Terrapass, perhaps the best-known carbon-emissions offset program in the US (they use your contributions for clean energy and other climate-friendly projects) now offers a nifty little web page that lets you estimate your climate-warming emissions from any airplane flight, domestic or international.
It’s a nifty little Google Maps “mashup” that calculates the round-trip mileage for any plane trip, and estimates the CO2 emissions that result from taking the trip. At a minimum, that gives you some more information to help decide whether the trip is really worth it—and if it is, what it might take to mitigate your climate impact.
The neat thing, to me, is that their calculations take a pretty good stab at adjusting for the complexities of air travel. Shorter plane trips tend to have more emissions per mile than longer ones, since planes use a lot of their fuel while taxiing and gaining altitutde; and the CO2 calculator takes that into account when calculating per-mile carbon impacts.
Of course, in the case of air travel, carbon emissions aren’t the only climate impacts to worry about; contrails may also contribute to warming trends, especially from wintertime red-eye flights (see here and here for more). It looks like the calculator doesn’t take that into account. Perhaps a future version will.