This simulation shows the world-wide air traffic over a 24-hour period. Watch as day dawns across the globe: The hubbub of activity created each morning in the skies gradually tapers off in the dead of night, only to come roiling back the following morning.
According to NATCA, the National Air Traffic Controllers Association: "On any given day, more than 87,000 flights are in the skies in the United States ... and in one year, [U.S.] controllers handle an average of 64 million takeoffs and landings. (I have yet to find reliable numbers for flights worldwide, though flightaware.com provides some interesting views of flight activity, as well as a lively discussion board that's gotten a lot of attention in recent weeks.)
The airline industry denies that air traffic contributes much to GHG emissions, nevertheless price increases in jetfuel over the past year have generated increasing interest in finding a stable fuel supply that just happens to be more carbon neutral.
Hat tips to John Wagner and Gene Coyle.