Explainer Series | Measuring air-travel emissions with Radiative Forcing Index

Understanding the interplay between human activities and the earth can help us measure emissions more accurately and take appropriate climate action. One area to look at is the aviation sector, which is responsible for 2.5% of global emissions.

The Radiative Forcing Index (RFI), a multiplier that factors in gases emitted to the atmosphere when flying at altitude, is necessary when measuring air travel-related emissions. Commercial aircraft have transformed the global economy since World War II, and accurately measuring their emissions is crucial to better understanding the footprint of the aviation industry.

Figure 1: the aviation sector is responsible for 2.5% of global carbon emissions, and air travel is a common emission source of many organisations’ inventories. Reference: Microsoft stock imagery

What is the RFI, and how is it used?

Known gross emissions from aircraft are multiplied by an RFI to give a more representative figure. There are two components to consider here:

• Firstly, we have radiative forcing, which involves the balance between the sun’s radiation reaching the earth, and heat leaving the earth from anthropogenic and natural factors.
• Secondly, greenhouse gases emitted at altitude result in more potential heating than if the same amount was emitted at sea level, as higher altitudes affect fuel combustion and emission characteristics.

Accounting for the interaction between these phenomena at altitude requires accurate measurement methods. This is where the RFI comes in.

Often an RFI is only applied if the flight reaches high altitude (25,000 feet or 7.6km). For example, if a plane was on an 800 km flight and its altitude was above 25,000 feet for 250km, the RFI would only be applied to 250km of the flight.

Estimations for the RFI from different literature range from 1.0 to 3.0. However, there is no consistent recommendation on which RFI value should be used. In its programmes, Toitū Envirocare takes a conservative approach and applies an RFI of 1.9 to the entire flight journey. This approach is informed by our current understanding of the science, published literature and international best practice.

Below are examples of different multipliers being applied under voluntary carbon programmes and compliance reporting:

Is this applied to all greenhouse gases or just CO₂?

Different gases have different radiative forcing values, as they have unique characteristics and various effects on the earth’s atmosphere.

The Effective Radiative Forcing (ERF) of different gases is shown in the table below. The impacts of non-CO₂ aircraft emissions at high altitudes received increased attention following a 1999 publication by the International Panel on Climate Change (IPCC) on aviation. The study found that the total climate effects of flying are estimated to be two to four times larger than CO₂ emissions alone.

Table 1: Effective Radiative Forcing for different gases with confidence intervals. Source: Lee et al. (2021)

Is the RFI of 1.9 still relevant?

With Voluntary Carbon Markets becoming more prominent, transparency and accuracy around emission reporting, including the use of an RFI, is essential. The IPCC finding that aviation emissions are more harmful than CO₂ emissions alone is over two decades old. How the radiative forcing effects are weighted and the timescale used (weeks, months, decades etc.) will alter the results considerably. Understanding how multiple factors influence warming at a local to global scale is an ongoing challenge for calculating RFI values.

The RFI of 1.9 stems from the often-cited paper from Grassi and Brockhagen (2007). Furthermore, ICROA notes that:

“If accredited organisations perform any footprinting measurement for air travel, then; they shall publicly disclose and justify (e.g., on their website) what Radiative Force Index (RFI) they apply when calculating air travel emissions.”

With this in mind, we should see more disclosures of RFI being applied to an organisation’s aviation emissions total. Moreover, this outlines the importance of an up-to-date analysis of all literature and any new literature published. This is crucial for inferring if the RFI value being used is appropriate. Without an analysis of the latest literature, we generally see an incomplete and sometimes inconsistent picture. As a result, policymakers are not able to make evaluations and decisions coherently, and emission totals would be being measured with an unwanted degree of inaccuracy.

Toitū Envirocare’s Position

Toitū Envirocare uses RFI adjusted emission factors for all relevant forms of air travel, including:

• Scheduled passenger flights
• Airfreight
• Chartered flights
• Organisations that own or operate aircraft, jet engine aircraft, light aircraft >25,000 ft.

Typically, helicopters do not fly to heights of 25,000 feet, so they have been omitted. Science and our understanding of issues can often change with the improvement of technology and applied methods.

Toitū Envirocare currently uses an RFI value of 1.9 to ensure alignment to leading best practice, including the ICROA code of best practice.

Keeping an eye on international research and the trends of radiative forcing may see this value change, particularly as the planet continues to warm. Toitū Envirocare will continue to observe the science around RFI and adjust our position in line with the most accurate and up to date science. This approach will include observing what values industry leaders use, such as SBTi and ICROA, and their rationale.

References

CarbonBrief website Explainer: The challenge of tackling aviation’s non-CO2 emissions published March 2017, accessed December 2021: https://www.carbonbrief.org/explainer-challenge-tackling-aviations-non-co2-emissions

Carbon Offset Guide website Total Climate Impacts from Aviation accessed December 2021: http://www.offsetguide.org/understanding-carbon-offsets/air-travel-climate/climate-impacts-from-aviation/total-climate-impact-from-aviation/

Climate Care website Calculator Methodology accessed December 2021: https://www.climatecare.org/calculator/calculator-methodology/

Cox, B. & Althaus, H. (2019). How to include non-CO 2 climate change contributions of air travel at ETH Zurich Editorial Information How to include non-CO2 climate change contributions of air travel at ETH Zurich. 10.13140/RG.2.2.30222.10565.

Etminan, M., Myhre, G., Highwood, E. J., and Shine, K. P. (2016), Radiative forcing of carbon dioxide, methane, and nitrous oxide: A significant revision of the methane radiative forcing, Geophys. Res. Lett., 43, 12,614– 12,623, doi:10.1002/2016GL071930.

Grassi, H. and Brockhagen, G. 2007 Climate forcing of aviation emissions in high altitudes and comparison of metrics: an update according to the Fourth Assessment Report, IPCC 2007

Highskyflying website Why are jet engines more efficient at higher altitudes accessed December 2021: https://www.highskyflying.com/why-are-jet-engines-more-efficient-at-higher-altitudes/

International Air Transport Association website IATA Carbon Offset Program FAQ Published April 2020, accessed December 2021: https://www.iata.org/contentassets/922ebc4cbcd24c4d9fd55933e7070947/icop_faq_general-for-airline-participants.pdf

International Civil Aviation Organization website Carbon Emissions Calculator FAQ accessed December 2021: https://www.icao.int/environmental-protection/CarbonOffset/Pages/FAQCarbonCalculator.aspx

Jungbluth, Niels & Meili, Christoph. (2018). Aviation and Climate Change: Best practice for calculation of the global warming potential. 10.13140/RG.2.2.26044.90245.

Lee, D. Fahey D, Skowron, A. et al. 2021 The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018. Atmos Environ. doi:10.1016/j.atmosenv.2020.117834

Ministry for the Environment Measuring Emissions: A guide for organisations accessed January 2022: https://environment.govt.nz/assets/Publications/Files/Measuring-Emissions-Quick-Guide-2020-final.pdf

Murlis, J. 2021 Guidance to Natural Capital Partners on the Treatment of Offsetting for Air Travel in the CarbonNeutral Protocol – Aviation Impact Factor and Sustainable Aviation Fuels/biofuels: accessed January 2021 https://www.carbonneutral.com/pdfs/Murlis_Aviation_Guidance_2021.pdf

Our World In Data website CO2 emissions from aviation accessed January 2022: https://ourworldindata.org/co2-emissions-from-aviation

Ramaswamy, V., et al., 2001: Radiative forcing of climate change. In: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmntal Panel on Climate Change [J. T. Houghton, Y. Ding, D. J. Griggs, M. Noquer, P. J. van der Linden, X. Dai, K. Maskell and C. A. Johnson (eds.)]. Cambride University Press, Cambridge, United Kingdom and New York, NY, USA, 349-416

Sredford website Radiative forcing rationale – bringing together the arguments for the different RF factors accessed January 2022: http://sredford.github.io/co2ohno/RF_rationale.html

Yale University – School of the Environment website: How Airplane Contrails are helping make the planet warmer ­Accessed December 2021: https://e360.yale.edu/features/how-airplane-contrails-are-helping-make-the-planet-warmer

Zhao, Wei & Ge, Yunshan & Li, Jingyuan & Liu, Le & Wang, Yuwei. (2021). Effect of altitude on the emission characteristics of a DI diesel engine. E3S Web of Conferences. 268. 01049. 10.1051/e3sconf/202126801049.