Is it really greener to fly?

Until the other week, I’d assumed that it was com­pletely clear that fly­ing (par­tic­u­larly long-haul, but all fly­ing to some extent) was a really Bad Thing in terms of its con­tri­bu­tion to cli­mate change. I’ve not stud­ied the cli­mate change sci­ence in much detail, but I con­stantly hear from those who have that fly­ing is a major con­trib­utor of the emis­sions that are caus­ing cli­mate change. And of course that’s a major motiv­a­tion for our dec­sion to go to Greece by train this summer.

So I was a bit taken aback to read an art­icle in New Scientist a couple of weeks ago, entitled Train can be worse for cli­mate than plane. Could we really be hop­ping on a budget flight to get to Athens in just a few hours while sav­ing the planet, our bank bal­ances and our green credentials?

Here’s the abstract from the ori­ginal paper the New Scientist art­icle is based on -
Mikhail V Chester and Arpad Horvath, Environmental assess­ment of pas­sen­ger trans­port­a­tion should include infra­struc­ture and sup­ply chains:

To appro­pri­ately mit­ig­ate envir­on­mental impacts from trans­port­a­tion, it is neces­sary for decision makers to con­sider the life-cycle energy use and emis­sions. Most cur­rent decision-making relies on ana­lysis at the tailpipe, ignor­ing vehicle pro­duc­tion, infra­struc­ture pro­vi­sion, and fuel pro­duc­tion required for sup­port. We present res­ults of a com­pre­hens­ive life-cycle energy, green­house gas emis­sions, and selec­ted cri­teria air pol­lut­ant emis­sions invent­ory for auto­mo­biles, buses, trains, and air­planes in the US, includ­ing vehicles, infra­struc­ture, fuel pro­duc­tion, and sup­ply chains. We find that total life-cycle energy inputs and green­house gas emis­sions con­trib­ute an addi­tional 63% for onroad, 155% for rail, and 31% for air sys­tems over vehicle tailpipe oper­a­tion. Inventorying cri­teria air pol­lut­ants shows that vehicle non-operational com­pon­ents often dom­in­ate total emis­sions. Life-cycle cri­teria air pol­lut­ant emis­sions are between 1.1 and 800 times lar­ger than vehicle oper­a­tion. Ranges in pas­sen­ger occu­pancy can eas­ily change the rel­at­ive per­form­ance of modes.

What this seems to be say­ing is that you can’t just meas­ure emis­sions from the “tailpipe” (as they put it) — you have to take into account the whole life cycle of the trans­port (build­ing the train or plane, etc) and the infra­struc­ture (the roads, air­ports, rail­ways, etc). Fair enough, but the other ana­lyses I’ve seen seem to be doing that as well.

Here’s an AirportWatch brief­ing for example: Aviation Emissions and Climate Change — An Overview. Among other start­ling fig­ures they say that:

Air travel is the world’s fast­est grow­ing source of green­house gases like car­bon diox­ide, which cause cli­mate change. Globally the world’s 16,000 com­mer­cial jet air­craft gen­er­ate more than 700 mil­lion tonnes of car­bon diox­ide (CO2), the world’s major green­house gas, per year. Indeed avi­ation gen­er­ates nearly as much CO2 annu­ally as that from all human activ­it­ies in Africa. One per­son fly­ing a return trip between London and New York gen­er­ates between 1.5 and 2 tonnes of CO2.

They also men­tion an Intergovernmental Panel on Climate Change (IPCC) report which sug­gests that:

  • Aircraft released more than 600 mil­lion tonnes of CO2 into the atmo­sphere in 1990.
  • Aircraft cur­rently cause about 3.5% of global warm­ing from all human activities.
  • Aircraft green­house emis­sions will con­tinue to rise and could con­trib­ute up to 15% of global warm­ing from all human activ­it­ies within 50 years.

Another art­icle (from the David Suzuki Foundation) says:

Although avi­ation is a rel­at­ively small industry, it has a dis­pro­por­tion­ately large impact on the cli­mate sys­tem. It presently accounts for 4 – 9% of the total cli­mate change impact of human activity.

But at a time when we urgently need to reduce our green­house gas emis­sions, emis­sions from avi­ation con­tinue to grow. For example, since 1990, CO2 emis­sions from inter­na­tional avi­ation have increased 83%. The avi­ation industry is expand­ing rap­idly in part due to reg­u­lat­ory and tax­ing policies that do not reflect the true envir­on­mental costs of fly­ing. ‘Cheap’ fares may turn out to be costly in terms of cli­mate change…

Compared to other modes of trans­port, such as driv­ing or tak­ing the train, trav­el­ling by air has a greater cli­mate impact per pas­sen­ger kilo­meter, even over longer dis­tances… It’s also the mode of freight trans­port that pro­duces the most emissions.

So where does the truth lie in all of this? I asked my friend Julian (who is a cli­mate change sci­ent­ist) what was going on, and he explained that the situ­ation is rather com­plex. The impact of avi­ation doesn’t just come from CO2 emis­sions, but from other green­house gases, such as nitro­gen oxides (NOx) as well. The effect of these is also increased because they are released at high alti­tude. There is also water vapour in the form of con­trails (the long thin clouds which form behind air­craft). Various cal­cu­la­tions are used to take account of these factors, but as far as I can see there’s not much agree­ment among the experts on how to do these cal­cu­la­tions. For example, many cal­cu­lat­ors for assess­ing the impact of air travel use a mul­ti­plier called the Radiative Forcing Index to take account of these non-CO2 factors — but the val­ues of RFI they use vary quite widely. (This is all grossly sim­pli­fied! If you want to read a full dis­cus­sion, the Stockholm Environment Institute (SEI)  has a paper on Carbon Offsetting & Air Travel which explains things very well.)

I think it’s import­ant to bear in mind that all sci­entific research is pro­duced by people with par­tic­u­lar interests, who are attached to par­tic­u­lar the­or­ies which they wish to defend. (It needn’t be any­thing as crude as being paid by the avi­ation industry to come up with par­tic­u­lar fig­ures — although of course that sort of thing does hap­pen. I don’t know much about Chester and Horvath’s fund­ing: it looks as though some of the money comes from Volvo.) The research­ers who come to the con­clu­sion that avi­ation is par­tic­u­larly dam­aging also have their interests. And those of us who read the research have interests too. I notice my imme­di­ate reac­tion to this research is to look for the flaws, since it doesn’t hap­pen to back up my pet the­ory that air travel is a prob­lem. I’m not so crit­ical when I read the SEI report…

The Quaker Advices and quer­ies has a sec­tion that I found a useful:

Do not allow the strength of your con­vic­tions to betray you into mak­ing state­ments or alleg­a­tions that are unfair or untrue. Think it pos­sible that you may be mistaken.

It’s import­ant to speak truth, and to act on the basis of that truth. But the real­ity is that dis­cern­ing the truth is far from easy! We need, I think, to keep reas­sess­ing our com­fort­able truths, keep think­ing it pos­sible that we may be mis­taken. But we mustn’t get bogged down either in so much debate  that we don’t ever get round to acting.

For the moment, I think the evid­ence is strong enough that air travel is a major con­trib­utor to cli­mate change, and I’m not about to swap my inter­rail pass for a plane ticket.

2 thoughts on “Is it really greener to fly?

  1. One factor that is often over­looked which is in trains’ favour fol­lows from the way that the “non-tailpipe” infra­struc­tural emis­sions for planes include only(!) the air­ports, since they need no tracks en route. The (envir­on­mental) cap­ital invest­ment in rail includes not just the sta­tions, but the tracks and lin­eside infra­struc­ture too.
    What this means is that the rel­at­ive dam­age caused by planes and trains can be improved in trains’ favour by using the tracks more intens­ively — and many rail routes are not used at full capa­city.
    So, if there is a plane-to-train modal shift, then the arith­metic works out so that whatever advant­age this brings about accord­ing to the pre-existing com­par­ison is — in real­ity — even bet­ter.
    Of course all this leaves aside the basic­ally more human nature of train travel — and if sav­ing the planet is about human inter­ac­tions as well as car­bon dioxide…

  2. Pingback: Is it really greener to fly? | PassengerPlus.Com

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