The whole strategy behind current flight carbon offset programs is to average out all the contributing factors and offer you a most-probable amount of emissions to offset in advance of you making your journey. This has the advantage that you can carbon offset your flight at the same time as buying the ticket.
The disadvantage is that so much information is unknown at the time of purchase, for instance how full the plane will be, what route it will fly, how much cargo will be carried, what type of aircraft it will be. As we have seen in this project, even employing the smartest algorithms this leads to huge uncertainties on the amount of emissions that an individual passenger is responsible for.
I wonder, couldn’t the offsetting be offered post flight, when all the data is known? Once the flight has been flown, the airline knows how much fuel the plane burned, they know how much cargo the plane carried and how many passengers travelled. They even know what everyone’s baggage weighed. All the uncertainties regarding flight path, load factor and aircraft type could be resolved, the actual emissions can be measured and the per-passenger responsibility calculated.
Passengers could receive an email (numbers invented): "Thanks for your recent flight with Swiss. You and your bags (15.2 kg) flew economy class with 89 other passengers, 621 kg baggage and 2.6 tonnes of freight on an Embraer 190 from BHX to ZRH, burning a total of 2018 kg of fuel. We calculate you are responsible for 0.65% of the carbon released into the atmosphere on this journey, or 240 kg of CO2. You are welcome to offset this amount by donating to our scheme, recommended contribution 13 CHF, or with another scheme of your choice."
It might be a way forward to resolve the enormous uncertainties surrounding current flight carbon emission offsetting.