How can aviation possibly meet climate targets? 

How can the avi­a­tion indus­try meet the objec­tives of the Paris Agree­ment? In a report pub­lished at the end of 2021, the Insti­tut Supérieur de l’Aéro­nau­tique et de l’E­space (ISAE-SUPAERO) draws up prospec­tive sce­nar­ios for the indus­try. Using an inno­v­a­tive mod­el­ling tool, the authors pro­vide quan­ti­fied esti­mates to shed light on the decar­bon­i­sa­tion debate. Only com­mer­cial avi­a­tion – pas­sen­gers and freight – is con­sid­ered because of its impor­tance for the cli­mate foot­print: it is respon­si­ble for 88% of aviation’s CO₂ emis­sions in 2018¹.

What are the main find­ings of your study?

Jérôme Fontane. Today, the avi­a­tion sec­tor accounts for 2.6% of glob­al CO₂ emis­sions and the annu­al growth in air traf­fic is esti­mat­ed at 3.1% between 2019 and 2050². Even with the most ambi­tious pro­jec­tions for tech­no­log­i­cal improve­ments and dis­rup­tions, it is not pos­si­ble to meet the glob­al warm­ing tar­get of 1.5°C by 2100 with­out mak­ing cer­tain com­pro­mis­es: either reduce the growth rate of air traf­fic and/or increase the sector’s share of the glob­al car­bon budget.

Even with the most ambi­tious pro­jec­tions, it is not pos­si­ble to meet the glob­al warm­ing tar­get of 1.5°C by 2100 with­out mak­ing cer­tain compromises.

If we aim for a 2°C tar­get, the result is more nuanced. Our mod­els show that an ambi­tious decar­bon­i­sa­tion sce­nario makes it pos­si­ble to main­tain the fore­cast growth rate of air traf­fic as well as the sector’s share of the glob­al car­bon bud­get. This sce­nario implies tech­no­log­i­cal break­throughs to improve fuel effi­cien­cy and sig­nif­i­cant decar­bon­i­sa­tion using low-car­bon fuels for the entire fleet. 

You men­tion the car­bon bud­get, a key notion in your prospec­tive sce­nar­ios. Can you go into more detail on this?

J.F. We chose to think in terms of a car­bon bud­get, unlike oth­er mod­els pub­lished before our study. The car­bon bud­get is the only rel­e­vant sci­en­tif­ic mea­sure when work­ing on cli­mate change mit­i­ga­tion tra­jec­to­ries, as indi­cat­ed by the IPCC.

It cor­re­sponds to the max­i­mum cumu­la­tive amount of CO₂ that can be emit­ted into the atmos­phere before reach­ing car­bon neu­tral­i­ty in order for glob­al warm­ing to remain below a giv­en tem­per­a­ture. For exam­ple, we cal­cu­late³ that the glob­al medi­an car­bon bud­get between 2020 and 2050 is about 380 GtCO₂ if we want to keep warm­ing to 1.5°C. This is the max­i­mum amount of net CO₂emis­sions that should not be exceed­ed over this peri­od. For a warm­ing main­tained at +2°C, this val­ue amounts to 860 GtCO₂. 

In con­crete terms, what does this mean for the avi­a­tion sector?

Thomas Planès. The avi­a­tion indus­try is respon­si­ble for 2.6% of glob­al man-made emis­sions. If we con­tin­ue with this share, the sec­tor will have a car­bon bud­get of 10 to 22.8 GtCO₂ by 2050, depend­ing on the lev­el of warm­ing tar­get­ed (+1.5°C to +2°C, respectively). 

With­in the dif­fer­ent sce­nar­ios being test­ed, the rate of decar­bon­i­sa­tion of the fleet is the major fac­tor for reduc­ing the car­bon foot­print. Why is this? 

T.P. The avi­a­tion indus­try has already made sig­nif­i­cant improve­ments in oth­er areas, such as fuel effi­cien­cy and air­craft loads. The ener­gy vec­tor – paraf­fin – has nev­er changed, it is one of the main means of reduc­ing car­bon emis­sions today. Var­i­ous more or less devel­oped options exist: elec­tric­i­ty, hydro­gen and syn­thet­ic fuels (elec­tro­fu­el or bio­fu­el). In 2018, alter­na­tive fuels – almost exclu­sive­ly in the form of bio­fu­els – account­ed for only 0.004% of world fleet consumption.

Is it real­ly pos­si­ble to replace the fuels used?

J.F. Depend­ing on the sce­nario under con­sid­er­a­tion, the decar­bon­i­sa­tion rate varies from 0–75%. It is like­ly that the real­i­ty falls between the two. The most pos­i­tive sce­nar­ios are nev­er­the­less lim­it­ed by the avail­abil­i­ty of ener­gy resources. We have esti­mat­ed – at first glance – that avi­a­tion will account for a very sig­nif­i­cant share of demand in 2050. We must there­fore bear in mind that an opti­mistic sce­nario in which the entire fleet is decar­bonised implies direct­ing these resources towards avi­a­tion, to the detri­ment of oth­er sectors. 

Part of the decar­bon­i­sa­tion process involves improv­ing ener­gy efficiency.

T.P. We must also bear in mind that part of the decar­bon­i­sa­tion process involves improv­ing ener­gy effi­cien­cy. This involves both incre­men­tal improve­ments (aero­dy­nam­ics, propul­sion, lighter air­craft) and rad­i­cal improve­ments (inno­v­a­tive archi­tec­tures). The devel­op­ments in our sce­nar­ios are real­is­tic, but the chal­lenge is to scale up. Tech­no­log­i­cal break­throughs are not enough to sig­nif­i­cant­ly reduce car­bon emis­sions: the speed of deploy­ment and renew­al of the fleet is a very impor­tant para­me­ter, con­di­tioned by the pro­duc­tion capac­i­ty of man­u­fac­tur­ers. In our sce­nar­ios, we have assumed annu­al effi­cien­cy gains of between 1–1.5% per year through fleet renewal.

These prospec­tive sce­nar­ios only con­sid­er CO₂ emis­sions. Yet you point out that non-CO₂ effects account for about two-thirds of the cli­mate impact of aviation!

T.P. CO₂ emis­sions are the only ones for which we have a reli­able met­ric – the car­bon bud­get – and they are fair­ly well quan­ti­fied today. This is not the case for non-CO₂ effects: their impact on glob­al warm­ing is still uncer­tain. Because of their sig­nif­i­cant cli­mat­ic weight, we have car­ried out a syn­thet­ic study includ­ing them. We have extend­ed the car­bon bud­get to include non-CO₂ effects, and trans­formed the non-CO₂ effects into CO₂ equiv­a­lents. If we con­sid­er a sce­nario that includes mea­sures to mit­i­gate these effects (e.g. changes in flight paths or engines), it is pos­si­ble to reduce the sector’s cli­mate impact by a fac­tor of 3 by 2050. This is a major means of reduc­tion, but it does not make CO₂ mit­i­ga­tion use­less: non-CO₂ effects only have a short-term impact on the cli­mate, unlike CO₂.

Where should we start today to effec­tive­ly decar­bonise aviation?

T.P. As far as tech­ni­cal fac­tors are con­cerned, it is essen­tial that the ener­gy sec­tor be strength­ened to accel­er­ate the tran­si­tion to alter­na­tive fuels. The speed of fleet renew­al is also a fac­tor to be increased today, if possible.

J.F. Our work shows that no one axis is a pri­or­i­ty, it is nec­es­sary to imple­ment all these mea­sures in par­al­lel. This prospec­tive study is the first aca­d­e­m­ic report on the sub­ject, and we have focused main­ly on GHG emis­sions, one of the only reli­able indi­ca­tors to date. This does not take into account cer­tain aspects of the tran­si­tion that are essen­tial to con­sid­er. For exam­ple, oth­er envi­ron­men­tal issues such as changes in land use and the avail­abil­i­ty of ener­gy resources. A lot of work also remains to be done in the socio-eco­nom­ic sphere, such as rethink­ing the use of aeroplanes.

Interview by Anaïs Marechal