Understanding the global heating effect of a flight can be useful to help you plan your own climate emergency response. It is especially useful to understand how flying compares to other activities in your life.
A number of emissions footprint calculators exist online. Here are just a few:
But how are the footprints calculated, how does it compare to other emissions sources, and why do some emissions calculators vary?
Some calculators measure carbon emissions, others measure carbon dioxide emissions. In addition, others measure carbon dioxide equivalent emissions (CO2-e).
A CO2-e footprint is a measure of the heating of all emissions from a source, not just its CO2 emissions. It is measured as the amount of CO2 emissions that would have the equivalent heating effect as that of all emissions.
For any given source — cars, power generation, aviation, food production, domestic household, etc — the C or CO2 or CO2-e footprint can vary because, for example, not all cars generate the same emissions. A “Household” can vary in size, a “Bus” can have different levels of occupancy. Their engines can operate at different levels of efficiency and with different fuels.
When embedded emissions (such as those created in the process of producing cars) are included, the specific footprint again varies.
There are added complications in calculating aviation emissions. The CO2-e emissions from a litre of jet fuel burnt at altitude generates between 2 to 5 times more warming than when burnt on the ground. If you include contrail cirrus — a unique warming source generated by jet plane emissions — this again changes the global heating footprint calculation. The global heating contribution of contrail cirrus is considered likely to dwarf that from CO2 alone. One 2019 report, Contrail cirrus radiative forcing for future air traffic, reported warming due to contrails to be twice that from CO2 alone.
But just because we can’t give it an exact number doesn’t mean we should discount it when discussing footprints.
The point is to be aware of these differences when comparing footprints. Read the fine print and don’t mistake apples for oranges.
That said, we can unequivocally say that flying has the most damaging effect on the climate of all transport modes, per kilometre travelled. One return flight to London increases your annual emissions by a staggering 50%. A plane 80% full creates 11 tonnes CO2e, excluding contrail cirrus, per economy passenger, which in 2016 was half the 22 tonnes CO2e figure for Australia’s total emissions divided by the population.
Specific footprint calculations should not entice us into thinking that emissions from one source are ok because those from another source have been reduced. We need to reduce all emissions to zero within ten years or so. If we don’t, there is an unacceptable risk that ongoing emissions will trigger catastrophic global heating.