I condemn flying. I think the environmental impact is huge and in many cases there are very valid alternatives. However, I have to admit I'm not familiar with the exact numbers. Maybe I have been exaggerating?

Flying

Not the main question, but interesting anyway. How much does the average Dutch person fly anyway?
The institute for mobility department of the ministry of infrastructure made a great report on this.

Some interesting takes, over the period of research:

• 50% of people didn't fly at all (so all flights are made by half the population)
• Only 13% of all people are responsible for 50% of all flights (so frequent flyers fill most seats)
• The average is 1.1 plane trips per person per year, corresponding to 2.6 individual flights on average per person per year (at 2.35 flights per trip)
• 81% of flights are for private reasons (vacation, visiting family, etc.)
• The average flying distance is 3'300 km, but this is skewed by fewer much longer trips - the mode of flight distance seems to be around 1'750 km (distance to final destination, summing any transfers)
• Half of all private flights are shorter than 2'350 km while for business trips half are shorter than 1'250 km

The Numbers

Let's focus on a modal flight of 1'700 km. That could be from Schiphol, Amsterdam (AMS) to Lisbon, Portugal (LIS), which is 1'848 km. It's a 3h direct flight. How much CO2 does this emit?

Trying to book this flight with Google Flights I see a CO2 emission of a single trip of 125 to 141 kg (flying economy, of course). It says 142 kg is typical for this trip.
The exact calculation for this is missing, though Google has written a page about where the numbers can come from. I tried booking a flight through the companies KLM, Air Portugal and Transavia, but they don't show CO2 numbers before you need to pay.

Let's see what other online calculators say. MyClimate.org says it's 382 kg of CO2, just about the double the number. This is tricky: the group trying to sell flights will want to report less eCO2, while the group trying to receive compensation donations will want to report more. MyClimate.org does have an extensive overview on how the calculation is made, including a final formula. In their number they claim to include:

• CO2 from fuel burned
• Emissions from production of kerosene
• Equivalent CO2 emission from other effects, like NOx
• Equivalent CO2 emission from RFI (Radiative Forcing Index, releasing CO2 high in the atmosphere is much more damaging than releasing it down here)
• Aircraft and infrastructure (equivalent) emissions

This sounds thorough. But let's redo their math.

Our Math

Note the MyClimate.org calculation info above.

We're pretending to fly with a common Boeing 737-800 (Winglets, passenger/BBJ2), still from Amsterdam to Lisbon.

Distance and fuel

The global distance is 1'848 km. But there are some inefficiencies because of traffic control and such. In practice on average 10% extra distance is made, but according to some standard we should estimate just 95 km extra. So 1'943 km total. MyClimate.org reports the fuel consumption formula for take-off, cruise and landing is f(x) + LT0 = ax² + bx + c . With their numbers for a, b and c this is:
0.00016 (1'943 km)² + 1.454 1'943 km + 1531.722 = 4'960.9 kg of fuel (though the unit isn't entirely clear).

Note that the 737-800 has a max fuel capacity of 20'000 kg (26'000 L) for a max range of over 6'000 km. So about 5'000 kg of fuel for a 1'800 km flight seems about right to me. Also, our number corresponds to a milage of 2.55 kg of fuel per km, which is close to this calculation.

Fuel emissions

Then MyClimate.org uses 3.16 kg of eCO2 per kg of fuel, which sounds about right. Next comes a big one: MyClimate.org says to multiply the number by 3 for effective CO2 due to RFI. Recent studies are linked on their page.
And there is some pre-production emission, set at 0.538 kg of CO2 per kg of fuel.

Total emission due to fuel is then:
4'960.9 kg (3.16 kg CO2e / kg 3 + 0.538 kg CO2e / kg) = 49'698,1 kg CO2

Aircraft deprecation

Now for the airplane itself. Boeing says their 737 series lasts for 55'000 flight hours. Our 3 hour flight is some fraction of this. The hard question is how much CO2 was emitted for the production of our plane. This is a number that doesn't seem to be published at all. A number that is available is the so-called 'aircraft factor', which MyClimate.org puts at 0.00034 kg CO2e per km per person. For the fun of it we could back-calculate to a lifetime number:
0.00034 kg CO2e / km / person 189 people (55'000 hours / 3 hours * 1'943 km) = 2.3 million kg of CO2 per aircraft

This seems like the right ballpark for an entire airplane, so let's continue with this 'aircraft factor':
0.00034 * 1'943 = 0.66 kg CO2e

This is actually a completely insignificant contribution.

Infrastructure

Lastly, MyClimate.org adds 11.68 kg CO2e to the sum for airport infrastructure. The exact content is not clear to me, it's probably taxing across the track and from things like energy for baggage belts to lights in the waiting rooms. This is also a number that seems about right but I couldn't directly verify.

Final sum

Our 737-800 has 189 economy seats. But MyClimate.org counts 148.0 instead, maybe to account for the number of seats occupied on average. They also assign 23% of emissions to cargo instead and multiply by 0.796 as the "passenger load factor". I'm also not entirely sure of this number. It could be accounting for empty seats, or maybe for baggage.

Dividing the total fuel emission by the total number of seats:
49'698,1 kg CO2 / 189 seats = 263 kg CO2 / seat

We could add the aircraft deprecation and airport infrastructure to this, but the we end up somewhere between the number of Google Flights and the calculation of MyClimate.org regardless. I do feel I have much more understanding of the emission calculations now.

Note that MyClimate.org distinguishes between short-haul and long-haul, each having different parameters. They take a weighted average between the two calculations based on actual flight distance. We have just assumed short-haul instead. This should explain most of the gap in our numbers.

Conclusion

Hard to say what we might conclude here.

• Flying does suck - a single short return flight will already by a significant portion of an average person's CO2 emission
• By how much it sucks exactly will be hard to say, though I feel that is not the point

So please consider alternatives for flying! Surely you can do your meeting for work online instead. Or you can take a train.
And yes, the available trains might suck. But if nobody takes them and everybody just keeps flying we are not pushing the system to change.