How bad are commercial flights for the environment?

Many figures and opinions circulate on the subject, let’s decrypt.

Some consider that the carbon emissions attributed to commercial flights are relatively insignificant and that average fuel consumption of most recent planes has reduced so the value per 100 km per passenger is lower than a car. They also report that the massive aircraft fleet grounding due to Covid had negligible impact on global Green House Gas (GHG) release in the atmosphere.

Others argue that CO2 emissions are not the sole vectors affecting the atmosphere warming and that only one Paris – New York return ticket accounts for about 65% of the annual carbon emission budget per person within a carbon neutral world (2050 objective) – See note 1-.

Guess what? All those arguments are true. This most probably explaining why the debate is tricky not to say polarized.

What is the aviation carbon footprint ?

Worldwide, the commercial aviation released 1,077 Gt of CO2 in the atmosphere (not including the effects of CH4 & NO2 emissions -see note 2-) in 2018. This is considering a 3,019 kg of CO2 emission per liter of kerosene combustion. This value takes not only the combustion but the overall upstream process of oil extraction, refining & transportation -note 3-. This accounts for 2,6% of the total WorldWide (WW) carbon emissions estimated being equal to 42,1 Gt. Let’s keep in mind 1 Gt CO2 as the absolute and 2,6% as the relative aviation direct contribution to WW carbon emissions.

But there are also some indirect effects. As we all have already observed, planes are leaving white streak against the sky in certain flights conditions – their name is contrails-.

Those line shaped clouds are the results of the agglomeration of NOx, soot particles and water vapor released within the engine exhaust gases. Depending on atmospheric conditions, contrails may clear out quickly or remain persistent evolving into cirrus clouds. In that later case, contrails are impacting atmosphere temperature though an added radiative forcing effect. The mechanisms involved in contrails built up are complex and still being studied. However, there is a consensus on their contribution to the atmosphere warming. While the precise extent is still being debated, The Atmospheric Environment revue published in 2020 by Lee & al stands as the reference on contrails warming effects -note 4-. The below conclusive extract is showing that the indirect effect of flying a jet aircraft in conditions favorable to contrails built up can be twice as much important that the sole result of burning kerosene.

As a conclusion the net global aviation radiative forcing effect is about three times the one usually attributed to the sole kerosene combustion, which is far to be insignificant.

How does Aerospace compare to other transportation sectors?

Undoubtedly, aircrafts design made huge progress toward sobriety but fleet renewal takes time to penetrate the market. And mostly, the fuel burn efficiency gains (about 15% per decade) were largely offset by traffic growth (+ 7% per year between 2014& 2019). The net overall effect is a constant increase of GHG emission within the sector (+100% between 1990 & 2018 in Europe- note 5-). In the same period, other transportation means managed to contain or reduce their environmental foot print. Aviation has clearly the lowest mark in the class (see yellow curve in below graph).

Evolution of GHG emissions from transport in the EU-28

Social acceptability

Social regard toward aviation is somehow ambivalent nowadays. While low fares are seen by some as an opportunity to discover the joy of travelling, others are pointing the 1% air travelers being responsible for 50% of the GHG direct emissions of the whole sector.

If we add the growing consciousness to act in minimizing the anthropic environmental changes within the population, it looks clear that the aerospace industry needs to take a wing turn toward environmental responsibility. Indeed, commercial aviation cannot remain the exception in a world where all sectors have to reduce annually their emissions by 5% between 2020 & 2030 to have a chance to limit the average warming to 2°C by the end of the century.

There are many & different options on the table, discussion to be continued in a further release!

Notes

Note 1. Taking an average 3,5 l /100 per pax, 80% occupation seats, 3,07 kg CO2 per liter of kerosen and 2t of CO2 per person the not to exceed annual carbon budget in a carbon neutral world.

Note 2. Source: « June 2020 » in IATA, Airline Industry Economic Performance – June 2020 – Data Tables.

Note 3. Combustion of one liter of kerosene emits 2,54 kg of CO2. Considering the process of oil extraction, refining & transport, this value gets up to 3,019 Kg Of C02. Adding the CH4, N2O liberated by the combustion, one liter of kerosene is emitting 3,075 kg of global CO2 equivalent. Source: ADEME Carbon Database (given per kg of kerosene).

Note 4. D.S. Lee, D.W. Fahey, A. Skowron, M.R. Allen, U. Burkhardt, Q. Chen, S.J. Doherty, S. Freeman, P.M. Forster, J. Fuglestvedt, A. Gettelman,
R.R. De León, L.L. Lim, M.T. Lund, R.J. Millar, B. Owen, J.E. Penner, G. Pitari, M.J. Prather, R. Sausen, L.J. Wilcox, The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018, Atmospheric Environment, 2020.

Note 5. Source: Environmental European Agency, Feb 3rd 2020 release.