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I have used both Kiwi.com and GoToGate to fly from the UK to India with multiple flights (4) on multiple carriers - it does work, and is cheaper than a regular flight with a single carrier, though makes for a long journey.

I am about to travel from the UK to the US via Kiwi.com again - the itinerary is London Luton (LTN) -> Reykjavik (KEF) -> Washington DC (BWI) -> Denver (DEN) - San Diego (SAN).

I am thinking of carbon offsetting my flight as I feel bad about making such a long journey. I wonder if these kind of "Virtual Interlining" itineraries are worse for the environment in terms of carbon usage than a regular flight (e.g. LGw - SAN direct with Virgin Atlantic) as there is a much greater number of miles covered from origin to destination, or are they better because they use seats on less popular routes that would otherwise be empty?

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Unfortunately this is a very complicated question and there is no easy answer.

Are multi-carrier flights booked with .. services like Kiwi.com or GoToGate better for the environment than regular flights or worse?

There is no first order difference in how you book. Kiwi offers non-stop flights and mainline airlines offer some really convoluted itineraries as well. Ironically the more complicated itineraries are often cheaper since they are less convenient for the customers. Airlines price by supply and demand, not by actual cost.

What matters primarily for carbon emission for an itinerary are the number of legs and the distance flown, not who you book it with. In this regard shorter itineraries with less legs are better. On short hauls, takeoff itself can take up to 40% of the entire fuel consumption, so having a non-stop is often the most emission efficient option. Fuel efficiency depends on distance flown with "mid range" distances being the most efficient. For a Boeing 777-200 the optimum is around 4500km. In some cases it can be more efficient to break up a very long flight into two medium sized segments.

A secondary driver is the load factor of the plane. The incremental emissions from carrying one more passenger are actually quite small. A full plane is more efficient in terms of "carbon per passenger mile" simply because there are more passengers for almost the same fuel.

This being said, moving one passenger from flight A to flight B makes no overall difference. It just makes flight A less efficient and flight B more efficient. The total emissions stay the same (more or less).

The real win of load factors only comes into play when capacity and/or demand shifts enough so that entire flights are added or removed from the schedule. If and how this is impacted (or not) by booking through Kiwi vs. (say) United is anyone's guess, but I doubt that there is a significant correlation.

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  • Thanks for this detailed reply, it certainly contains food for thought but as you say maybe there is no easy answer! I guess the last paragraphy of your answer is the crux of the matter - are extra flights added because of Kiwi etc or not?
    – drkvogel
    Dec 19, 2022 at 22:09
  • If extra flights are not added to the schedule because of Kiwi.com, then it could be argued that such intineraries ("Virtual Interlining"/VI or "self-transfer", as I believe they are called) are significantly more carbon-efficient than regular flights, as they are using seats on existing flights that otherwise would be empty, and the total carbon cost of a VI seat would be due to just the extra weight of the passenger and luggage - is my thinking.
    – drkvogel
    Dec 19, 2022 at 22:16
  • However, as these VI itineraries get more popular, they possibly are creating more demand for flights - I guess only the airlines would know for sure. Even so, perhaps the existence of VI is at least making the most efficient use of existing routes and lessening demand for new direct flights.
    – drkvogel
    Dec 19, 2022 at 22:21
  • @drkvogel: it's complicated. You can just as well make the opposite argument: It's better to fly on an almost full flight so the empty ones stay empty and get eventually cancelled. That's the ONLY event that will bring emissions down significantly. When you fly, you create the same amount of incremental emissions regardless of how you book it and how much you pay.
    – Hilmar
    Dec 19, 2022 at 22:41

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