The CarbonNeutral Protocol Index

2.5.1 Calculating the climate impact of aviation

The purpose of this guidance is to set out how The CarbonNeutral Protocol considers the global warming impact of aviation, and to clarify the accounting method to be applied to the emerging use of Sustainable Aviation Fuels (SAFs).

How The CarbonNeutral Protocol addresses climate impacts from aviation

The CarbonNeutral Protocol recognises the strengthening scientific consensus that high altitude climate impacts from aviation are greater than the impact of recognised GHG emissions alone. It deploys an Aviation Impact Factor (AIF) as a multiplier applied to the GHG emissions from aviation in order to take account of the wider impacts of aviation on climate. This includes but is not limited to: short and long-term impacts from GHGs alone and others with global warming influence (for example, soot particles and aviation induced clouds); and direct and indirect impacts (for example, the interaction of NOx with methane gases and ozone at high altitudes).

Guidance on accounting for the global warming impact of emissions from aviation

From 2014, The CarbonNeutral Protocol invited clients to consider a precautionary factor, the Aviation Impact Factor (AIF) greater than 1 to more fully reflect non-GHG contributions to global warming, also referred to as Radiative Forcing. In 2021, The CarbonNeutral Protocol introduced the requirement to apply a minimum AIF to all emissions from aviation.

The AIF was introduced and mandated at a time of lack of guidance and clarity in existing standards and frameworks. Whilst the science of impacts of aviation continues to evolve, there is increasing consistency in standards and frameworks for its treatment in corporate GHG accounting. For example, the Science Based Targets initiative removed the requirement to account for non-GHG warming effects in its target setting methodologies. As such, it has become clear that the application of a separate AIF is misaligned.

As a result, The CarbonNeutral Protocol no longer requires an adjustment to emissions from aviation with the application of an AIF. Instead, organisations should consider how conversion factors applied during the calculation of emissions from aviation account for Radiative Forcing.

For example, the emissions factors from the UK Government (UK Government, 2022, Greenhouse gas reporting: conversion factors 2022, link) publish factors for emissions from aviation including Radiative Forcing. The United States Environmental Protection Agency (EPA) published its most recent version of its Emission Factors Hub in March 2023, which bases the aviation-related factors on guidance from the 2022 Guidelines to Defra / DECC’s GHG Conversion Factors for Company Report.

The CarbonNeutral Protocol highly recommends that organisations use emissions factors that include impacts of Radiative Forcing.

Climate Impact Partners first reviewed the science underpinning the climatic impact of aviation in 2009, when it commissioned Professor John Murlis to provide guidance on the issue. The 2009 review, and its subsequent updates (Climate Impact Partners, 2021, Aviation Impact Factor and Sustainable Aviation Fuels/biofuels, link), highlighted that complex atmospheric chemistry associated with high altitude emissions of GHGs, other gases and effects, such as short- lived contrails and cloud formation, supported the view that the impact of aviation on climate may be greater than from recognised GHGs.

The CarbonNeutral Protocol recommends but does not require organisations to account for Radiative Forcing for two main reasons:

  1. The scientific evidence, although strengthening, is still associated with some uncertainty in its ability to take accurate account of the wider impacts of aviation on climate. Although knowledge of the processes at play is strengthening, the scale of impacts remains in some important cases, subject to wide confidence limits. This is particularly the case for impacts of contrail induced cirrus clouds.
  2. There is no publicly accessible record of climate regulations or compliance regimes applying an AIF greater than one for emissions from aviation. The EU’s Emission Trading Scheme for aviation considers only emissions of carbon dioxide. DEFRA, the UK Government ministry responsible for environment affairs, has provided internationally recognised guidance in support of a multiplier factor of 1.9. This factor is not actively applied within UK regulatory programmes, nor to any voluntary action on climate mitigation by the UK Government and its ministries. The aviation sector’s plans for a global carbon offset scheme to ensure carbon neutral growth from 2027 – the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) – also considers only carbon dioxide emissions.

The CarbonNeutral Protocol will continue to review the impact of aviation annually to align to best practice, including the outcomes of the ongoing consultations to The GHG Protocol Standard and related guidance.

Accounting for the use of Sustainable Aviation Fuels

The guidance above is based on the use of the conventional liquid hydrocarbon fuels (LHF) available widely for aviation. However, in light of the Paris Agreement's 1.5⁰C warming target, the aviation industry, in partnership with the International Civil Aviation Organisation (ICAO), has now adopted a set of goals to reduce aviation’s climate impact.

The measures required to reach these goals include operational changes to achieve more fuel-efficient routing of flights, more fuel-efficient aerodynamic aircraft design and changes to the aviation fuels in use. Of these, it is expected that changes to aircraft fuel will produce the greatest contribution to reduction targets, with the progressive reduction of the proportion of conventional LHF used through the introduction of Sustainable Aviation Fuels (SAF).

SAFs come in many forms, including hydrocarbons produced from renewable or waste feedstocks and a range of alternative fuels including hydrogen or electricity. Although both hydrogen and electricity are seen as potentially important fuels for the future, considerable further development is required to engines and airframes before they can be widely used. In the short term, SAFs most commonly take the form of blends of conventional LHF and chemically equivalent fuels processed from waste oils, agricultural wastes and biomass feedstocks that can immediately replace LHF.

SAF displaces conventional LHF, replacing the fossil carbon with renewable carbon so that the direct impacts of flights are reduced proportionally to the amount of SAF in the blend. However, the secondary effects of aircraft flights, including impacts of non-CO2 engine emissions and of the flight itself (contrails and induced cirrus), are currently recognised as of a similar order to their direct impacts — emerging evidence suggests that future assessment may put them on an order of twice the direct impacts of total engine CO2 emissions. This dilutes the direct benefits of SAF by factor of approximately 2 today, but possibly more in future. There are, then, direct Scope 1 gains from the use of SAF, but at current blending levels, they are relatively modest.

While the development and deployment of SAFs is currently limited, its use in commercial flights is growing and expected to increase over time. Clients able to access SAF fuelled flights can account for their impact under the guidance provided in Guidance 2.7, subject to availability of reliable use data and appropriately adjusted AIFs.

Clients pursuing increased deployment of SAFs to reduce emissions from their air travel should make themselves aware of the wider sustainability issues associated with the production of SAFs (see Murlis 2021 guidance – and seek assurances about the adequacy of environmental safeguards applied to the production of SAF feedstocks.

2.5.2 Determining aviation emissions from flight distances

Where exact fuel consumption data is not available for GHG emission calculations, passenger kilometres travelled should be used as a basis for calculation instead. Depending on flight distances, different emissions factors are applicable and are often classified as domestic, short haul, medium haul or long haul. Due to the extreme variability in country sizes, the use of “domestic” classification can be counter-productive when applied to flights within a particular country, using emissions factors provided for use within a different country.

This applies particularly when using DEFRA emission factors for air passenger transport conversion figures in countries other than the United Kingdom.

Therefore, for the purposes of consistency, the following classifications should apply:

  • Short haul: Flight distance of less than 785km (DEFRA emission factors for “domestic” should be applied)
  • Medium haul: Flight distance between 785km and 3,699km inclusive (DEFRA emission factors for “short-haul international” should be applied)
  • Long haul: Flight distances of 3,700km or greater (DEFRA emissions for “long-haul” should apply)

For clarity, these distance classifications should be applied when calculating emissions arising from passenger flights (passenger km) and/or air freight transportation (tonne km). These distance categories must be applied internationally, in the absence of robust, country-specific factors.