Jet-setting across the country can take a major toll on the environment. Carbon emissions from flying contribute to global warming by trapping heat in our atmosphere. One round-trip flight adds about a half a ton of pollutants into the environment -- per person. But if you must fly, try to choose flights that go during the daytime. The reason is surprisingly simple: The white vapor exhaust clouds that form behind planes are trapped at night, but they're radiated by the sun during the day. It's these vapor trails that contribute to global warming.