Flying Green: 5 Sustainable Airlines Leading on CO₂ Efficiency

I love a comfortable flight. At 5'9", I appreciate a little extra legroom, who doesn't? But when it comes to an airline's environmental impact, I want to look beyond the onboard experience.

If you're looking for the most sustainable airlines, there isn't a simple label that tells the whole story. Some airlines use fuel more efficiently than others. The aircraft, route, and cabin you choose also matter.

This guide looks at five airlines that led Cirium's 2025 ranking for CO₂ efficiency, followed by practical ways to compare the flights you can actually book.

What makes an airline more sustainable?

For this list, I'm using Cirium's 2025 EmeraldSky review, published in April 2026. It compares the world's 100 largest scheduled passenger airlines using CO₂ emissions per available seat kilometer.

That means comparing emissions with the number of seats an airline flies and the distance it covers. A lower score means better CO₂ efficiency on this measure. It is an airline-wide average, rather than the emissions of your particular ticket.

The ranking gives us a useful starting point. It doesn't measure every part of sustainability, from working conditions to waste, or mean these flights have no climate impact. I'd use it alongside a comparison of the specific flights available for my trip.

Five airlines leading on CO₂ efficiency

These are the top five in Cirium's 2025 global efficiency table. The figures below are grams of CO₂ per available seat kilometer, not total emissions per passenger.

1. Scoot

Scoot took first place in the review. Based in Singapore, it's a low-cost airline with flights across Asia-Pacific and beyond. Its fleet includes Boeing 787 Dreamliners, Airbus A320-family aircraft, and Embraer E190-E2s.

Cirium's analysis of Scoot's performance shows why fleet age alone isn't enough to judge efficiency. The way an airline uses its aircraft and seating capacity also matters. If Scoot serves your route, compare its flight with the other options instead of assuming the ranking settles the decision.

2. Wizz Air

Wizz Air ranked second globally and first in Europe. According to its 2026 announcement, its performance reflects an efficiency-focused operating model and a young fleet, largely made up of Airbus A321neo aircraft.

It's one to compare for European trips. I'd look at the actual route, departure airport, and flight estimate alongside the fare. A low airline-wide score is helpful, but it doesn't tell you how every possible itinerary will perform.

3. TUI Airways

TUI Airways, the UK airline, placed third. This result applies to TUI Airways specifically, rather than every airline operating under the wider TUI brand.

TUI says it uses Boeing 737 MAX aircraft for short- and mid-haul trips and 787 Dreamliners for long-haul holidays. Those aircraft are more fuel-efficient than the planes they replaced in its fleet. It's a useful option to compare when booking a holiday from the UK, while still checking the exact flight and cabin.

4. Air Europa

Air Europa came fourth. Its Madrid hub connects with the Americas, and its fleet includes Boeing 787 Dreamliners and Boeing 737 aircraft.

That makes it worth checking for trips involving Spain or a transatlantic journey. Look at the full itinerary when comparing it with other airlines. A connection through Madrid may work well for your trip, but the airline's average ranking shouldn't replace a comparison of your actual options.

5. Frontier Airlines

Frontier placed fifth globally and led the North American regional ranking. For US travelers, it's a useful airline to include in the comparison.

In its April 2026 announcement, Frontier reported that fuel-efficient A320neo-family aircraft made up 85% of its fleet. As with the other airlines here, I'd compare the flight estimate, schedule, and total trip cost before deciding. Efficiency is one part of choosing a flight that works for you.

How to choose a lower-emission flight

A good airline ranking helps narrow the search. The next step is comparing the flights for your route, dates, and cabin.

Compare individual flights

Search your route on Google Flights, then expand a flight to see its estimated emissions. You can also sort by emissions or use the filter for flights with lower-than-typical emissions on that route. Google's flight emissions guide explains how to find these options.

Check the whole itinerary

Compare a nonstop flight with the connecting options. A nonstop may be more convenient, but it isn't automatically the lowest-emission choice. Google notes that, especially on long routes, a connection using more efficient aircraft can have a lower estimate. Check the full journey, including the return trip.

Compare cabins

Premium economy, business, and first-class seats take up more space, so they receive a larger share of a flight's estimated emissions. If you're comfortable flying economy, compare that option too.

Consider the train

For trips with a practical rail alternative, check the train before booking a flight. Google Flights sometimes displays train options, and rail can have significantly lower emissions. This is particularly worth considering when airport transfers and waiting time would eat into the time saved by flying.

Make the most of each trip

I'd also think about how the trip fits into your wider travel plans. If you want to visit several places in the same region, could you combine them into one longer trip and use trains or buses between stops? It's a practical way to avoid booking separate round-trip flights for every short getaway.

Choosing the flight is only one part of the picture. My responsible travel tips cover other choices you can make once you arrive.

What about sustainable aviation fuel?

Sustainable aviation fuel, usually shortened to SAF, can reduce emissions over the fuel's lifecycle. That includes producing, transporting, and burning it. SAF still produces emissions when it's burned, and its overall benefit depends on how it's made.

The International Civil Aviation Organization's lifecycle framework includes those different stages when assessing fuel emissions. I wouldn't treat a mention of SAF in an airline's marketing as proof that my flight is impact-free. I'd look for clear information about the fuel used and what the claim actually covers.

Flying green FAQs

Which airline is the most sustainable?

There isn't one universal winner across every environmental and social measure. Scoot led Cirium's 2025 review for CO₂ efficiency among the 100 largest scheduled passenger airlines studied. Your own flight may perform differently, so compare the options on your route before booking.

Why do Google Flights estimates differ from this ranking?

They measure different things. The table above shows an airline-wide CO₂ figure per available seat kilometer. Google Flights estimates lifecycle greenhouse gas emissions for the passengers and flight you're selecting, expressed as CO₂ equivalent. Google explains its emissions methodology here. The two figures shouldn't be compared directly.

Do these estimates include every climate effect?

No single headline number covers everything. For example, Google Flights displays a flight's estimated contrail impact separately in the flight details. Treat emissions estimates as a useful comparison tool, rather than an exact prediction of your trip's entire climate impact.