Environmentalists have warned that 100,000 deserted ghost flights will be flown across Europe this winter with few or no passengers on board.
Such flights, known as ghost flights, have been branded 'absurd and revolting' by Greenpeace because of their wasteful impact on the planet.
Why would airlines choose to fly empty planes? The reason: EU law dictates that they must operate a percentage of scheduled flights to keep their valuable takeoff and landing slots at major airports. This is also known as the 'use it or lose it' rule.
Pre-pandemic, the use it or lose it quota was 80%, meaning an airline needed to operate 80% of its total planned flights to keep a particular slot for itself. But the European Commission temporarily suspended this rule when Covid-19 brought travel to a standstill.
In October it was reinstated at a lower benchmark of 50%, asking airlines to operate at least half of planned flights to maintain a slot. In March it is set to rise again to 64% in time for the summer season, until October 2022. That's despite air traffic still being dramatically down on normal levels.
Heathrow Airport recorded passenger numbers of just 3 million last December - less than half of the traffic for the same period in 2019.
Lufthansa says the 'use it or lose it' rule means it will be forced to fly the equivalent of 18,000 empty planes during winter.
The German carrier has already announced around 33,000 cancellations, but predicts it will have to operate unnecessary flights to hold onto its runway slots.
A Lufthansa spokesperson revealed that just 45% of its flights were full between January and March 2021.
'If we wouldn't risk the loss of slots in certain airports in Europe, we probably would have cancelled them and put them together with other existing flights,' they added.
Other airlines have not communicated the number of ghost flights they operate. However, Greenpeace used Lufthansa's proportion of empty flights in relation to its 17% market share to calculate an estimate for other European carriers.
It predicted that 100,000 ghost flights would take-off this winter, emitting up to 2.1 million tons of greenhouse gas emissions. That's the same amount produced by 1.4 million cars over the course of a year.
The research assumed an average flight time of 90 minutes on a 200-seater plane (Boeing 747-400) over a distance of 800-1,000km.
Herwig Schuster, spokesperson for Greenpeace's European Mobility For All campaign said: 'We're in a climate crisis, and the transport sector has the fastest-growing emissions in the EU - pointless, polluting u201cghost flightsu201d are just the tip of the iceberg.
'It would be irresponsible of the EU to not take the low-hanging fruit of ending ghost flights and banning short-haul flights where there's a reasonable train connection.'
Each passenger on a return flight from London to Singapore accounts for around three tonnes of CO2. That's the equivalent of heating a family home for a year.
However UK airports largely want to see the 80:20 rule reintroduced. They argue that it maximises competition and keeps airfares low.
Ryanair called on Lufthansa to sell its empty seats to consumers at low prices, accusing the carrier of 'crying crocodile tears about the environment' to protect its slots.
Ghost flights are also bad news for consumers affected by travel disruption. Airlines can operate ghost flights amid travel bans, meaning they don't need to cancel flights even if booked passengers can no longer travel. In turn, that means consumers miss out on refunds.
This is because the Denied Boarding Regulation (EU261) says that delayed travellers must be reimbursed or re-routed if departing from the UK or EU, or landing in the UK or EU when flying with a UK or EU carrier.
They may also be entitled to claim compensation of up to u20ac600 (£500) depending on the extent of the disruption. Even for a handful of passengers, this can result in a hefty bill for airlines.
Both Skyscanner and Google Flights will highlight the greenest flight for your journey when you use their search tools. Our research shows you can slash your emissions by up to 45% by switching carriers.
Business and first class are responsible for up to four times more CO2 per passenger.
Choose greener aircraft
Newer and more fuel-efficient models are better for the environment. Look out for the Boeing 787-9 Dreamliner, and Airbus A350-900 and A320neo.
Avoid indirect flights where possible. Taking off uses more fuel than cruising.
A heavier plane guzzles more fuel.
Take the train
Consider travelling by rail for short-haul journeys. The Eurostar emits up to 90% less carbon.