Sydney to London flight emissions can be halved depending on route, plane and seating

A flight between Sydney and London can have dramatically different carbon emissions depending on whether it stops over in Asia, the Middle East or Africa.

Flying across the world but want to limit your emissions? Experts say the size of an economy passenger’s carbon footprint can vary dramatically depending on the route and model of plane. Even when flying on the same plane type between the same locations, average passenger emissions can vary widely depending on how the airline lays out seating across its cabins.

The industry standard modelling of per passenger emissions is displayed when booking through travel agents and websites such as Skyscanner, Google and Expedia.

Experts warn this cannot capture all of a flight’s actual footprint – a multitude of factors determine a flight’s emissions, which means there are few reliable rules that climate conscious travellers can follow that don’t require hefty research. But some basic parameters offer a rough guide.

Fuel-efficient planes and stopovers

Industry-standard modelling combines the distance between departure and arrival airports, an aircraft’s rate of fuel consumption, the number of passengers across different classes, how much floor space passengers take up in each seating class, average passenger and cargo loads on the service and the fuel needed to carry them.

In addition to the fuel burned during the flight the methodology factors in emissions generated during the production, processing, handling and delivery of the jet fuel needed for the flight.

Based on this modelling, an analysis of major airlines’ offerings on the Sydney-London Heathrow route found that an economy passenger could travel with a carbon footprint as low as 1,072kg of CO₂ if they chose a Cathay Pacific route via Hong Kong.

This is much lower than the typical footprint on the route of 1,491kg of CO₂ for an economy passenger, because on both legs Cathay Pacific operates Airbus A350s, a new generation widebody plane that burns up to 25% less fuel per seat than other long-haul aircraft.

Conversely, one Singapore Airlines service from Sydney to London via Singapore uses Boeing 777s – planes that first flew 20 years ago – creating a carbon footprint of 1,825kg per passenger.

Sydney-to-London flights via North America are among the most polluting options. A United Airlines service via San Francisco, with both legs operated by Boeing 777s, creates a carbon footprint of 2,154kg per economy passenger, more than double that of the most efficient Cathay Pacific itinerary.

The growing emissions gap between old and new aircraft underscores the importance of fleet renewal for airlines in a competitive industry in which fuel can account for more than a quarter of operating costs.

Qantas offers several options between Sydney and London. Economy travellers who transit through Perth before taking the airline’s more than 17-hour non-stop service to Heathrow have an estimated carbon footprint of 1,262kg, largely due to the efficiency of the Boeing 787 Dreamliner used on the longer leg.

However, the historic QF1 route via Singapore has an emissions footprint of 1,498kg per economy passenger, due in part to the Airbus A380 used on both legs.

Carriers have largely moved away from the A380 and its four fuel-thirsty engines, which have become uneconomical to run on routes where there is no consistent demand for its cabins that can seat up to 600 passengers.

Qantas also operates an Airbus A330 with 297 seats between Sydney and Singapore, with an economy passenger carbon footprint of 528.7kg compared with the A380’s 544.2kg on the same leg.

There is a mammoth difference between the footprint of economy passengers and those booked in premium, business and first class.

This is due to the much larger floor space a premium seat takes up, which could be used more efficiently to carry more economy passengers.

The typical carbon footprint of a business class traveller between Sydney and London is 5,854kg, significantly more than the 1,491kg of a typical economy passenger.

Nevertheless, airlines are increasingly configuring their aircraft to carry more business class passengers.

‘Burn fuel to carry fuel’

Each plane has a maximum number of passengers and cargo it can carry, calculated according to its destination and the weight of the fuel it must carry to fly that distance.

Ian Douglas, a senior lecturer in aviation management at the University of New South Wales, previously worked to reconfigure seating on the planes Qantas flew between Los Angeles and Sydney.

“We had an absolute limit on the weight, we couldn’t put in any more economy seats and their passengers’ weight, so we increased the size of the [business] upfront,” Douglas says.

“If you think about square metres on a plane, a business class suite takes up about as much space as three economy seats, but an airline makes more from a business class ticket than three economy air fares.”

That means an airline can make more money by taking less passenger weight, which in turn means less fuel is needed to power the flight.

A change in seat class is a quick way to reduce emissions
Felicity Burke, Flight Centre

But it’s a balancing act, Douglas says. Fitting out a plane with only business class seats is a seriously inefficient way to transport passengers and leads to a much larger carbon footprint.

Complicating the equation for climate-conscious travellers is the trend towards ultra long-haul routes.

Flying one leg instead of two halves the number of take-off movements – the most fuel-intensive part of a flight. However, ultra long-haul services must waste more of their maximum payloads on carrying fuel – the “burn fuel to carry fuel” factor.

These calculations are front of mind for Qantas, which plans to run non-stop flights from Sydney and Melbourne to London and New York by 2026.

Qantas has worked with Airbus on configuring A350-1000s with about 170 fewer seats than the maximum they can carry, to accommodate the extra fuel needed to fly the roughly 17,000km to London.

Ultra long-haul flights will be much less efficient users of fuel per passenger, costing airlines more. Qantas has said it will charge about 30% more for tickets on these routes.

Susanne Becken, a professor of sustainable tourism at Griffith University, says the ultra long-haul trend contradicts climate goals.

“Stopping over on some routes is actually more efficient,” Becken says. “When you’re looking for an optimal distance, the magic number is about 4,000km. Flying anything over this means you need to carry a lot of fuel.”

Of course, opting to fly in an economy seat on a route with the lowest emissions won’t stop the higher-polluting flight from taking off or the airline swapping out business seats, Douglas says. He also notes that if fewer seats are filled than expected, each passenger’s true footprint increases. The averages are just estimates.

Crucially, the estimates are based on a direct measurement between origin and destination airports. This doesn’t account for diversions or holding patterns due to bad weather.

Nor do the estimates factor in the massive detours many airlines have had to incorporate to avoid Russian airspace since 2022 – a headache for European carriers competing with Chinese competitors who fly directly over Russia.

Emissions from Qantas’s Perth-London flights increased when the airline wasforced to introduce a stop in Singapore to observe a buffer around Iranian airspace during missile threats this year.

The use of sustainable aviation fuel is also not factored into the estimates. However, due to the cost and scarcity of the green alternative, its use has been limited to just 0.2% of jet fuel burned, according to 2023 figures.

Flying accounts for 2% of all global emissions – a growing concern for travellers.

Felicity Burke, the Asia-Pacific director of Flight Centre’s consulting division, says professional services companies in particular are increasingly taking emissions into account when booking flights.

But corporates are not yet giving up the front of the cabin on long-haul flights from Australia. Over the past year, business and first-class bookings through Flight Centre have increased by 23% and 7% respectively.

“A change in seat class is a quick way to reduce emissions,” Burke says. “However, on a 24-hour journey that has an impact on traveller wellbeing during and after a flight … it’s certainly [just one] consideration to make on long-haul travel.

“Creating a more sustainable travel program doesn’t necessarily mean travelling less or travelling in a more expensive manner; it means travelling smarter.”