For the purpose of argument, let's say that a plane emits 3/4 as much carbon dioxide per mile as a car. This seems to be roughly what the graph is showing in your linked post.

A single plane trip from California to Europe is 5,500 miles. Double that for the round trip, and you've got 11,000 miles. Multiply by 3/4 and you've got 8250 miles, which is easily what an average person might drive in a year. If you add another trip (notice I said "one or two" in my post), then you're definitely emitting more CO2 than the average American emits by driving for a year.

Of course, if the car person in question drives a Prius or an electric car (very common in the Bay area), the numbers look even worse for the plane person. Not to mention the fact that the plane efficiency improvements are created by packing more and more people on the plane, which makes for a miserable flight. The car efficiency improvements are generated by newer technology.