So a nonstop flight from San Francisco to Boston would create 487 kilograms of carbon dioxide emissions per person (2,708 miles x 180 g/passenger mile), or almost 1 ton for a round-trip. .... We therefore need to apply a "radiative forcing index" of 2.7 to correct for this
So, that is 2.7 tons. (though there is quite a lot of discussion on the RFI, numbers way lower has been mentioned).
Again, according to the Greenhouse Gas Protocol, your emissions would be 300 grams per mile (equivalent to a 30-mpg vehicle). Since you can't drive a straight line all the way across the country, the trip is going to be about 3,100 miles, releasing 930 kg of greenhouse gases, or less than 2 tons for the round-trip.
Flying isn't that bad, you just go go go more. (And this is for a 30-mpq car , the average is lower than that for all cars across the US)