific time period, relative to the impact of carbon dioxide (CO₂). GWP values are based on scientific estimates of the heat-trapping capability of each gas and the length of time it remains in the atmosphere. GWP values are typically calculated for a specific time horizon, such as 20, 100, or 500 years, to provide a standardized comparison. For example, methane (CH₄) has a higher GWP than CO₂ but has a shorter lifespan in the atmosphere. Over a 100-year period, methane has a GWP of 28, meaning it has 28 times the warming potential of CO2.

To illustrate the above with an example, let us look at the emission factor for diesel. Direct emissions from burning a litre diesel are 2,56 kgCO2e. This emission factor can be split into 2,52 kgCO2e from CO2 and 0,04 kgCO2e from N2O.

Dimension 2: life-cycle emissions

The life-cycle approach examines emissions associated with an activity throughout its entire lifespan, from raw material extraction to disposal or recycling. For example, in the case of a product, it considers emissions from manufacturing, transportation, and product use. By accounting for the entire life cycle, emission factors provide a more accurate representation of the environmental impact, enabling companies to identify opportunities for emission reductions across the supply chain.

Applied to fuel and electricity consumption, it implies that part of the life-cycle emissions falls under scope 1 or 2, and partly under scope 3. The direct emissions from burning fuel or generating electricity fall under scope 1 and scope 2, respectively (for companies purchasing electricity, for electricity generating companies the direct emissions from generating electricity are scope 1 emissions). Those direct emissions are also referred to as tank-to-wheel emissions. Besides, there are upstream emissions related to acquiring, processing and transporting the fuel and electricity. Those emissions are scope 3 emissions and are also referred to as well-to-tank emissions.

Let us illustrate the above again with the example of diesel fuel. Diesel has direct emissions or tank-to-wheel emissions of 2,56 kgCO2e per litre (scope 1). On top, diesel has indirect upstream emissions or well-to-tank emissions of 0,61 kgCO2e per litre (scope 3). This adds up to total life-cycle emissions or well-to-wheel emissions of 3,17 kgCO2e per litre.

Dimension 3: out of scope emissions

While carbon emission factors primarily focus on greenhouse gas emissions that are accounted for within the reporting scopes, a complete emission factor also accounts for additional effects that may be classified as “out-of-scope.” These include biogenic emissions, which result from natural processes like decomposition or the combustion of biogenic carbon stocks like wood, as well as emissions from other greenhouse gases not covered in the traditional carbon footprint calculations such as Montreal Protocol gases. Accounting for out-of-scope emissions ensures a more comprehensive evaluation of the environmental impact.

A special note must be made here about the additional climate impact caused by airplanes emitting water vapor and other aerosols at high altitude. This effect—which often incorrectly is abbreviated to the “air travel radiative forcing effect”—is caused by the release of these aerosols, and therefore does not technically classify as a greenhouse gas emission that requires reporting in-scope (see dimension 1). However, because of its importance in the current climate debate, this additional radiative forcing effect is commonly included within reporting scope emissions, especially regarding those from business travel and freight transport.

Let us go back once more to the example of diesel fuel. On top of the total life-cycle emissions of 3,17 kgCO2e per litre, diesel also results in biogenic emissions of around 0,1 kgCO2e per litre. This is due to the fact that diesel is typically a mix of mineral diesel and a small share of biodiesel. The direct CO2 emissions from biodiesel are biogenic and therefore out-of-scope emissions.

Dimension 4: market vs. location-based

This fourth dimension only applies to emission factors related to the purchase of electricity. When calculating emissions associated with electricity consumption, two approaches are commonly used: market-based and location-based. The market-based approach considers emissions based on the average emissions intensity of the electricity mix contract. In contrast, the location-based approach accounts for emissions based on the specific emission intensity of the electricity generation at the country or region where it is consumed. The Greenhouse Gas Protocol requires to report emissions from electricity consumption using the location-based method and highly recommends using both approaches in tandem (i.e., a dual reporting requirement).

Where to find good emission factors?

This brings us to the question of where correct and detailed emissions factors can be found, covering all the above dimensions. At Carbon+Alt+Delete, as part of our emission factor management we rate the quality of emission factor databases on 6 metrics:

  1. GHG Protocol completeness: does the database cover (almost) all 21 GHG Protocol activities?
  2. Emission factor completeness: does the database cover all 4 emission factor dimensions?
  3. Internal consistency: is the database internally consistent in terms of calculation methodologies, definitions, and sources?
  4. Geographical completeness: can the database be used for several countries, beyond the home country of the publisher?
  5. Recency: does the database provide updates on a recurring basis (typically annually)?
  6. Ease of access: is the database available for free and in a structured, machine-readable format?

The figure below gives our scoring of some of the most used emission factor databases in Europe.


By incorporating these dimensions, carbon emission factors provide a nuanced understanding of the greenhouse effect of activities for the reporting organization. They enable you/us/users to identify emission hotspots, prioritize mitigation efforts, and track progress over time. Moreover, these factors facilitate comparisons between different activities and industries, fostering a collective effort towards reducing carbon footprints and mitigating climate change.