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Presuming the A320 is at least 80% full (of passengers) and comparing it to a typical road car with 2 people inside, which one is more environmentally friendly, per passenger mile?

I have attempted to do some rudimentary calculations, but then was reading that because aircraft release their emissions at altitude, this offsets (?) some of the environmental impact.

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    $\begingroup$ Define environmentally friendly. Are you referring to global warming or air quality (e.g. fine particulates)? $\endgroup$ – Sanchises Jul 29 at 16:12
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    $\begingroup$ (also, the typical road car has one person inside, sadly) $\endgroup$ – Sanchises Jul 29 at 16:14
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    $\begingroup$ This probably depends on the distance. I cannot believe that flying 50 km or about with an airliner could be more environmentally friendly. $\endgroup$ – h22 Jul 30 at 8:00
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    $\begingroup$ "More environmentally friendly" or "less environmentally damaging"? $\endgroup$ – thosphor Jul 30 at 8:06
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    $\begingroup$ I think the scope and definition of this question needs to be nailed down a lot better - at the moment it's impossible to give a decent factual answer to it (and good, factual answers are what the stack exchange network is all about) $\endgroup$ – Caius Jard Jul 30 at 9:42
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Overall, the two very different methods of transportation have surprisingly similar amounts of emissions. The exact circumstances make each better in some scenarios, but overall airplanes are slightly better for the environment for a 2-person trip.

Expert Sources

The general question here about cars vs planes has been studied in great depth by experts, so I'll refer to them wherever I can. The EPA, for example, published a detailed report1 on vehicle efficiency that's regularly updated. I've reformatted a table from that report here and made the assumption that only two passengers are in the vehicle.

Table showing a passenger car has a CO2 factor in g/passenger mile of 171.5 while a short haul airplane has 225 and a medium haul airplane trip has 136 The GHG's 2018 GHG Conversion Factors for Company Reporting reports similar numbers but with a different methodology.

Your question asked specifically about an A320. According to the the GHG's report, an A320 is about average in load factors and emissions except for (rare) long-haul. So if you chose an A320 and not an A380, your long-haul numbers would be even better than suggested by the table above.

If, And's, and But's

There are lots of considerations here, like vehicle fuel efficiency, extra hotel visits, cargo, etc. However, we could just be getting nitpicky, so let's look at how big a factor those are. For example:

  • If you're in a car, your typical trip along some of the busiest airplane routes is about 1.08 to 1.27 times longer than a trip directly point-to-point (my own estimates)

  • An extra 50 lb. suitcase would require about 33 g CO2 per mile 1

  • An extra passenger in an automobile adds minimal extra emissions, but a lot of extra emissions on a plane

  • Cars produce several times more CH4 per passenger mile than airplanes do, and a little more N2O. By my estimates looking at [1] this makes cars ~15% worse than if the CO2 or gallons fuel burned is considered alone.

  • Car emissions can vary a lot, with a 1984 SUV producing 8 times as much CH4 and 20 times as much N2O compared to a recent sedan.1.

  • Getting to and from the airport might take an extra 20-60 miles of travel each way

  • Staying in a hotel room (you're not really going to drive from San Diego to Portland in one day, right?) creates an extra 15.13 kg CO2 2

If we graph these considerations together we can compare their size. These are rough estimates and your exact emissions will vary.

Rough estimates of size for each method of transportation

Some of these considerations are huge, although some other considerations like staying in a hotel room or the trip to and from the airport are pretty minor. It makes little sense to say air travel is better for the environment without adding that many automobile trips are, in fact, better because there are 3+ people travelling or the trip is short.

It's an apples to oranges comparison

However, just comparing emissions per mile between the two is a little misleading. There are some considerations like

  • Automotive traffic contributes to smog
  • Many people go places by plane they would never go to by car
  • Air travel often requires other travel considerations at the end like not being able to travel as freely or in your favorite vehicle

Contrails' climate effect makes for a similar apples-to-oranges comparison. Contrails cause enough heating for their effect alone on global temperatures to be measurable. However, contrails also go away after just about a day, while CO2 emissions stay around for decades, some of it taking several millennia to dissipate.

Sources:

1. "Emission Factors for Greenhouse Gas Inventories", EPA, last updated 9 March 2018

2. CHP in the Hotel and Casino Market Sectors, EPA

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I'm going to simplify and assume that jets and cars burn the same fuel, and output the same exhaust, CO2, NOx and all. I'm going to compare only short-haul flights against cars.

According to Wikipedia, an A-320-NEO does 1.95L/100km per seat. Assuming flying at 80% capacity, that gives us 2.4L/100km per seat. According to The Car Guide, a 2019 Honda Civic does 6.5L/100km. Assuming an average car carries 2 people, that gets 3.2L/100km. It would seem like flying is slightly better, using this simplistic measure.

Keep in mind that the longer you fly, the better the mileage, since the takeoff and climb are the fuel guzzling segments.

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Federico Aug 1 at 5:44
  • $\begingroup$ This simplification is so great that I believe it ends up being inaccurate. Not only do they not burn the same fuel and output the same exhaust, but they release exhaust at different altitudes. Fine particulates don't really matter miles in the air, whereas water vapor does. In an urban city, it's the exact opposite. $\endgroup$ – forest Aug 3 at 10:25
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It depends on what you mean by "environmentally friendly." Just for an example, let's consider a 1000 mile trip.

An A320 burns about 5 gallons of fuel per seat per hour, and with 150 seats this comes to 750 gallons per hour. A 1000 mile flight will take about 2.5 hours, so this comes to 12.5 gallons per seat, or 1875 gallons total. This means that 2 seats worth of fuel would be about 25 gallons total.

For driving, if you assume a fairly efficient car will get 50 miles per gallon, and the drive is not as direct and is 1300 miles, this comes out to 26 gallons total.

So, on the face of it, they are approximately equivalent in fuel usage.

However, if the flight is only 80% full, then if you choose not to take the flight, it's still going to happen. This means the difference in fuel usage will be from the reduction in weight of two people and their luggage. The fuel usage will decrease by about the same amount as the weight reduction compared to the total aircraft weight. If you approximate 2 passengers plus luggage at 400 lb, and aircraft weight at 140,000 lb, based on the 1875 gallon fuel consumption from before, the additional fuel burned by the two people choosing to fly is about 3 gallons total. This makes it much better than driving.

But not all emissions are equal. The aircraft emits most of the exhaust higher in the atmosphere, where it will create contrails that help trap solar energy.

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    $\begingroup$ There's a bias in your calculation. 50 mpg means they're driving the most efficient ICE cars in the US tipping the calculation in favor of cars. Whereas US average for a new 2017 model is 25 mpg, almost 30 if you leave out small trucks. Using a more realistic, but still high, 30 mpg gives 43 gallons for the car trip or 21.5 per seat; a rather different result. The calculations might be different elsewhere in the world. $\endgroup$ – Schwern Jul 30 at 1:19
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    $\begingroup$ @Schwern, nothing in the question restricts the answer to the US. In Europe where the average car size is typically much smaller the average MPG are much higher ... I do 60mpg without breaking a sweat on a mix cycle with my little car (skoda 1.4d). And some cars do even better ... $\endgroup$ – Hoki Jul 30 at 8:10
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    $\begingroup$ This stuff makes it sound like the canard "flights kill the environment" is a bit too simplistic. If you're going to travel, it isn't clearly much more/less green to take a car versus a plane. If flights are "less green", it's more because they let you rack up a lot more distance a lot faster . It's the total miles/kilometers traveled in a span of time, say a year (~32 megaseconds), not the method, that really counts for more than the per-distance efficiency in these cases. $\endgroup$ – The_Sympathizer Jul 30 at 8:10
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    $\begingroup$ Which of the great Imperial unit countries uses this 10-pound gallon? Myanmar, Liberia, or USA? noting that fuel in the UK is sold in litres... $\endgroup$ – Harper - Reinstate Monica Jul 30 at 14:07
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    $\begingroup$ @Harper fuel in the UK is sold in liters, but car fuel economy in the UK is measured in miles per (imperial) gallon. $\endgroup$ – Peter Green Jul 30 at 23:18
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While not specific to an A320, nor a make/model of automobile, these averages may help put your question into perspective. How any of it relates to "environmentally friendly" is purely subjective.

"...the average fuel consumption in 2017 was 34 pax-km per L (2.94 L/100 km [80 mpg‑US] per passenger)..."

from https://en.wikipedia.org/wiki/Fuel_economy_in_aircraft#Flight_distance

"...The average fuel economy for new 2017 model year cars, light trucks and SUVs in the United States was 24.9 mpgUS (9.4 L/100 km)..."

from https://en.wikipedia.org/wiki/Fuel_economy_in_automobiles

Note the airliner value is per passenger, the automobile is per vehicle. if you put 4 people into the car it will 4x the passenger miles, which in this example is 99.6 (if the car has 4 passengers)

For pure fuel efficiency, the automobile, on average, would be more efficient, if both travel at the airline average of 80% passenger capacity.

This sadly isn't reality, there are too many factors left unaccounted for...the cargo transported with each flight that an automobile cannot carry. the fact that flying can go places you cannot drive, and therefore the plane is infinitely more fuel efficient. the fact that automobiles probably do not average 4 passengers, but the airline calculation was based on actual passenger and fuel consumption values. there is no mention of the emissions associated with the different types of fuel (jet vs 87 octane). there is no mention of the environmental impact of building and maintaining an airport, or building and maintaining x miles of road. etc etc etc

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    $\begingroup$ Welcome to the site. $\endgroup$ – Koyovis Jul 30 at 22:20
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I convinced my friends not to fly to Croatia from Germany for our holidays and make a road trip instead for climate reasons. I calculated the climate footprint beforehand and results were a lot different from the ones presented here. Reading this, I became unsure whether I my recommendation was right, So I asked some scientists about the above calculations. One of them is Dr. Bernhard Stoevesandt, the Head of Departement Aerodynamics, CFD and stochastic Dynamics at Fraunhofer Institute for Wind Energy Systems IWES. That is the most renown institute in Germany on that subject. This is the translation of what they said:

The climate impact of air traffic is to a large degree caused by changes in cirrus cloudiness resulting from the formation of contrails. Following the guidelines of the german environmental ministery the climate impact of airplanes is to be estimated around three times the impact of the actual CO² emissions of the plane due to the formation of contrails. For details, here is the recent research paper inclunding a summary of relevant studies: https://www.atmos-chem-phys.net/19/8163/2019/acp-19-8163-2019.pdf

I guess, the above calculations have to be adapted accordingly.

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Aircraft fuel consumption is also quite dependent on the actual distance traveled - while in cruise the engines are quite efficient, this is quite different when climbing - where they are operated near their maximum thrust. So long haul flight might get a slight advantage over a car - but on short hops, a placne can consume significantly more fuel than a car...

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Depends.

Are you bringing stuff?

The problem is that the A320 quickly becomes prohibitive, or even impossible, if you are transferring

  • too much stuff for common luggage, or, too much stuff to drag from claim to car rental
  • balky outsize stuff that won't handle as baggage
  • fragile stuff that won't survive as baggage
  • Hazmat or prohibited stuff that shouldn't fly
  • Valuable stuff that's a high theft risk

For instance you can't purge chainsaw gas enough to make it flying safe, and the long lumberman-tier bar will be too fragile and balky. The chest of Snap-On tools will go home with a baggage handler. Also, Hertz will not want that in their rental car.

The other factor is the weight of substantial stuff adds proportionately to the A320's fuel burn (i.e. an A320 with 1% more gross weight burns 1% more fuel) -- but it does not do so to an automobile at cruise. the automobile has slightly higher rolling resistance, zero acceleration energy since it's at cruise, and zero effect on aerodynamic drag. The last one dominates fuel consumption at freeway cruise. And that's my direct experience talking; I've driven my car long distances both empty and laden in comparable conditions.

Will you be extensively using a car at the other end?

In this case, you may be better off bringing your efficient car, rather than renting whatever run-of-rental-shop they happen to hand you. In my experience, most of the time, the efficient cars are gone, and they cheerfully "upgrade" you to a bigger car with abysmal fuel economy. I have often sat in the waiting room waiting for the efficient car I asked for to be cleaned and prepped, because I would be putting a lot of miles on it.

Early this decade, they were charging a significant premium for boldly-efficient cars like the Toyota Prius.

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  • $\begingroup$ All good points. Bulky, excess, or valuable stuff could also be shipped separately, where the incremental environmental cost is still pretty low. $\endgroup$ – fooot Jul 31 at 15:45
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The answer is that it is dramatically more environmentally friendly in terms of CO2-equivalent greenhouse gas release to drive any given distance than to fly it. The average American drives three times the distance from San Diego to London, yet the relative contribution of that year of driving versus a single flight from America to London is less than 150% more (just eyeballing the difference in the plot in the link below).

And as others have pointed out, aircraft burn through fuel at a precipitous rate during take-off and landing, so for shorter distances this disparity is likely to be even greater as the total ratio of cruise altitude efficiency becomes lower.

The dramatic impact that aircraft contrails have on this matter really can't be overstated.

Comparison of air versus road travel: https://phys.org/news/2017-07-effective-individual-tackle-climate-discussed.html

Average distance driving: https://www.fhwa.dot.gov/ohim/onh00/bar8.htm

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    $\begingroup$ Neither link makes any data driven comparison between air travel and car driving. The links do not confirm your statements. $\endgroup$ – Koyovis Jul 31 at 21:24
  • $\begingroup$ That is not true. Link one establishes a comparison between a single flight and a year's worth of driving. I use the second link to get an estimate of how many miles one drives in a year. The average is ~15K/year and the distance between London and San Diego (among the longest of flight options between the two continents of sizable traffic. If the two forms of travel were equivalent mile-for-mile, then the savings of not driving for a year in link 1 would be roughly three times the size of not flying to europe once, which it certainly is not. Very generous approximations were made. $\endgroup$ – LasagnaMuncher Jul 31 at 21:55
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    $\begingroup$ Also, if you want more details on Link one, you are encouraged to follow the link provided's link to the Environmental Research Letters academic article in which the findings are initially reported. However, because there are often restrictions on access to academic journals, it is common practice to use these second-party summaries of original scientific work when interacting with the general public. $\endgroup$ – LasagnaMuncher Jul 31 at 22:12
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In regards to lbs of fuel consumed per passenger nautical mile the A320 unquestionably is.

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Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

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