Singapore-based Scoot took top honors this year as the world's most emissions-efficient airline in aviation analytics company Cirium's annual rankings, released Wednesday. The Singapore Airlines low-cost subsidiary dethroned Hungarian ultra-low-cost-carrier Wizz Air, which came in second this year.
Cirium's 2025 EmeraldSky Annual Review ranks the 100 largest scheduled passenger airlines based on the amount of CO2 they emit per available seat kilometer.
Scoot's score was 51 grams of CO2 per ASK, with Wizz Air at 52.9 grams. In third was U.K.-based TUI Airways at 53.6 grams. The only U.S. carriers to make the top 15 were Frontier Airlines with 54.1 grams, placing it fifth, and Spirit Airlines, coming in 12th with 56.8 grams.
The scores were calculated using Cirium's EmeraldSky platform, which was launched in May 2024 and uses measured aircraft emissions and fuel burn to tabulate. The platform has ISAE 3000 Reasonable Assurance from PwC and accreditation from the Rocky Mountain Institute, according to Cirium.
Cirium's methodology includes aircraft variances, flight weight estimations, fuel burn while taxiing and in the air, cabin configuration and seat dimensions, and the fuel-efficiency attributes of an aircraft, such as engines and winglets. It then adds a carbon allocation to determine the carbon emitted per seat.
The report reviews CO2 emissions per seat in 2025, 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 shows the CO2 emissions per ASK difference from the prior year. Of the top 15 carriers, 12 improved their emissions scores year over year.
In addition to the global list, the EmeraldSky report ranks the top 10 largest airlines by CO2 per ASK, and 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.
Of the largest global airlines, Qatar Airways ranked No. 1 with 60 grams of CO2 per ASK, followed by Ryanair at 62.7 grams and Turkish Airlines at 64.2 grams. Southwest Airlines (67.8 grams), United Airlines (69.9 grams), Delta Air Lines (72.1 grams) and American Airlines (75 grams) each made the top 10.
For North America, Frontier was first with 54.5 grams of CO2 per ASK, followed by Spirit (57.4 grams) and WestJet (67 grams). In fourth place, Southwest's score for the region dropped 1.7 percent to 68 grams per CO2 per ASK, the largest such emissions decline. Half of the North American list showed slight increases in their emissions scores year over year, including WestJet, Alaska Airlines, Air Canada, Delta and American.
Western Europe rankings had Wizz Air on top at 53.1 grams of CO2 per ASK, followed by Jet2 (57.9 grams) and Transavia (59.9 grams). Only two of the top 10 carriers had higher scores from the prior year—Eurowings and Vueling. British Airways had the largest emissions score decline, 3.8 percent lower than in 2024.
RELATED: Cirium's 2024 Flight Emissions Review