Cirium’s Emissions Rankings: Newer Jets, Denser Cabins, Smarter Route Planning

‍LONDON — Cirium’s 2025 EmeraldSky Annual Review names Scoot (TR) the world’s most emissions-efficient airline, surpassing 2024 leader Wizz Air (W6). The ranking is based on passenger CO2 per available seat kilometer among the 100 largest scheduled passenger airlines. Cirium attributed TR’s lead to high seat density and longer average flight sectors. Overall, the results favored airlines with younger fleets and denser configurations.

That makes the top of the ranking somewhat unsurprising. Low-cost carriers often perform well on emissions intensity because they tend to operate younger fleets, fit more seats into each aircraft, and deploy those aircraft on sectors that maximize efficiency. In that sense, Cirium’s headline ranking largely confirms what the industry already broadly understands.

The more revealing part of the report is not simply who tops the table, but the route-level data showing how airlines are improving emissions performance in practice through fleet renewal and better aircraft deployment.

Low-cost, largest carriers and regional leaders

Wizz Air placed second, followed by TUI Airways (BY), Air Europa (UX), and Frontier Airlines (F9), highlighting the strong performance of low-cost carriers. F9 was the highest-ranked U.S. airline at fifth, while Spirit Airlines (NK) ranked 12th. Cirium categorized airlines into Gold, Silver, and Bronze tiers, with the top five earning Gold status.

Cirium also ranked the largest carriers by available seat kilometers, with Qatar Airways (QR) leading, followed by Ryanair (FR) and Turkish Airlines (TK). Regionally, Frontier led intra-North America, W6 led Europe, JetSmart (JA) led Latin America, Virgin Atlantic (VS) topped the transatlantic category, and Air Canada (AC) led the transpacific category, with Delta Air Lines (DL) in second.

Where airlines are actually cutting emissions

Cirium’s “closing the gap” table recognized airlines that improved emissions intensity through fleet renewal, not just those already operating efficient fleets. Korean Air (KE) achieved the largest long-haul year-on-year improvement on Incheon–Seattle (ICN-SEA), reducing emissions by 27.4% after switching from 777-300ERs to 787-9/10s.

American Airlines (AA) also performed well, with emissions on New York JFK–Delhi (DEL) down 20.4% and JFK–São Paulo Guarulhos (GRU) down 19.3%, both due to 787-9 deployment. Delta ranked in the top 10 with improvements on Boston (BOS)-London Heathrow (LHR) and Minneapolis/St. Paul (MSP)-Heathrow after upgrading from A330-200s to A330-900neos.

What the report ultimately shows

Our main takeaway from Cirium’s report is that emissions efficiency depends more on operational decisions, such as using newer aircraft, increasing seat density, and optimizing route-aircraft matching, rather than branding or net zero promise slogans.

The 2025 EmeraldSky review is Cirium’s second annual edition. The methodology uses flight-level operational data, is independently assured under ISAE 3000 by PwC, and assesses whether airlines are increasing capacity faster than emissions and their current position on the efficiency curve.

See the full report here.