Contrails from jet engines can add to the warming effect of flying
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If you’ve been using carbon footprint calculators to work out the impact of the flights you take, brace yourself. The true impact of your flying could be several times higher than commonly used CO₂ calculators suggest.
“The numbers are shocking,” says Jhuma Sadhukhan at the University of Surrey in the UK. She and her colleagues compared the figures from four established calculators with one they created.
For instance, suppose you fly first class from Singapore to Zurich on a B777 plane. At the time the study was done, the International Civil Aviation Organization (ICAO) and International Air Transport Association (IATA) flight calculators suggested this would produce the equivalent of around 3000 kilograms of carbon dioxide. The Google Travel Impact Model (TIM) said around 5000 kg while MyClimate put it at around 8000 kg. But according to a flight emissions calculator created by Sadhukhan and her colleagues, the actual number is more than 14,000 kg.
“The impact is considerably higher,” says team member Eduard Goean of the University of Surrey and Therme Group, a resort company based in Austria.
The new calculator, called the Air Travel Passenger Dynamic Emissions Calculator (ATP-DEC), differs from existing ones in two ways. Firstly, rather than assuming a flight takes the ideal route, it uses past flight data to estimate the most likely route, flight time and time spent taxiing before take-off and after landing, as well as how full an aeroplane is likely to be.
Unlike other calculators, this one is dynamic in the sense that the past flight data is continually updated, says Goean. For instance, many flights are taking longer routes because of Russia’s war on Ukraine – other flight calculators still don’t take account of this, the researchers say.
The second key difference is that ATP-DEC takes account of all the known ways in which flying can affect the climate, including the formation of contrails, nitrogen dioxides and water vapour. Contrails, for instance, can have a greater warming effect than the CO₂ emissions from an aircraft.
Other flight calculators either don’t include these factors at all, or use an averaged value. “They don’t vary with the aircraft, or the fuel conditions, or the external conditions,” says Sadhukhan. “Ours is more comprehensive.”
Goean says the team will be making their calculator available to others, as well as producing an app that will come out early next year. “If an airline company wants to integrate ATP-DEC, we can start tomorrow,” he says. But it could take weeks or months to set up the necessary data feeds.
Flight emissions calculators sometimes offer passengers the chance to voluntarily pay a small fee to “offset” their emissions. However, some studies have concluded that many offsetting schemes do not deliver on their promises.
A spokesperson for ICAO said the methodology behind the ICAO Carbon Emissions Calculator can be found on its website. “The calculator does not quantify the climate change impact of aircraft emissions using the Radiative Forcing Index [a measure that accounts for non-CO2 gases] or other such multipliers, as scientific consensus has not yet been reached,” they said in a statement.
“The TIM is an accurate, transparent, and free resource to help consumers choose less-emitting flights,” says Dan Rutherford at the International Council on Clean Transportation, a nonprofit that helps advise Google on how to refine its CO₂ calculator. “We continue to improve the model, including the incorporation of short-lived climate pollutants like contrails, in order to maximise its usefulness to the flying public.”
“We appreciate this study as a valuable addition to the ongoing discussion,” says Kai Landwehr at MyClimate. Uncertainties about the warming effects of, say, contrails mean no method can claim to be absolutely right, but using better and more up-to-date data will enhance accuracy, Landwehr says. “We are planning an update to our calculator in the coming couple of months and intend to incorporate the best practices and current knowledge highlighted in this study.”
IATA was also contacted for comment.
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