Hungarian low-cost carrier Wizz Air took top honors as the world's most emissions-efficient airline in aviation analytics company Cirium's inaugural Flight Emissions Review, released Wednesday.
The report ranks airlines based on the amount of CO2 they emit per available seat kilometer.
Wizz's score was 53.9 grams of CO2 per ASK and was followed by Frontier Airlines at 54.4 grams and Turkey-based Pegasus at 57.1 grams. The only other U.S. carrier beside Frontier to make the top 20 list was Spirit Airlines, which was ranked seventh at 58.4 grams of CO2 per ASK.
The scores were calculated using Cirium's EmeraldSky platform, which was launched in May 2024 and uses measured aircraft emissions and fuel burn. The platform has ISAE 3000 Reasonable Assurance from PricewaterhouseCoopers and accreditation from the Rocky Mountain Institute, according to Cirium.
Cirium's methodology takes into account flight weight estimations, such flight operations as time spent in the air and taxiing, and a fuel model to determine fuel burn. It then adds a carbon allocation to determine the carbon emitted per seat.
The report looks at CO2 emissions per seat in 2024, the number of flights, the average number of seats per flight, the weighted average age of the aircraft and the average flight distance in kilometers. It also compared the number of flights and CO2 per ASK to 2023 data to show the year-over-year change for those two columns. For instance, Frontier increased its flights by nearly 14 percent from 2023 to 2024, but its CO2 per ASK score declined 1 percentage point, indicating its carbon efficiency also increased.
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In addition to the global list, the report ranks the top 10 largest airlines by CO2 per ASK, "to show how scale affects emissions intensity—a critical component of understanding industry performance," according to Cirium. Ryanair was ranked first for this category, followed by four U.S. carriers: Southwest Airlines, Delta Air Lines, American Airlines and United Airlines, in that order.
"Here's what makes this data both encouraging and sobering: while these airlines excel at reduced emissions intensity, total emissions continue rising with demand growth," Cirium chief marketing officer Mike Malik wrote in the report. "Even the most efficient operators can't offset demand growth through operational improvements alone yet. … Seat density and fleet age matter more than almost any other factor airlines can control."
The report also provides intra-region performance breakdowns. Airline scores for regions may differ from their global scores because only those flights flown for the region were counted in those categories.
For North America, Frontier led, followed by Flair and Spirit. Southwest was in ninth. None of the three U.S. legacy carriers—American, Delta or United—made the list. For Western Europe, Wizz again was on top, Ryanair was fifth, with flag carriers TAP Air Portugal, Norwegian, Finnair and SAS taking seventh through 10th.
In addition, Cirium looked at intra-regional operators. For the North Atlantic, French Bee was on top, followed by Azores Airlines and Norse Atlantic Airways. The North American carriers that made the top 10 list included WestJet in fourth, JetBlue in eighth and Air Canada ninth.
The Cirium report also provided route-level improvements, identifying airport pairs with the greatest efficiency gains, and operational insights. Cirium plans to update the Flight Emissions Review quarterly.