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 accounts for the global warming impact of aviation, and to clarify the accounting treatment 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 (including 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 calculating the global warming impacts emissions from aviation

Table 11 below sets out an historic record and forward schedule of rising AIF values to be applied to aviation from the period 2021 to 2030. This is based on independent guidance1 provided by John Murlis, scientific advisor and Advisory Council member to The CarbonNeutral Protocol, and reflects the strengthening scientific evidence that the climatic impact of aviation needs to consider secondary impacts of aircraft on global warming. The Murlis guidance developed in December 2020 concludes that the case for applying an AIF (Aviation Impact Factor) to the carbon emissions to account for secondary effects is now strong enough to revise the approach in the Protocol. It proposes an AIF of 3 as an emerging best estimate and recommends that this should be considered as a target value.

In response to the revised Guidance, clients should consider the evidence for secondary aviation impact, following which they may elect to adopt a value higher than the default AIF. Additionally, the default value will be increased from the starting value of 1.0 in 2020 to 3.0 over the 10 years to 2030 in equal annual increments of 0.2 to allow progressive adaptation to the higher value. This will ensure that by 2030 all clients must apply an AIF of 3 to reflect the direct engine emissions of carbon, the climate forcing impacts of non-carbon engine emissions and other secondary impacts due to flight operation (for example, contrail formation).

Table 11: The Evolution of Recommended and Mandated AIF Factors as Applied to CarbonNeutral Certifications

Interpreting guidance on impacts on climate from aviation into The CarbonNeutral Protocol

Natural Capital Partners first reviewed the science underpinning the impact on climate from aviation in 2009, when it commissioned Professor John Murlis to provide guidance on the issue. The 2009 review 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. To take account of these additional warming impacts, it was recommended that The CarbonNeutral Protocol introduced an “Aviation Impact Factor” (AIF) as a multiplier of the direct carbon emission impacts.” However, the science was not well enough understood to provide clear guidance as to how such additional effects should be calculated. Therefore, The CarbonNeutral Protocol calculated carbon footprints for aviation directly from aviation GHG emissions. Clients were free to apply an AIF of greater than one.

In 2014, John Murlis updated the 2009 guidance. The updated guidance recognised strengthening scientific evidence indicating that the full impact of aviation on climate may be greater, by a factor of two, than from recognised GHGs alone. However, the scientific understanding of the higher factor was still poor to fair, and the evidence for quantifying the effect of contrails, which are a large part of the added impact, was particularly poor. Therefore, for the purposes of CarbonNeutral certifications, The CarbonNeutral Protocol required that clients specify whether or not they elected to apply an AIF of 2 (or any other factor >1) based upon their review of the evidence.

In 2019, John Murlis updated the 2014 guidance, concluding that: “It is now recommended, taking a precautionary view in response to the strengthened evidence and the urgent need to reduce impacts of all kinds of economic activity on the climate system, particularly those showing high growth, that the AIF multiplier of 2 should be considered as a target multiplier, to be adopted over a period to 2025. Clients should be encouraged to continue to take regard of the evidence and to elect to apply higher multipliers in the longer term if in their view the evidence warranted it. The current evidence suggests this would extend to a multiplier of approximately 2.5 to take account of the best estimate of total impact, including currently highly uncertain impacts on cloud processes.”

In 2021, John Murlis updated the 2019 guidance, concluding that: “The new assessment suggests that, mainly following the re-evaluation of contrail-induced cirrus, non CO2 warming should now be considered as a more significant factor in overall estimates of aviation impact on the climate system, approximately twice the value of the CO2 term. This would imply that the overall impact of flight is equivalent to approximately 3 times the CO2 emission. Although this is not accepted as current practice, it may become so in future….”(Natural Capital Partners, 2021, Guidance to Natural Capital Partners on the Treatment of Offsetting for Air Travel in The CarbonNeutral Protocol, link).

The CarbonNeutral Protocol does not immediately mandate an AIF of 3 for three 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, 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.
  3. The CarbonNeutral Protocol’s provision that clients may elect to apply an AIF greater than the default in Table 11 above recognises the voluntary nature of the CarbonNeutral certification, while also encouraging clients to take account of the strengthening case for different accounting for aviation emissions in their carbon management strategies and plans.

Natural Capital Partners continues to keep this issue under review. Specifically, the plans by the International Standards Organisation (ISO) to develop internationally applicable guidance on “Radiative Forcing Management—Guidance for the quantification and reporting of radiative forcing-based climate footprints and mitigation efforts”.

The Murlis 2021 updated guidance in full is available here.

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, the aviation industry, in partnership with ICAO, the International Civil Aviation Organisation, has now, in the light of the UNFCCC Paris Agreement temperature goals of 2⁰C and 1.5⁰C, 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 the reduction targets, with the progressive reduction of the proportion of conventional LHF in use 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. For the short term, SAFs are expected to be in 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.

In use, SAF displaces conventional LHF, replacing the fossil carbon with renewable carbon so that the direct impacts of flight are reduced proportionally to the amount of SAF in the blend. However, the secondary effects of aircraft flight, including impacts of non-CO2 engine emissions and of flight itself (contrails and induced cirrus) are currently recognised as of a similar order to the direct impacts and emerging evidence suggests that future assessment may put them of an order twice the direct impacts of total engine CO2 emissions. This dilutes the direct benefits of SAF by factor of approximately 2 now 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.