I will soon be going to Atlanta, GA, USA from Copenhagen, DK, and I would like know which of the following opportunities you think is the most climate-friendly way to travel:

Flying from CPH to Atlanta with transfer in Amsterdam (4.750 miles/7.650 km). Taking train/bus from CPH to London (aprrox. 950 miles/1500km). Then flying from London to Boston (3.250 miles/5.250km). Then taking train/bus from Boston to Atlanta (1.050 miles/1.700km). Distance in total: 5.200 miles/8400km. This trip is roughly 500 miles/1.000 km longer, but it’s a 1.500 miles/2.400km shorter flight-trip. What do you think the difference in the CO2 emission will be?

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A bus consumes about 40 liters of diesel fuel per kilometer. If there are 20 occupants, that's 2 liters of fuel per kilometer. However, with 10 occupants it would be 4 liters of fuel per kilometer.

An airplane consumes about 3.5 liters of jet fuel per passenger kilometer (assuming the airplane is full, which it usually is).

If you replace 2400 km of flying with 3200 km of bus traveling, that's probably not an overall good idea although if the bus is fully occupied, it will save a small amount of fuel (but maybe waste a lot of your time).

If you on the other hand replace 2400 km of flying with 3200 km train trip, that's an excellent idea. Trains today almost always use electricity, and practically all new electricity production installed today is carbon neutral. There is a clear pathway to make both electricity production carbon-free (onshore wind power, offshore wind power, solar power, electrolysis, hydrogen storage, combined cycle hydrogen power plants, hydropower) and also make short bus trips carbon-free (battery-electric buses) but for long bus trips and airplanes, we are very far from making them economically carbon-free. Also in many countries today such as France, Norway, Sweden and Finland, carbon dioxide emissions of the whole existing generation mix are very small, far below 100 g / kWh.

If you want to be climate friendly, prefer train and maybe electric buses that are only economically feasible in transportation inside cities, not between cities. Don't prefer long-distance diesel buses.

If you don't live near a train station, it is not a major crime to take a short bus trip to a train station. That trip is presumably so short that it would be a prime candidate for electrification if the bus making that trip isn't already electrified.

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