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Is a carbon-free aviation industry really possible?

“We must reduce air travel to meet the Paris Climate Agreement”

Cécile Michaut, Science journalist
On February 2nd, 2021 |
3 min reading time
Aurélien Bigo
Aurélien Bigo
Research Associate of the Energy and Prosperity Chair at Institut Louis Bachelier
Key takeaways
  • Even though fuel consumption per passenger has decreased fourfold since 1960, emissions are still increasing due to the long-term growth of the aviation industry.
  • In his PhD thesis for École Polytechnique, Aurélien Bigo has studied the ways in which France’s transport industry can meet its 2050 carbon neutral targets.
  • He believes that technical advances will not be enough – a profound change in the way we travel is required.
  • He suggests thinking in terms of emissions per transport time – 90kg CO2/h for air travel versus 0.6kg CO2/h for rail travel.
  • According to his study, only by reducing traffic can we decrease emissions and meet the Paris Climate Agreement.

Your the­sis is about the ways in which the French trav­el indus­try can meet the 2050 car­bon neu­tral tar­gets. Can you tell us about the sit­u­a­tion for air trav­el as it cur­rent­ly stands?

Aurélien Bigo. Over the past few decades, the num­ber of pas­sen­gers has dou­bled every fif­teen years. We have also seen an increase in medi­um-haul flights. Aver­age fuel con­sump­tion per pas­sen­ger has decreased four­fold since 1960, due to tech­no­log­i­cal advances and a high­er load fac­tor for planes. How­ev­er, this progress has been bal­anced out – and then some – by the increase in traf­fic. This means that the green­house gas emis­sions of the avi­a­tion indus­try are con­tin­u­ous­ly on the rise. This growth has only been hin­dered, for a while, in times of cri­sis, such as the post 9/11 peri­od and the cur­rent Coro­n­avirus-relat­ed eco­nom­ic crisis.

Is the avi­a­tion indus­try capa­ble of reduc­ing its car­bon footprint?

For this, we will need dis­rup­tive inno­va­tion so that the sec­tor may one day shake its reliance on fos­sil fuels. These inno­va­tions should be assessed accord­ing to sev­er­al dif­fer­ent cri­te­ria: envi­ron­men­tal impact (on CO2, but not exclu­sive­ly), cost, and poten­tial roll-out date. Accord­ing to these cri­te­ria, no cur­rent tech­nol­o­gy is com­pat­i­ble with the French Cli­mate Plan’s goal of reach­ing car­bon neu­tral­i­ty by 2050, or with the tar­get of lim­it­ing glob­al warm­ing to 2°C. The risk is that efforts may shift to oth­er industries.

What about biofuels?

On aver­age, the cur­rent, first-gen­er­a­tion bio­fu­els emit just as much green­house gas­es as petrol does, if we look at their life cycle assess­ment (LCA)1. This is most­ly due to changes in land use – areas are defor­est­ed to make place for bio­fu­el crops, which them­selves emit green­house gas­es. Sec­ond-gen­er­a­tion bio­fu­els, made from crop residue and biowaste, emit much less green­house gas. How­ev­er, they are in lim­it­ed sup­ply. Their poten­tial is insuf­fi­cient to replace fos­sil fuels across the entire aero­space indus­try. And let us not for­get that oth­er indus­tries, such as road and sea trav­el, are also invest­ed in these kinds of fuels.

Is hydro­gen the solution?

The French gov­ern­ment recent­ly announced a nation­al strat­e­gy to devel­op decar­bonised hydro­gen2. The objec­tive is for the first hydro­gen planes to be launched by 2035, but the prob­lem is that fleets are only replaced every 20 years, at best. This plan would also not com­ply with the Paris Cli­mate Agree­ment. To stay with­in 2°C of glob­al warm­ing, we would have to reduce our green­house gas emis­sions by at least 2.7% every year, which will be impossible.

What’s more, hydro­gen has a major dis­ad­van­tage – it is very hard to store. That means it can only be used for short- and medi­um-dis­tance flights, where­as it is long-haul flights that con­tribute the major­i­ty of the industry’s car­bon emis­sions. Final­ly, the cost of hydro­gen is very like­ly too high for this fuel source to be viable. In any case, green­house gas emis­sions are not the avi­a­tion industry’s only prob­lem when it comes to the cli­mate. NOx emis­sions, con­den­sa­tion trails and induced cir­rus 3 also have a neg­a­tive impact, and nei­ther bio­fu­els nor hydro­gen would solve that.

What are the solutions?

Tech­no­log­i­cal solu­tions are not nec­es­sar­i­ly the answer. We have to think dif­fer­ent­ly. The avi­a­tion indus­try often empha­sis­es that planes do not use more fuel per pas­sen­ger and kilo­me­tre than cars do. But if we look at CO2 emis­sions per hour, it is a com­plete­ly dif­fer­ent sto­ry. The his­to­ry of mobil­i­ty shows that trav­el times have stayed the same – one hour per day on average.

Our mobil­i­ty has been shaped by the speed of modes of trans­port. We have a seen the rise in faster meth­ods, which can take us 40–50 km per day on aver­age, com­pared to the 4–5 km com­mute from a cen­tu­ry or two ago. For longer dis­tances, the same rea­son­ing applies. If we look at emis­sions per trans­port time, planes pol­lute way more than oth­er trans­port meth­ods – we are talk­ing about 90kg CO2/h, ver­sus 7kg CO2/h for cars, and 0.6kg CO2/h for trains. When we look at it in terms of trans­port time, we see that planes are the most pol­lut­ing means of trans­port, both due to high emis­sions per kilo­me­tre, and by pro­mot­ing trav­el­ling the longest distances.

So, what can be done to make the avi­a­tion indus­try to dras­ti­cal­ly reduce its emissions?

Only by reduc­ing air traf­fic will we be able to lim­it our emis­sions. But this solu­tion is clear­ly taboo in the cur­rent cli­mate, as politi­cians are count­ing on future growth, with air­port exten­sions planned for Paris-Charles de Gaulle Ter­mi­nal 4, Nice, Caen and Lille. The sim­plest way would be to take fur­ther action on domes­tic flights by clos­ing the routes that are least used and can be trav­elled by train. The cur­rent mea­sure of elim­i­nat­ing domes­tic flights that can be replaced by 2.5 hour-long train trips is not hav­ing a big enough impact. For inter­na­tion­al flights, mea­sures should be con­sid­ered on a glob­al lev­el to have a sig­nif­i­cant effect. A fuel tax could be an option. Com­pa­nies could also opt to trav­el by rail for trips under 4 or 5 hours and use video con­fer­ence tools to reduce the num­ber of long jour­neys. That being said, 75% of trips are made for per­son­al, not busi­ness, reasons.

The­sis defense on 23 Novem­ber 2020. The­sis avail­able here.

To learn more about this top­ic, see Aurélien Bigo’s arti­cle in The Con­ver­sa­tion (avail­able in French only).


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