Take one round-trip flight between New York and California, and you’ve generated about 20 percent of the greenhouse gases that your car emits over an entire year. If you’re flying, you’re adding a significant amount of planet-warming gases to the atmosphere — there’s no way around it. But there are some ways to make your airplane travel a little bit greener. The most effective way to reduce your carbon footprint is to fly less often. If everyone took fewer flights, airline companies wouldn’t burn as much jet fuel. ...according to some calculations, a round-trip flight from New York to San...