Against single-occupant cars, airliners win big. Airliners typically see 80-100 passenger-miles per gallon, versus about 36MPG for the average American car.
But if you get a more efficient car and add more people to it, airliners don't win anymore. Put four people in a new Prius and you're looking at about 200 passenger-miles per gallon. Long-distance busses and trains do really well too. And of course you can power surface transportation with alternative energy sources much more easily. Electric trains are common and electric cars can make long trips now, while nobody really even knows how to build a useful electric airliner.
I don't understand the emphasis put on air travel when it comes to emissions, and it's definitely better than typical car habits, but surface travel in general can be a lot better than air travel.
No.
Air travel is generally a separate category of travel that's mostly independent from other forms of travel. It directly competes with long range high speed rail, but it's far worse than rail travel. http://www.seat61.com/CO2flights.htm
Air travel sit's between single passenger cars and buses in terms of pollution. The break-even is generally ~3 people in a car depending on MPG and how out of the way the airport is.
Of course as it becomes a major mode of travel, its worth addressing. But for now, air travel is actually the preferable mode.