A few years ago, a question popped up that spurred me to do some research and crunch some numbers (https://sustainability.stackexchange.com/a/6204/4886):
I just calculated some more specific numbers for the opportunity cost
of a single passenger on an A 300-600 over a 2,000km trip. Assuming
the plane is half-full of just about everything (fuel, cargo,
passengers, etc.) it works out to be ~26L of aviation fuel. That's it.
You getting on the flight will only add 26L to the amount of fuel
consumed by the plane over the entire trip. That's about 69kg worth of
CO₂ emissions — a far cry from the 1.3 tonnes you are worried about.
How far can you drive with 26L of petrol in your car?
The numbers vary, of course, according to the type of plane, the distance flown, the direction and speed of the wind, temperature, humidity, and a whole host of other factors. If you wave away the ones beyond your control, play the averages game, and reduce it down to a simple CO₂/passenger figure, the answer I got was 69kg.
I can't remember what the airline's average gross passenger weight (passenger, carry-on and luggage) was, but wouldn't be surprised if it was around the 95kg mark. So each kilogram of mass being transported on a 2,000km international flight by one of those Airbuses works out to about (69/95=) 726 grams of CO₂.
Over a 2,000km trip, therefore, the emissions would be about (726/2000=) 0.36 grams of CO₂ per kilometer flown per kilogram of mass transported.