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On Google Flights, I searched the FRA-JFK route:

screenshot with three flights

It surprises me that a flight with a layover emits so much less CO2 than a direct flight, as many times I've heard the suggestion that the takeoff is the most polluting phase of the flight and so flights with layovers are more polluting than direct flights.

Might there be something wrong with Google's assessment or is that "rule of thumb" bad advice?

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    Are they all the same class of plane for the longer distance? In the KLM listing I see cityhopper which usually does the within Europe flights.
    – Willeke
    Mar 30 at 9:18
  • 1
    If the AF and KLM flights use more recent planes like a 787 or 350 that probably makes a big difference. But apparently the most efficient one is a direct flight Condor flight (not shown here) using an A330neo
    – jcaron
    Mar 30 at 9:41
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    There is literally an (I)nfo button in your screenshot. Click on that and it'll tell you why this is the case (hint: it includes things like type of aircraft/speed/number of passengers/etc, not just distance). This doesn't deserve an answer when the answer is right there...
    – Doc
    Mar 30 at 11:36
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    Also consider outright lies by the airline - they may "use a different calculation" because having a lower figure is more attractive to end-users. You're looking at these numbers and making a potential purchasing decision here. Cynical, yes. Plausible, also yes.
    – Criggie
    Mar 30 at 20:50
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    The one with the layover doesn't have to carry the fuel for the whole journey from the beginning which may make a difference? Mar 31 at 18:51

5 Answers 5

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Three reasons, in order of ascending importance.

TL;DR version: With long-haul flights, some extra CO2 comes from lifting more fuel, some more CO2 from flying a bigger airplane, and a lot more CO2 from commercial cargo payload, which large airliners carry and small ones don't.

  1. While it's true that takeoff and climb take the most energy in very short flights, aircraft are equipped with an energy storage device, similar to a hybrid car. That device is the entire aiplane: they accumulate kinetic energy as velocity and potential energy as altitude.

An airliner at 35,000 ft and Mach 0.8 (10 km and 230 m/s) carries an energy equivalent of traveling ~100 miles at cruise conditions, largely what it consumed during takeoff and climb. The only energy irreversibly wasted by the takeoff-landing cycle, aside from engine efficiency losses, is that burnt during taxi and that absorbed by brakes during the landing.

enter image description here

This means that takeoff/landing losses are fairly small. Only takeoff and taxi fuel is spent in addition to cruise fuel for an additional stop.

Meanwhile, fuel costs fuel to carry. The fuel cost of carrying that fuel is approximately 2/3 of the plane's fuel weight % at takeoff. To illustrate, if you take off with 45 tons of fuel at total weight of 100, then 30% of these 45 tons, or 13.5 tons, is spent on lifting and carrying fuel. If you take off with 15 tons, that's only 10% out of 15 tons, or 1.5 tons. For long flights, this is more significant than taxi and takeoff burn.

  1. Large airplanes capable of traveling long distances weigh more per passenger than small airplanes used for short-haul flight. To compare apples to apples and exlude the 737 due to its outdated design, let's take A319 and A350. The A319 carries 125-156 pax in a 40-ton plane, or 260-310 kg per passenger. The A350 at 440 pax limit is a minimum of 320 kg per pax, and realistically 350-450 kg when not at densest possible sardine-class configuration.

It's not all extra fuel tanks. Large airplanes are also more capable in other ways. They have higher tailwind and crosswind limits, they have to be more reliable to get that ETOPS certification, they have better radars and equipment. They offer more room per passenger, both in larger seats, larger overhead bins, more aisle space, more galley and restroom space, and a lot more everything for business and first class seats... yes, we're getting there.

And most importantly, they are much more capable of carrying cargo. Virtually all air freight is containerized. The 737 can't carry containers at all, so it can't really carry cargo, except maybe the occasional urgent parcel. The A320 and the 767 carry special small containers, made for either the A320 only (LD3-45) or the 767 only (LD2). A320 containers are relatively rare, but the 767 is a mainstay of the air freight industry. Anything larger, such as the A300, A330, A340, A350, A380, B747, B787, B777, is specifically designed to fit industry-standard LD3 containers, introduced for the 747, easily swapped between aircraft types.

Air cargo is very important today. Marine shipping is extremely fuel-efficient, but at over a month per trip, it's simply too slow for direct consumer purchases. Almost everything people buy on Amazon or other online marketplaces has to be carried by air, to provide a short enough time between ordering and delivery to trigger the right satisfaction mechanisms that make online shopping attractive. And unlike marine shipping, it's not just from China to the rest of the world; large companies keep their stock in distribution centers in the US and Europe, but there's too many countries to have a warehouse in each one.

  1. They say there are three kinds of lies. The CO2 figures you see per passenger are of the third kind.
    Virtually all large airplanes carry cargo in addition to passengers, and virtually all ultra-long-haul flights carry first-class passengers, which take a lot more of the airplane's space than economy pax. Meanwhile, the CO2 calculations are done by simply dividing expected fuel load by passenger capacity.

And up to half of that CO2 can be produced carrying freight. That's why long-haul aircraft will almost always show higher fuel burn per passenger. They simply carry more things. Besides spending more fuel to carry fuel, they are almost always loaded with cargo.

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  • It would have been better if you had kept the TL;DR right in the start of your answer. Or may you kept in the bottom of your answer purposefully. ;-) Apr 2 at 10:50
  • @NaveedAhmed I usually put it at the bottom, because many people tend to scroll to the end, and conclusion at the end is a more scientific style. But this is Travel, not Aviation, so I guess at the top makes sense.
    – Therac
    Apr 2 at 18:08
  • Very nice that you brought a bar graphic, that is very clear, thanks! Where did you take it from?
    – LoremIpsum
    Apr 6 at 17:03
  • @LoremIpsum researchgate.net/figure/…
    – Therac
    Apr 7 at 1:15
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As the comments said: the CO₂ estimates depends a lot on the aircraft and other factors. The actual calculation is rather complicated and so Google (and others) have to make a lot of simplifying assumptions.

For the aircraft, it's the fuel efficiency but also the seating arrangement. For example, a Boeing 777 can have 3-3-3 or 3-4-3 seating in economy. Obviously 3-4-3 is more efficient since you cram more passengers into the same space, but it's also a miserable flying experience, so pick your poison.

This being said the numbers that Google provides here seem unrealistic to me. Fuel is the second largest expense for airlines so they are highly incentivized to optimize fuel efficiency. I doubt that KLM can operate a transatlantic flight 40% more efficient than Delta. If that were the case, Delta would be out of business by now.

and so flights with layovers are more polluting than direct flights.

Surprisingly that's not always the case. Above a certain distance it's more efficient to add a fuel stop. For a Boeing 777 that's about 4500km.

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    Wikipedia tell us: A220-300 = 2.02 l/100 km/seat, 767-200ER in 181 seats configuration = 3.34 l/100 km/seat. Which would result in savings of exactly 40%. Given the aircraft involved here it should be less, though, but there may be differences in the way this is computed.
    – jcaron
    Mar 30 at 13:21
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    KLM is using some (expensive) synthetic fuel mixed in, and also offers carbon compensation by planting trees. Maybe that's incorporated here? Mar 30 at 21:29
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In this specific case, the savings are most likely due to the type of aircraft involved. More recent aircraft such as a 787, 350, or 330neo can be a lot more fuel efficient than older aircraft. This comes mostly from more efficient engines, but also from improvements in aerodynamics (the winglets and other tip-of-wing optimisations have surprisingly important effects), or weight optimisations (787s and 350s use a lot of composite materials and are lighter).

There could also be differences in cabin configuration. A denser configuration (smaller row pitch or more seats per row) will result in better efficiency per seat (that's possibly one of the reason the Condor flight on that route shows such a higher efficiency).

If one wants to go into details for a very specific flight (i.e. on a specific date), the exact route flown, and the influence of jet streams can radically change things as well. You could also consider saturation at the airports involved (both in the air and on the ground).

In the more general case, the computation is complex, and it's hard to say that, even with the exact same plans, a direct flight is always necessarily more fuel efficient than one with a stop: while the take-off phase is indeed quite fuel-hungry, and having one rather than two should be better, a direct flight requires more fuel to be carried on board, which weighs a lot, and which will have an impact on the overall fuel consumption.

For instance, an A350-900ULR has a maximum take-off weight of about 280 tons. Out of that, there could be up to 135 t (nearly half!) for fuel, for a flight at max range/payload. The quantity of fuel depends on the distance (but it's not linear as indeed there's a larger need during take-off), so the aircraft would be lighter during most of the flights, and would use less fuel. I know the question has been raised again recently with the recent and upcoming Europe-Australia direct flights, and I believe the answer was that it would actually be more efficient to stop (but we're really at the extremes of aircraft capabilities here).

This answer over at Aviation.SE tells us that the take-off and climb phase uses about as much fuel as one hour of cruise, but doesn't get into details based on how much fuel is on board/the total weight of the aircraft. You could probably get more detailed figures over there.

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  • "and which will have an impact on the overall fuel consumption": especially because the portion of the flight where the plane is heavier is the beginning of the flight, i.e., during takeoff, the energy-hungry phase. A flight from Frankfurt to JFK needs enough additional fuel to cruise for roughly 350 km more, plus the fuel required to get the first lot of fuel from the runway to cruising altitude. A flight from Frankfurt to Amsterdam only needs enough fuel to fly for roughly 370 km (including safety margins of course).
    – phoog
    Apr 1 at 20:39
  • The take-off phase is more hungry, but water vapour emitted in the upper troposphere (when cruising) has a significant climate impact even if the fuels were all carbon-neutral.
    – gerrit
    Apr 2 at 7:10
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Clicking on the "CO2e" figure on Google flights reveals it to be based on passenger seat miles.

Aircraft flying long haul flights have to carry sufficient fuel for diversions and emergencies in addition to the total amount of fuel needed to fly the plane and passengers to their destination.

On shorter haul flights the amount of fuel taken on board for the plane to reach its destination is less (it's a shorter flight) and weighs less. Allowing for requisite tradeoffs, airlines can reduce the amount of fuel taken on board and/or increase the number of seats allocated for the flight, both of which can reduce carbon emisions per passenger mile.

The savings come from not having to burn fuel to take off and carry a long way before it needs to be fed into the aircraft's engines. Or to put it another way, the fuel burned on the second half of a long distance flight would not need to loaded onto the aircraft if the flight only covered half the distance (roughly speaking of course).

If you break a long haul flight into two shorter flights and refuel at a stop over, you keep the fuel savings of the shorter legs, but add the cost of an additional landing, takeoff and hull pressurization cycle. From the figures this will affect CO2e estimations for the total flight, but they will also be influenced by aircraft design and efficiency.

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  • Doesn't any substantial flight need to carry fuel for diversions and emergencies?
    – LoremIpsum
    Mar 31 at 18:31
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    @LoremIpsum I would hope so and I didn't mean to imply the requirement didn't apply to shorter routes, although the amount of fuel required for diverging may vary depending on how far away from land or alternate airports the route reaches. I'm not a load master but believe it might also vary if the route direction has tail winds in one direction and headwinds in the other.
    – traktor
    Mar 31 at 22:36
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    This is why refueling stops (tech stops) are still very common on cargo flights, where passenger comfort isn't an issue. Cargo from Asia to North America refuels in Anchorage, whereas Dubai is a common stop on the way to Europe.
    – user71659
    Apr 1 at 22:34
  • FRA-JFK is only about 340 km more than AMS-JFK, so for these two routes the advantage of a refueling stop must be pretty minimal (compared to one with a stop near the middle of the route, for example at Rekjavik).
    – The Photon
    Apr 2 at 15:39
  • @ThePhoton I tend to agree - on reflection the 2 hour layovers in AMS and CDG probably indicate a change of both aircraft and aircraft type. I find it really hard to believe the KLM figure, of over 25% less CO2e than either of the other carriers, is as straight forward as it seems.
    – traktor
    Apr 2 at 22:43
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KLM was sued by an environmental company over alleged "greenwashing." I think your gut instinct is correct in that this is not truly possible. Their score may be lower due to the purchase of carbon offsets. IMHO carbon offsets are a bit of a farce. And it appears that KLM is still making inflated claims about their CO2 emissions and I expect a carbon emissions lawsuit will be next.

https://www.theguardian.com/world/2024/mar/20/dutch-airline-klm-misled-customers-green-claims-court-rules

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    Carbon offsets are the only remaining solution to climate change if you assume that the world's biggest polluters will never agree to reduce emissions (in practice, not on paper) and that consumers will never agree to reduce consumption. I wouldn't call it a farce.
    – JonathanReez
    Apr 1 at 19:59
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    @JonathanReez the fact that anything is the only remaining solution to a given problem does not imply that it isn't a farce; there is also the possibility that the solution, being farcical, is ineffective. This seems rather likely in the case of carbon offsets. They don't seem to be having much of an impact on carbon dioxide levels.
    – phoog
    Apr 1 at 20:52
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    @phoog yes, only a tiny percentage of global carbon emissions are receiving an offset, so you won't see an impact just yet. There are numerous global scale geoengineering ideas (such as this one) that could eventually offset emissions on a much bigger scale. But just because the current scale is small doesn't mean that its a 'farce'.
    – JonathanReez
    Apr 1 at 20:56
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    @JonathanReez no, what makes it a farce is that planting a tree doesn't actually offset carbon emissions unless there is some provision to ensure that the carbon extracted from the atmosphere by the tree never returns to the atmosphere. I'm not aware of any such programs. It is also arguable that present-day offset programs, even if they are true offsets, are farcical precisely because their scale is insufficient to have an impact on the problem. The possible development of a future non-farcical sequestration program doesn't make ineffective present-day programs effective.
    – phoog
    Apr 1 at 22:11
  • Farcical meaning absurd. Apr 2 at 6:23

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