Deep dive: why emissions numbers change and how to handle emission changes

Explore the detail behind why datasets for carbon calculations are published months after travel, and how your business should handle this.

The key takeaways

This insights post is a technical deep-dive. If you would like a summary, these are the key takeaways:

  1. Data sources for carbon emissions are updated in the months and years after travel, which can lead to your carbon numbers changing if you refresh your data. This is normal and part of the scientific process.
  2. Communicate a ‘reporting date’ so your carbon auditors can clearly understand when your carbon numbers are updated.
    • If you are able to refresh your carbon numbers, we recommend that you refresh on the 2nd of the month in which you start compiling your final carbon emission reporting for that year.
    • If you are not able to refresh your carbon numbers, your reporting date is most likely whenever the final calculation was made against each travel segment.
  3. Have a recalculation/rebaselining criteria for republishing emissions. This is required by GHG Protocol, and we recommend you set 5% of your entire business’s emissions from all scopes as your threshold. If you recalculate a previous year’s emissions and they change less than your rebaselining criteria you do not need to take action.
  4. Use a regularly-updated methodology to ensure you do not have unexpected changes to your emissions that might require a recalculation.


The calculation of travel-related carbon emissions is a complex and ever-changing process that relies on a wide range of data sources. The carbon emission from a given source (e.g. airplane, hotel, car, train), is often sliced and diced based on factors such as the number of people and empty seats/rooms, freight ratios, and seat proportions.

Each of those factors relies on separate data sources or live research. One of the challenges of accurately measuring carbon emissions is the lag in underlying data sources or the evolving nature of the research, which can cause calculated emission numbers to change over time.

To give some examples of how this can change:

  • The ‘load factor’ and ‘freight factor’ of an airline is typically reported to ICAO, IATA or BTS. In almost all cases, this data is held back from immediate release for competition reasons. Some airlines report as recently as 3 months ago, others are 13+ months overdue.
  • The carbon emissions of a hotel room are normally based on ‘averaging’ research by the Cornell Hotel Sustainability Benchmarking Index, which happens annually, or from data directly released by a hotel chain at any point in a year.
  • Seat proportion data is normally calculated using research using IATA, but this is an ‘industry’ average, and as any flyer knows, the size of a seat can differ wildly between airlines. Research will almost certainly change these multipliers over the next few years.
  • The indirectness of flights can vary hugely based on geopolitical factors (e.g. when airspace over a recently declared war zone causes flight diversions).

Ultimately, by adapting to the ‘moving landscape’ of carbon accounting - and working to improve data collection and analysis - we can better understand the impact of our actions on the environment and take steps to mitigate the effects of climate change.

The key dates for carbon emission numbers updating

The lifecycle of a carbon emission in travel has a few key dates where the emissions will update:

  1. At time of booking, theestimated emissions will be based on the most recent available data for the supplier. Some suppliers provide data updates monthly, so this could be an extremely recent calculation.
  2. Any changes to equipment pre-trip (e.g. aircraft type changes), will lead to a newly calculated emission for the new aircraft.
  3. 48 hours post-trip, the first ‘static’ data will arrive. This will normally be data related to the true distance travelled rather than estimated (e.g. if the plane took a non-standard route plan, or needed to circle an airport).
  4. 3 months post-trip, accurate ‘load factor’ and ‘freight factor’ data will arrive for US carriers.
  5. 6 months post-trip, accurate ‘load factor’ and ‘freight factor’ data will arrive for non-APAC carriers.
  6. 12+ months post trip, semi-accurate ‘load factor’ and ‘freight factor’ data will arrive for APAC carriers (some APAC carriers are only publishing average load factors for the year, rather than monthly data).
  7. At any time, the leading research could also be updated. At least once every five years you should expect a research change that may lead to a change in ‘best practice’ methodology. An example of the last time this happened was when DEFRA switched from no radiative forcing, to recommending businesses use radiative forcing, an uplift of 90% in emissions.

How to deal with the uncertainty of emissions factors changing

Quite simply, we suggest choosing a ‘reporting date’ and sticking to it. This provides you with an automatic future-proofed paper trail documenting how your calculations were carried out over time.

Your carbon auditors will be well aware that the best-in-class emissions calculations will change over time and that the Greenhouse Gas Protocol (the most commonly accepted rules for carbon accounting) actively encourages the most accurate possible carbon accounting possible across all emissions scopes, including Business Travel, given the available data and science.

Our suggested reporting date is the second day of the month in which you draw up your final emissions report. You can then publish in your report that your emissions calculation is accurate based on data available on the “2nd [month] [current year]” (e.g. 2nd June 2022).

Other reporting schemes that are also acceptable to carbon auditors include:

  • The emission calculation based on time of booking
  • The emission calculation based on time of invoice
  • The emission recalculated 48 hours after travel
  • The emission recalculated 6 months after travel
  • The emission calculated on the 1st of Jan in the year following travel

Again, the reporting scheme you choose will depend on the technical capabilities of your carbon reporting supplier. For clarity, the Thrust Calculator supports the recalculation of all historic data based on any desired interval, including those examples listed above

Can I use a methodology that doesn’t change?

Some datasets change very rarely (e.g. elements of DEFRA have not been updated since 2018), but almost all will occasionally be updated based on the latest scientific evidence and academic research. Examples include:

  • DEFRA - who currently base their air emissions calculations based on ‘average’ load factor and freight factor data,from 2018 data sources
  • ICAO - who use 2016 datasources if you are using their core datasets
  • Google - use modern datasets for US carriers, but 2019 worldwide averages for other carriers.

You may choose to select a methodology from a specific year in the past and stick with it, but it is important to remember that the resulting calculations will be based upon old data which is not as reflective of your true emissions. This may or may not be acceptable to your auditors.

What if there are emissions changes after my reporting date?

The most commonly accepted rules for carbon accounting are published by the Greenhouse Gas Protocol. These rules include guidance on ‘rebaselining’ which is where you recalculate your comparison year; you should apply the same ‘rebaselining’ logic to all subsequent reporting years too.

The key to ‘rebaselining’ is having a threshold of your entire business emissions which is your rule for triggering a recalculation. For most businesses, this is 5%; so if your overall business emissions would change more than 5% from the original calculation, you should recalculate.

Therefore, understanding how much your business travel emissions need to change in order to cause a rebaseline is crucial. If business travel is 1% of your overall emissions, they would need to increase 500% to cause a 5% change that triggers a rebaseline; whereas for a services company where business travel is 50% of the company’s emissions, only a 10% change to business travel emissions would justify a rebaseline.

Your account manager at the business that provides your carbon calculations should provide guidance for your specific business.

Can updated numbers due to data-source lag trigger a recalculation

Up until the end of 2022, this was a possibility due to COVID-19, but moving forward we think this is unlikely until the next major world-changing event.

Load factors, freight factors, and other data sources changed dramatically month-to-month during the COVID-19 pandemic, and the final numbers are only just becoming known.

Outside of COVID-19, load factors and freight factors typically change by a few percent over the course of a year, and average out on a global scale, which therefore means you are very unlikely to hit your rebaselining criteria.

Companies that are using data that relies on regularly updated methodologies, such as Thrust Carbon, are least likely to be affected by large unexpected changes that could trigger the need to re-calculate.

Confirming you are on the right track

It is important to make sure you are on the right track to accurate emissions. If you are a direct Thrust Carbon customer, our success team would be delighted to help you understand your calculation approach.

If you use us via an intermediary (e.g. a TMC), please reach out to your account manager who will connect with the right person at Thrust Carbon.

If you don’t use Thrust Carbon data at all, please reach to our team. Our Thrust Calculator can help you automatically calculate emissions with best-in-class data, and Thrust Advisory can provide the bespoke services your unique business needs.

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