But are the per-person emissions in a passenger aircraft less than 1/2.7th compared to a car for the same distance traveled?
Also, electric planes are theoretically coming.
Also, electric planes are theoretically coming.
The key is the frivolous word in the GP comment. Aircraft actually are fairly efficient, it's just that the mileage of a single flight is a substantial fraction of most people's total annual driving. That old Jevons paradox again.
As for electric aircraft: think "powered glider" rather than "emissions-free 747". Roughly half an airliner's takeoff weight is fuel, and that's burnt up over the course of the flight. Battery storage densities are on the order of 1/100th to 1/50th of petrol or kerosene (latter is essentially jet fuel). The trend's been slight on improvement, and though there've been some battery-powered aircraft, they're comparable to a very-small-engined aircraft with a tiny fuel tank -- think 20-40hp and less than a gallon of fuel (about 15-30 kW and 2-3 litres, for rational people).
If I remember correctly, he said that the energy use per-person in a (full) commercial aircraft flying from A to B is half as much as the energy use per-car if you drove from A to B.
This means that if two people carpool from A to B, you're as efficient per-person as flying from A to B; if four people carpool, it's twice as efficient per-person as flying.
Please call me out if I'm wrong or if I misunderstood the passage from Without Hot Air.
EDIT: Source: See table 5.3 on https://www.withouthotair.com/c5/page_36.shtml
1. Aircraft have heaviest fuel use during takeoff and ascent. Short flights are the least efficient. I'm not aware of a good modeling for this, but you might consider that there's a fixed takeoff/landing/taxi cycle and a variable cruise component to most flights. Also, larger aircraft (at capacity) tend to be more efficient than smaller ones, though that's a fairly varyable dynamic.
2. Automobiles offer numerous other capaibilities (and limitations). Generally, more cargo, and more direct transit. With airport delays adding 1-3 hours to a flight, that's 150 miles / 240 km of ground travel possible at highway speeds.
3. More luggage capacity / fewer restrictions. You can carry items on a car you couldn't on an airplane.
4. Point-to-point service. Aircraft fly to airports. Cars drive to specific addresses. That's another consideration in on-ground time, possibly several hours, in a flight. Autos start looking viable for trips of 200-250 mi / 320 - 350 km.
5. There are larger autos availabe. Five adults in a small compact might not be reasonable, but might consider an SUV or van. And efficient choice here would make up for lower fuel economy with higher passenger capacity.
Aircraft still generally win on safety on a pro-rated distance basis.
But if you must make the comparison, automobiles are obviously more efficient than aircraft per passenger for shorter distances. You wouldn't burn anywhere near the amount of fuel a jet uses taxiing to drive a comparable distance, to illustrate the extreme end of the spectrum favoring automobiles.
Actually teasing out where exactly the inflexion point lies depends on the specific vehicles being compared, occupancy levels, and distance being traveled. These things have been studied and compared, Google it if you're actually interested in knowing when to drive instead of fly. It's far more prudent to simply cut down or eliminate air travel altogether, IMHO.
There are other difficult to measure impacts from flying. For example, most people I encounter in the SF Bay area like to travel frivolously by air and broadcast to the world that they're doing so in their in-person conversations and social media posts. This amounts to advertising and promoting more frivolous air travel throughout their audiences, and wouldn't really be feasible if they hadn't taken the flight in the first place. There's a greater collateral damage with air travel, because it's so accessible and convenient, all it takes is a photo of a wing or some beautiful destination you flew to and anyone who sees the photo in your friend circle can likely afford to repeat the activity - there are effectively no barriers. Social networking has exacerbated this effect _tremendously_, it's somewhat horrifying.