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.
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.
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.
Overall, the two very different methods of transportation have surprisingly similar amounts of emissions, so close it's tempting to oversimplify things to a statement like "planes are 20% more efficient than cars". The exact circumstances make it so you can't say one is better in many or even most circumstances. Even worse, different definitions of "environmentally friendly" can produce estimates that vary by 50 to 100%. At the end of the day, for a two-person medium-range trip, a plane is slightly better for greenhouse gas emissions.
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. It's also a tricky matter where slightly different assumptions can lead to noticeably different outcomes. For example, BBC published a great article on the subject3 based on the UK's BEIS reports. The EPA also 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 automobile.
The UK's Department for Business, Energy, and Industrial Strategy's 2019 GHG Conversion Factors for Company Reporting reports similar numbers but with a different methodology. These numbers do not include other emissions like contrails and soot, which can be according to the BEIS can be accounted for with a 1.9x multiplier for aviation.
Your question asked specifically about an A320. According to the the BEIS'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.
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. But by my estimates this makes a small difference of less than 1%
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.
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 traveling or the trip is short.
However, just comparing emissions per mile between the two is misleading. There are some big considerations like:
Contrails', soot, NOx, and other non-greenhouse-gas emissions effect on climate 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. High altitude NOx similarly becomes less potent much faster than CO2.
Despite all these caveats, you can try to account for such emissions with something simple like BEIS's 1.9x multiplier, which the UK's BEIS says is often necessary, but admits that "there is no suitable climate metric to express the relationship between emissions and climate warming effects from aviation, but this is an active area of research. Nonetheless, it is clear that aviation imposes other effects on the climate which are greater than that implied from simply considering its CO2 emissions alone."
Sources:
1. "Emission Factors for Greenhouse Gas Inventories", EPA, last updated 9 March 2018
2. CHP in the Hotel and Casino Market Sectors, EPA
3. Climate change: Should you fly, drive, or take the train?, 24 August 2019, Reality Check column
4. UK's Department for Business, Energy, and Industrial Strategy's 2019 GHG Conversion Factors for Company Reporting
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
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.
Note: the UK Govt report gives figure that is 150-200% higher than the USA's EPA agency, stating that it's "High Altitude Effects" of CO2... So, either one or the other report is wrong by 100-200 percent!
Here is another study which is at odds with the previous figures, because it includes high altitude effects, where emissions are released at 8km altitude versus close to sea level... Nitrous Oxide is 300 times more potent than CO2 as a greenhouse gas, and it falls to the ground easier from road level, it's 1.5 times denser than air. NO2 emissions released at 8km altitude are significant greenhouse agents.
Source: bbc.com
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...
Depends.
The problem is that the A320 quickly becomes prohibitive, or even impossible, if you are transferring
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.
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.
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
