Frontier Airlines is the most efficient carrier in North America and the second most efficient in the world, according to a new emissions calculation methodology developed by the airline industry data company Cirium.
Frontier, with carbon dioxide emissions of 54.4 grams per available seat kilometer in 2024, was beat out only by Hungarian discount carrier Wizz Air, which emitted 53.9 grams of CO2 per seat kilometer last year, according to the Cirium analysis.
As a point of comparison, lifecycle emissions of a simple cup of black coffee are 21 grams of carbon, according to CarbonClick, with emissions sometimes ballooning to many times more than that depending upon cultivation techniques, the type of cup used and whether the coffee is consumed with cream or other additives.
Among the world's 10 largest airlines, as measured by total seat miles offered, Ryanair was the top emissions performer, emitting 63 grams of CO2 per available seat kilometer. The four U.S. carriers among the top 10 largest airlines ranked second through fifth respectively, with Southwest leading the way, followed by Delta, American and United.
Southwest was the clear efficiency winner among those U.S. behemoths, with emissions of 68.9 grams per available seat kilometer. The other three were bunched closely together, emitting between 74.4 and 75.4 grams of CO2 per seat kilometer.
How the ranking is calculated
Cirium joins several entities that have developed methodologies for measuring aviation and airline carbon emissions, including Google, IATA, the International Civil Aviation Organization (ICAO), the U.K.'s environmental department DEFRA and Cirium's competitor, OAG. But Cirium says its platform, called Emerald Sky, is the first to use real-time flight tracking data for its calculations instead of relying on projected or standard routes or forecasted route averages.
To achieve its emissions estimates, the platform also considers actual air time and atmospheric conditions of a flight; aircraft and engine types; aircraft age; seating configurations; passenger loads; cargo loads; and taxi times.
An airline's usage of sustainable aviation fuel (SAF) is not yet considered in the Emerald Sky formula because SAF accounts for only a tiny fraction of total jet fuel. Cirium chief strategy officer Kevin O'Toole said the company will include SAF in the algorithm once its accounts for 1% of global fuel consumption, which is about double its current contribution.
Cirim said the methodology had been independently reviewed and validated by PricewaterhouseCoopers and by the Rocky Mountain Institute, a Boulder, Colo.-based environmental nonprofit.
Why low-cost carriers are more CO2-efficient
Overall, the Cirium Flight Emissions Review showed that dense seat configurations and modern fleets are airlines' primary levers for efficiency.
All the 10 most efficient carriers have a low-cost model. Frontier achieved its second place ranking largely through its combination of densely configured cabins and a fleet that averages just four-and-a-half years of age.
The other U.S. airline in the top 10 is low-cost carrier Spirit, with a fleet-age average of 6.3 years of age.
Full-service airlines, which offer first and business class cabins, tend to have higher emission on an available seat kilometer basis.
"Seat density and fleet age matter more than almost any other factor airlines can control," chief Cirium marketing officer Mike Malik wrote. "While industry leaders are succeeding through these operational strategies, it also highlights how limited the levers for emissions reductions really are compared to the scale of the challenge."
The airline industry has pledged to achieve net-zero emissions by 2050.