Don’t get me wrong, flights are convenient. They’re fast and fun, and sometimes you score something duty-free. But especially if you’re buying last-minute, they can cost an arm and a leg. Furthermore, the amount of pollution a commercial jet emits in one flight is shocking. For example, flying round-trip New York to San Francisco stamps 0.9 metric tons of a carbon footprint per passenger, or about 20 percent of the average annual car emission.