tl;dr

Flight CO₂ emission calculators have huge uncertainties which users are not informed about

Travelers wishing to discover or offset their CO₂ emissions from flying may turn to an online calculator. Inputting their origin and destination airports, and perhaps their seating class, they receive a precise result. However, the uncertainty on that calculation is huge - changing some assumptions could easily half or quadruple the result.

There are two main sources of uncertainty:

Just the origin and destination airports is not enough. The distance between those two airports is very well known, but depending on the geography, airline and even date of the trip the flight path could be up to 10% longer. Flying via an intermediate airport, even if this doesn’t increase the length of the flight, will affect the emissions. The aircraft model used could change the amount of emissions by a factor of five.

The seating class, how full the plane is and how much cargo it carries are all used to adjust the amount of emissions each passenger is responsible for. However, in addition to this information probably missing from the calculation, there are in any case no agreed rules for how to account for these factors. Moreover, studies suggest that CO₂ in the upper atmosphere is worse than at ground level, leading to some calculators including radiative forcing factors up to 300%.

Input the same data into different calculators and users will get a range of results... What can be done? I see three options:

Everyone takes the average flight CO₂ emission. Without more information from airlines, or more work on the calculators, no other option makes statistical sense.

Since the airline knows how much fuel the plane used, how the seats were filled and how much cargo was carried, there is no lack of information. We still have to resolve how the total emissions are divided up between economy, business, first class passengers, empty seats and cargo, and agree on an RF factor, but the uncertainty is already massively reduced.

Users should be able to see the uncertainty on the result they are presented with. They should see where the uncertainty comes from, and should be able to input, change or remove data and see the effect. That’s the goal of this project.