When it comes to climate change, a small number of us have disproportionate impact. That’s especially true when it comes to air travel, since most humans have never set foot on a plane. Airplanes in the United States dump five times more carbon dioxide in the sky than all airplanes in Asia, even though Asia’s population is nearly 14 times ours. A similar pattern holds true domestically. Most American adults fly less than once a year, while just 19 percent take three or more round trips a year, according to a Gallup poll from 2015. If you're one of the frequent flyers with an outsized impact on the global atmosphere, here are a few things to know about being friendlier to those friendly skies. (Spoiler alert: The best way by far is to fly less.) 1. First class = Destroyer class.

A first-class seat is luxurious, coming with more legroom, butt room and even a hot meal. It's great, if you can afford it. But first-class seats also come with something the planet can’t afford: a jumbo-class carbon footprint. Because first-class seats take up more space and bring in more cash than economy seats, the passengers sitting in them shoulder more of the burden of getting that plane in the air. The World Bank (whose jet-setting executives seldom fly coach) estimates that a first-class seat takes up to five times more space than an economy seat. Some carbon calculators simply double the carbon footprint of a coach ticket to get a first-class footprint. Here's a real-world example. Environmental Protection Agency head Scott Pruitt likes to fly first class and bring multiple aides and guards along on his travels. Pruitt's business-class seats between New York and Italy last June took up about three times more space on the Emirates A380s he and his staff flew on. (Emirates has exceptionally luxurious business class: “I felt like a king on a throne from Dubai to Bangkok,” one travel blogger wrote after a long business-class flight on Emirates.) In addition to costing U.S. taxpayers $7,000, Pruitt’s tickets were responsible for dumping nearly six tons of carbon dioxide into the upper atmosphere. Not counting his entourage or the greater effects of high-altitude pollution (see No. 6 below), Pruitt's round-trip to Italy polluted the atmosphere as much as an average Italian citizen does in a year. Pruitt also had about three times the impact of each of his aides flying coach on the same flights.

2. Pick your airline carefully. Just like you can choose to buy a gas guzzler or a fuel-sipping car, you can choose to fly on a fuel-guzzling flight or ... well, an even-more-fuel-guzzling flight. The nonprofit International Council for Clean Transportation tracks airlines by their fuel efficiency, and Seattle-based Alaska Airlines has won for domestic flights seven years in a row. Flying on Alaska does about one-fourth less damage to the climate than flying on the dirtiest airline, Virgin America. For transpacific flights, you might want to avoid Qantas and Korean Airlines. They use 64 and 36 percent more fuel respectively than the cleanest airlines, Hainan and ANA, according to the ICCT. 3. Go nonstop if you can.

Fewer miles means less fuel burned, and stopovers can add greatly to the distances flown — as well as the time it takes to get where you’re going. Takeoffs are also the most energy-intensive part of flying, so fewer takeoffs equal less pollution. (The longest, intercontinental flights can complicate this general rule since those planes are burdened by thousands of gallons of fuel that they won't burn until several hours later.) 4. Be picky about your destination. It’s a pretty direct correlation: The more miles you fly, the worse your impact. So, if you have a choice between someplace far away and someplace closer, consider staying closer to home. From Seattle, sunny Phoenix or San Diego are less than half the nonstop distance to sunny Honolulu. Flying to Sydney or Bali (again, from Seattle) will have more than twice the climate impact of flying to Costa Rica, which has twice the impact of flying to Baja California. 5. Combining trips is a good idea. It's a tip that urban transportation planners often give to fight traffic: Combine trips so you accomplish more than one goal each time you set out from your driveway. In fact, there's a whole field dedicated to this concept; it's called transportation demand management. But it's a concept lacking in most discussions about the impacts of air travel, which assume that the volume of people flying will keep, um, soaring.

The nation's fastest-growing airport, Seattle-Tacoma International, plans to double its international flights in the next 20 years. Rather than managing that growth, airport officials say their job is to meet whatever level of demand the flying public brings. It might not be practical to combine some air trips, but here's one way it can be done: fewer, longer vacations by air. Instead of two separate, week-long vacations in faraway locales, you can cut your carbon footprint by half by enjoying a single two-week vacation in one faraway locale. 6. Height matters. If you’re traveling alone, a long flight in economy class can burn less fuel than driving alone in a typical American car, not to mention a gas-guzzling SUV. But if you’re choosing your mode of travel based on its climate friendliness, don’t forget to triple the effect of fuel burned at high altitude. Exhaust released above 30,000 feet helps form hazy clouds of ice crystals (think contrails or cirrus clouds). They trap extra heat in the atmosphere. The exact amount of this contrail effect is highly uncertain and the subject of atmospheric research, but the Intergovernmental Panel on Climate Change has estimated that high-altitude exhaust is two to five times worse for the climate than exhaust near the ground. (The German nonprofit group Atmosfair’s carbon calculator incorporates the contrail effect, while some other carbon calculators do not.)