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Calculating Flight Emissions
There are many different flight calculators on the internet and each can turn up with a different amount of emissions for the same flight. This is because there are several factors that need to be taken into account. Some calculators take none of these into account, where others do not take into account all of the factors.
What needs to be taken into account?
Different factors for short, medium and long haul flights:
There are different emissions per passenger per km for short medium and long haul flights due to the increased emissions required for take off, taxiing and idling (the shorter the flight the higher the proportion of flight fuel used for takeoff, taxiing and idling), but also due to the different passenger crafts used for different flight lengths.
The earth is curved and planes don't fly in vectors between destinations:
The distances are calculated using an approximation formula (the Great Circle Distance
Formula) described at:
How to calculate the distance between two points on the Earth [meridianworlddata.com].
The radiative forcing index:
An important consideration when dealing with aircraft emissions is that the effect of aviation is more than just that from CO2 alone. Effects of aviation on the atmosphere need to include the effects of other greenhouse gases including NOx compounds, ozone, methane, water, contrails and particles which are all emitted from aircraft exhausts at the same time as CO2. As these compounds are released directly into the stratosphere, their potential to effect the anthropogenic radiative forcing that naturally takes place in this region, is much greater than that for fossil fuel production on earth due to the longer residence times required for the latter. Such an effect is taken into account by the development of the radiative forcing index (RFI) which compares the total radiative forcing effect caused by aviation to that caused by CO2 alone.
The radiative forcing index is internationally agreed to be around 2.7. In simple terms this means that the effect of an aircraft's emissions when released in the stratosphere has around 3 times the global warming potential that the same emissions would have on the ground.
Economy vs Business and First Class:
Because it requires more aircraft space and resources to cater for business class passengers, and even more for first class passengers, you really need to take this into account when calculating your emissions. Business class passengers are responsible for twice the emissions of economy class, and first class are responsible for three times. What this means is — buy economy, or if you really do need that extra leg room — take responsibility for the extra emissions.
For a detailed explanation of all of the above concepts, including references for the research that they are based on, please check out the following pdfs: