Bsrgvty

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.

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.

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.

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.

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