I'm not one for extremes, like suggesting people should never fly or go on cruise ships because they generate so much pollution. But there's no excuse any more for not at least considering the environmental impact of your vacation travel and possible alternatives.
The Sierra Club has come up with a neat little quiz that highlights the climate impact of various choices. Take the quiz yourself for a fix on your travel/climate IQ, but here are a few of the conclusions I drew from it:
Jet fuel and gasoline both add about 20 pounds of CO2 to the atmosphere/gallon burned. But jets are 6 to 8 miles up in the air where the CO2 plus the water vapor and nitrogen oxides they emit create heat-trapping clouds. Non-stop flights pollute less than those that make stops.
The most environmentally-friendly way to travel is by train. Not such a great option in the U.S., but if you're going on a European tour, that's a viable choice.
Cruise ships are environmental villains. Not only do they emit three times as much CO2 as airliners, they pollute the oceans with sewage and garbage and--this is my opinion, not the Sierra Club's--exploit poor countries. The passengers get to frolic on unspoiled beaches for which the cruise lines pay the host countries little or nothing. What little money passengers do spend in their hours-long swarming of port cities goes mostly to luxury chain stores, not to the citizens of these very poor countries.
So, if you've got a fuel efficient car, pack in the family and head off on a road trip this summer. Allow a little time for serendipity, and this could be your most memorable vacation ever.