Here is some quick back of the envelope calculations:

26 g CO2/Mile for a small car

176 g CO2/Mile for a plane

I take one trip round-trip from San Francisco to New York City: 6,000 m x 176 = 1,056,000 g.

I drive all year in my small car: 10,000 miles x 26 = 260,000 g

4 times as much for one plane trip than for all my car travel for a year. Now imagine if I take 4 or more trips. It adds up.

Abstract calculators don't really explain the actual use per actual person, which is what Antony is talking about.