CO₂ calculation methodologies: Why is there such a big difference in estimates?

Have you ever wondered why CO₂ estimates vary across different calculation methodologies and how to choose the right one for your business? This article will explore different methodologies used to calculate CO₂ emissions and the broader CO₂e (CO₂ equivalent) impacts from aviation, and what factors contribute to each outcome.

Visualizing the Variances: The Emissions Graph

The graph above provides a comparative analysis of various methodologies used to estimate CO₂/CO₂e emissions per passenger based on the distance flown. The vertical axis represents emissions in tonnes of CO₂/CO₂e per passenger, while the horizontal axis shows the distance flown in kilometers. Each line represents a different methodology, highlighting how different factors, assumptions, and approaches can lead to varying emissions estimates for the same flight distance.

9 Key Methodologies Compared

The methodologies presented in the graph differ based on the parameters they consider, ranging from simple CO₂ emissions to comprehensive assessments that account for non-CO₂ effects, such as Radiative Forcing Index (RFI). Below, we delve into the notable methodologies featured:

1. DEFRA (Department for Environment, Food & Rural Affairs - UK): DEFRA uses detailed operational data, including aircraft-specific fuel burn rates and passenger load factors. This approach aims to provide a more granular emissions profile, especially for different types of flights. DEFRA's methodology also incorporates RFI, which accounts for the non-CO₂ effects of aviation, such as water vapor, contrails, and nitrogen oxides.
2. Umweltbundesamt (German Environment Agency): Umweltbundesamt follows a more streamlined approach, often relying on global averages and simplified inputs. However, it also includes RFI considerations to reflect the broader environmental impact. The resulting emissions curve shows a higher trajectory, especially for long-haul flights, compared to methods that exclude non-CO₂ effects.
3. Manatū mō te Taiao (Ministry for the Environment - New Zealand): Similar to DEFRA and Umweltbundesamt, this methodology incorporates RFI. It reflects the policy focus of New Zealand on comprehensive climate impact assessments, aiming to align with stringent environmental standards.
4. EPA (Environmental Protection Agency - USA): The EPA provides a more moderate estimate, focusing primarily on CO₂ emissions without including RFI. This methodology is often used in regulatory contexts where non-CO₂ effects are not mandated for reporting.
5. CO₂ Emissiefactoren (Emission Factors - Netherlands): This Dutch methodology integrates RFI, offering a more comprehensive view of aviation's impact on climate. It is designed to align with national climate policies that emphasize a broader spectrum of greenhouse gases beyond CO₂.
6. EEA/ACC/18/001 (European Environment Agency - Air Climate Change): This methodology reflects the European approach, which heavily incorporates regional regulations and standards for environmental accountability. Its estimates are shaped by EU climate policies and directives.
7. TU Chalmers (Chalmers University of Technology - Sweden): The TU Chalmers approach is characterized by its reliance on global averages and simplified inputs, resulting in a more generalized emissions estimate. It provides a moderate emissions curve suitable for broader, non-region-specific analyses.
8. ICAO (International Civil Aviation Organization): The ICAO method is based on a detailed database of flight and fuel consumption data. It does not include RFI but provides a relatively accurate representation of CO₂ emissions based on real-world flight operations.
9. French Ministry of Ecology: This methodology aligns with European standards and incorporates some elements of RFI, focusing on France's regulatory and environmental context.

Impact of Radiative Forcing Index (RFI)

Some methodologies go beyond just CO₂ to consider the Radiative Forcing Index (RFI), which captures the full climate impact of aviation emissions. RFI ranges from 1 to 3, meaning the overall climate impact is 1 to 3 times greater than CO₂ emissions alone.

This is especially important for long-haul flights, where non-CO₂ effects like contrails and water vapor play a significant role. DEFRA, Umweltbundesamt, Manatū mō te Taiao, and CO₂ emissiefactoren are examples of methodologies that include RFI, leading to higher emissions estimates. DEFRA, TU Chalmers, Manatū mō te Taiao, and CO2 emissiefactoren methodologies incorporate a Radiative Forcing Index (RFI) of 1.7, whereas the Umweltbundesamt methodology applies a higher RFI of 3. These approaches align with more stringent environmental policies and provide a more comprehensive representation of air travel's climate impact.

Choosing the Right Methodology

When selecting a methodology for reporting or compliance, several factors come into play:

1. Local Regulations: Your choice may be dictated by the regulatory environment of your country or region. For example, European standards significantly shape estimates by Umweltbundesamt, EEA/ACC/18/001, and the French Ministry of Ecology.
2. CO₂ vs. CO₂e Reporting: If you're in a region where the methodology isn't strictly regulated, you might need to decide whether to report just CO₂ or the broader CO₂e, which includes other greenhouse gases.
3. Incorporating Radiative Forcing Index (RFI): To account for the full climate impact of aviation, consider choosing a methodology that includes RFI, especially for long-haul flights.

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Understanding these variations is essential for ensuring your reporting truly reflects the full climate impact of air travel. By selecting the right methodology, aligned with both regulatory requirements and your sustainability goals, you can ensure a more accurate representation of the environmental impact of air travel.

Important note: While this graph provides a snapshot of some key methodologies, it is important to note that it doesn't capture every methodology out there. Other significant players, such as IATA, TIM, RDC, Cirium, PACE, GATE4, and others, also contribute to this evolving field.

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