STRENGTH IN NUMBERS

The make-up of methodologies

Carbon emissions methodologies come in three ‘flavours’ and levels of accuracy, according to Thrust Carbon (see table below): those based purely on business travel spend, those based on distance travelled or the number of hotel nights, and those based on fuel burn or ‘actuals’.

“Each emissions methodology has its strengths and weaknesses, and the choice of which to use depends largely on the specific goals, resources and data availability of the organisation,” says Horst Bayer, founder of business travel sustainability consultancy TravelHorst.

Spend-based methodologies are relatively easy to implement, require straightforward calculations and can be applied broadly, but are the least accurate option, says Bayer. Those that use distance travelled as their foundation are better for reporting and are the most popular among corporates, he says.

Despite the plethora of methodologies available, Park says DEFRA is useful “for setting a strong baseline and is essential in understanding both your emissions overall and any increases or reductions over time.”

The standard takes distance travelled and the cabin class in which a passenger flies as its core factors, together with a multiplier for radiative forcing which accounts for the non-CO2 impact of emissions (more on that later).

Many countries have similar methodologies that were domestically developed – such as France’s ADEME and the United States’ EPA – and then there are those considered more advanced, or granular, such as those of ICAO, IATA (CO2 Connect), TIM (the Travel Impact Model), and, a newcomer, Cirium’s Emerald Sky methodology which was launched in May this year. Advito, the consulting arm of BCD Travel, even has a proprietary methodology, Gate4.

These methodologies vary in complexity, considering factors like aircraft type, seat configuration, flight departure date and time, passenger load, and cargo allocation.

Despite advances in emissions calculations, there remains a place for the simplest, spend-based methodologies. “It’s very inaccurate but if all you have is the spend data – someone’s expensed a flight but provided no details – then these methodologies have a plus point,” explained Kit Aspen, founder of sustainability data specialist Thrust Carbon, on a recent webinar.

“DEFRA is good, but ‘actuals’ are the best because of all the differences that might occur,” he added. For example, one airline might fly a slightly longer route than another or arrive at a different time of day when it is more likely to circle an airport. Or, with more empty seats, emissions would be split among fewer passengers. And if an aircraft is carrying freight, that cargo is responsible for a portion of the flight's emissions.

“When you start looking into all that, then you end up with what's called a fuel burn calculation. These are the most complex calculations,” said Aspen. “They typically have 10 or 20 completely separate variables coming together and, in general, it's where the world wants to get to.

“If you want to make intelligent decisions about your travel programme, then you're going to be using actual data. Because if you want to know if airline A is more sustainable on a key city pair than airline B, then you need to know which planes those airlines are flying. What does their operational efficiency look like? How full are those planes?”

Sustainability specialist Squake, which helps companies track their travel-related carbon emissions, and compensate for them, offers some 30 different methodologies to its TMC, tech and corporate customers.

“Companies have their reasons for choosing a particular one and often it’s their ‘native’ methodology as reporting standards and penalties are applied in country-specific ways,” says Squake co-founder and CEO Philipp von Lamezan.

He also notes that even if your supplier, TMC and a third-party are providing you with emissions data using the same methodology, there can still be discrepancies because of the way, for example, that distance is measured, such as between airports or between city centres. “We often have situations where we need to synchronise the data between us and the corporate or TMC,” he says.

Predicting and reporting

Several experts with whom BTN Europe spoke stressed that emissions methodologies have different purposes at the point-of-sale (when searching and booking travel), and post-trip (for reporting purposes) – and furthermore that emissions figures displayed in booking tools are estimates, albeit of varying accuracy.

The former can be used to steer travellers towards more fuel-efficient trips – this could be because of more modern aircraft typically deployed on the route or higher historic load factors, for example.

“Pre-travel CO2 estimates can lead to more responsible choices and support less GHG emitting travel options. Clients should be able to rely on their TMC to provide this visibility,” says Pippa Ganderton, product director of travel management company ATPI's Halo sutainability solution.

For this, a more granular methodology must be deployed in the booking tool. That’s because the DEFRA methodology, for example, applies the same CO2 emissions figure to all flights on the same route, which is of little use when trying to influence travellers’ decisions.

“Granular emissions methodologies are more effective at influencing traveller behaviour at the point of sale,” says Park. “By providing detailed comparisons between different flight options, these methodologies enable travellers to make more informed decisions that favour more fuel-efficient and sustainable suppliers.”

Park’s company, Parexel, has had its SBTi targets validated and currently uses the DEFRA methodology for emissions reporting but is currently working on a project that would allow it to apply different methodologies based on the availability of data by country and transportation type. It is already using alternative methodologies during the search and book process to influence travellers' decisions.

Von Lamezan echoes Park's comments: “To have influence on your emissions you need to move into the pre-booking space.”

Nevertheless, even the most sophisticated pre-trip, point-of-sale emissions figure will only ever be an estimate. “A prediction can only ever be so good – a change of aircraft, one less passenger, the weather… they all have an impact on a flight’s actual emissions. We shouldn’t be expecting predictions to be able to reflect that,” says von Lamezan.

It is in the post-trip calculation and reporting of emissions where accuracy is most important, for granular methodologies enable companies to track their carbon footprint, set reduction targets and measure progress towards them.

“Even something like having two or three more people onboard shifts the distribution of emissions by one or two per cent on a Boeing 737,” says von Lamezan, by way of example.

 

The importance of RFI

Radiative forcing captures the full climate impact of aviation emissions, including non-CO2 effects “such as contrails and nitrogen oxides which are significant contributors to global warming,” says Park. As a result, it’s common practice to apply an ‘RF’ multiplier to CO2 emissions to account for these additional impacts, though there is ongoing debate about the exact value of this multiplier.

“There's lots of work being done at the moment to try and find that [exact figure] out, but we all know it exists and we all know that it probably multiplies emissions by a factor of two or three, and that it absolutely is best practice to include it when calculating carbon emissions,” says Ruggles-Brise.

“As somebody responsible for my company's emissions reporting, I don't want to be labouring under the false impression that our emissions are a third of what they actually are,” she adds.

Her recommendation is to apply a radiative forcing index (RFI) in your emissions calculations and to do so sooner rather than later: “If you’re not including it now, you will need to in the future so start planning for it.”

Squake’s von Lamezan concurs, noting from his experience that while some companies apply an RFI multiplier of x3, others apply none at all, and that a generally accepted figure is x1.7. “If you’re not applying a multiplier you need to have a strong rationale for that,” he says.

ATPI's Ganderton agrees that a radiative forcing multiplier should be included as standard, adding: “If auditors are to be comparing apples with apples this is one criteria that should be mandatory. Emissions at altitude should not be ignored, based on the science available to support the impact of aviation on radiative forcing.”

Looking ahead

Emissions methodologies for business travel are evolving rapidly while the number on the market simulaneously continues to grow, but not everyone sees the proliferation of models as a positive thing.

“Everyone is trying to reinvent the wheel with their own more sophisticated methodologies, but it doesn’t make them comparable,” says von Lamezan. “We must decide what is just reinventing the wheel and what is actually progress. We need to consolidate – we need to be able to compare apples with apples.”

With increasingly more companies reporting their scope 3.6 emissions under CSRD from 2025, there are concerns that those figures are indeed incomparable. This could be a cause for concern for companies whose customers increasingly assess them – and their competitors – through a sustainability lens.

However, it has not come to that yet, Ruggles-Brise believes, who says “business travel is just a line item within a line item within CSRD”. In spite of its perceived complexity, corporates should instead take heart that “actually, calculating business travel emissions is not so complicated [compared to other emissions categories] because we have good data and good methodologies that are well defined by the GHG Protocol.”

Nevertheless, the question remains: will the industry ever coalesce around a single solution? Several sources BTN Europe spoke to thought not, but suggested IATA’s methodology and the Travel Impact Model could emerge as market leaders. The latter, however, does not currently include a multiplier for radiative forcing.

That could be resolved sooner rather than later, according to one source, although a statement provided to BTN Europe by Travalyst, which oversees the methodology, was non-committal: “The TIM is a continuously improving model, based on the latest science. The TIM goes beyond just CO2 by accounting for some lifecycle emissions – specifically those associated with the production and transportation of aircraft fuel.”

The statement continued: “The aim is to eventually account for the full climate impacts of aviation as the science matures. Work is taking place within the TIM Advisory Committee to understand how the TIM can meaningfully account for more non-CO2 factors, including but not limited to contrails.”

One challenge that corporates face as emissions methodologies evolve – or when switching the methodology it uses – is that past data is suddenly askew. But there is a solution.

“Changes in emissions calculation methodologies can have a significant impact on a company's historic emissions data,” says Bayer. “To manage this, companies often need to restate previous emissions figures, ensure transparency in their reporting, and adjust their sustainability strategies if necessary. By doing so, they can maintain the integrity and credibility of their reporting, even in the face of evolving methodologies.”

‘Rebaselining’ is the technical term – that is retrospectively applying a new or updated methodology to historical data. “It is one of the realities of carbon accounting,” says Ruggles-Brise.

“It’s all fairly nascent in this area so as a sustainability community we are always trying to improve on measurement and accuracy. When a change is made, you have to recalculate your baseline – that’s [guidance that is] set out in the GHG Protocol which is the governing standard.”

As emissions methodologies for business travel continue to evolve and multiply, the industry faces a critical challenge: navigating the complexity to ensure accuracy and comparability.