Aviation is becoming increasingly familiar with Scopes 1, 2 and 3 of the Greenhouse Gas Protocol, which can be summarised as direct, indirect and supply/value chain emissions. But Scope 4 emissions, often called avoided emissions or positive impacts, are also important to understand — though certainly controversial in concept and in application. We sort the low-carbon wheat from the greenwashing chaff in the latest of our ab initio primer series.
After much work to calculate around Scope 3 emissions and their impact up and down value and supply chains, a further level to consider may not be immediately welcome. But the impact of calculating Scope 4 emissions is likely to be substantial, especially when driving change in industries difficult to decarbonise, like aviation — although it still remains somewhat nebulous, and calculations are complicated.
In its widest definition, Scope 4 is about avoided emissions, a calculation made on the basis of an organisation making products that generate fewer or lower-impact emissions, are more energy efficient, or otherwise better for the environment.
PwC defines Scope 4 as “reductions that occur outside of a product’s life cycle or value chain, but as a result of the use of that product”, but is clear that, as yet, “unlike scope 1-3 corporate and financed emissions reporting which follow clear standards under the GHG protocol and the Partnership for Carbon Accounting Financials (PCAF), there [are] no officially recognised agreed standards for the measurement and reporting of avoided emissions.”
In the aviation sense, the concept might be most readily imagined as the step change reductions in aircraft emissions per seat-kilometre within a comparative airframe market: the shift from a Boeing 737NG to a 737 MAX or an Airbus A320ceo to an A320neo.
Crucially, it’s important to understand that these reductions might be included as Scope 1 for an airline, but Scope 4 for an airframer or enginemaker.
The way that Scope 4 is calculated is thus critical to its use. Questions here include the baseline reference. In the aviation context, this might include the thorny old question of how to measure improvements in fuel efficiency, where airframers or enginemakers tout an impressive percentage improvement over previous generations of, but never define what the previous generation is.
Is this new 737 MAX 8, say, being compared to a 737-800, a 737-400 or an A319, A320 or A321? With or without winglets, and what kind? Which production tranche? Were any in-service efficiency improvements or performance improvement packages applied to the aircraft? How are key industry externalities that change over time, such as more efficient ATC routing, optimised descent profiles, or fuel calculations optimised for weight, being taken into account?
The World Resources Institute offers a framework, Estimating and Reporting the Comparative Emissions Impacts of Products, but without specific reference to Scope 4 as a concept.
The Science Based Targets initiative, meanwhile, recommends discounting avoided emissions [PDF, p31], but its definition of “avoided emissions” as “positive impacts” does not entirely correlate to some definitions of Scope 4.
Yet discounting Scope 4 risks diminishing the impetus for complex systems and difficult-to-decarbonise industries like global passenger aviation to make some of the most meaningful and impactful improvements to its overall emissions. At the very basic level, it is inherently beneficial that there be some incentive to develop more efficient products with lower environmental impacts.
Scope 4, at its core, currently asks more questions than it answers. As a concept, the inclusion or exclusion of these emissions from targets and reporting is still under contention, but there are persuasive arguments that accounting for them in some way is beneficial. As a result, engaging with Scope 4 within aviation’s work to standardise and report on its environmental impact and decarbonisation initiatives will be increasingly important.
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