In July, British tabloid the Daily Mail came out with a screaming headline: "EXCLUSIVE: Video shows Hillary Clinton boarding private jet just hours after launching global-warming push." Clinton's strategy to slash carbon dioxide emissions, the Mail gleefully reported, didn't preclude her from traveling on an aircraft that burns hundreds of gallons of jet fuel every hour.

Air travel by environmentalists has long been an easy punching bag for conservative pundits—and private jets like Clinton's probably deserve some sneering. But for those of us who have to make do with commercial airliners, flying is becoming much easier to defend. Michael Sivak, a transportation researcher at the University of Michigan, has found that from 1970 to 2010, the amount of energy consumed per mile, per passenger, on an average domestic flight dropped 74 percent. From 1968 to 2014, the fuel efficiency of new airplanes improved 45 percent, according to the International Council on Clean Transportation (ICCT).

For this good news, we can thank airlines' obsession with fuel, which accounts for roughly one-third of their expenses. At Boeing, the world's largest aircraft manufacturer, there's a rule of thumb: A 1 percent improvement in efficiency adds up to $1 million in fuel savings over the course of a single-aisle plane's 25-year life span.