Wealthy environmentalists such as Al Gore and Laurie David get a lot of stick for flying around the world in private jets. That’s because the positive greening effects of riding a bike or driving a Prius can be easily undone by using a Gulfstream IV, which burns tons of jet fuel just to move a few people a few hundred miles to a conference or vacation home.

But if members of the eco–jet set were to fly commercial—and join a few hundred other people sardined into coach—it might be a different story. It may seem counterintuitive, but a provocative study released earlier this year argues that in the U.S., flying from place to place actually consumes significantly less energy – and hence produces significantly fewer emissions—than driving does.

In the study, Michael Sivak, a research professor at the University of Michigan’s Transportation Research Institute, crunched data on passenger movements over the last 40 years and calculated the amount of BTUs needed to move a person one mile on a plane compared with the number used to move a person a mile. We should note at the outset that the study has not yet appeared in a peer-reviewed general, and that it covers large, aggregate sets of data that applies specifically to the U.S.

In 1970, Sivak reports, it took 10,185 BTUs to move a person a mile in an airplane, compared with 5,067 BTUs to move a person a mile in a car—making flying twice as energy intensive as driving. But as the chart below shows, the ratio came down steadily. By 1985, airline travel consumed only 4,950 BTUs per passenger mile, less than half the 1970 amount. Car travel became more efficient, too, but not so dramatically. By 2000, airline travel (3,892 BTU per passenger mile) officially became greener than driving (3,926 BTU per person, per mile.) The trend has continued so that in 2010, flying burned just 2,691 BTU per passenger mile—an improvement of 74 percent since 1970. That was 43 percent better than driving the average car, which gets about 21.5 miles per gallon (4,218 BTU per passenger mile).* It was better than buses as well.

“People still think the situation is still the same as it was in the 1970s, and they are constantly citing old data showing flying is worse for the environment than driving,” Sivak says. “That’s not the case.”

What accounts for the dramatic change? The answer has surprisingly little to do with regulations and standards, and everything to do with market forces, the peculiar dynamics of the airline market, and Americans’ rising propensity to drive solo.

“For a long time, fuel has been our No. 1 cost center,” says Nancy Young, vice president of environmental affairs at Airlines for America, the trade group for airlines. “Fuel is anywhere from 30 to 35 percent of an airline’s cost.” When any single input accounts for so much of total spending, companies have ample motivation to figure out how to use less of it. For airlines, reducing the use of energy used per passenger mile hasn’t been a matter of being environmentally correct; it has been a matter of life and death.

In an era when it was routine for big airlines to go bankrupt, controlling energy use has become something of an obsession. For years, airlines have been pressuring manufacturers to come out with more fuel-efficient versions. Given its new components, systems, and materials, a plane like the new 787 can be anywhere from 15 to 30 percent more fuel efficient than the plane it is replacing. Winglets—the little tips at the end of wings that can boost efficiency by between 2 and 6 percent—are becoming standard. “Planes today are even using more drag-resistant paint,” Young says. Since 1978, she says, the fuel efficiency of planes has more than doubled.

Airlines have also been busy making their operations less energy-intensive. Airlines use flight-optimization software to determine how planes can go from point to point while burning less fuel. On the ground, planes have started to use only a single engine while taxiing. Young notes that airlines are committed to boosting efficiency by 1.5 percent per year through 2020, at which point they would like their growth to be carbon neutral. Commercial aviation in the U.S. may account for only 2 percent of emissions, but the industry nonetheless wants to prepare for a time when emissions may be limited. “Our economic interests align with our environmental interests,” Young says.

The real revolution, however, has come in airlines’ greater efficiency in filling planes. Getting the airplane off the ground and moving it at speeds of 500 miles per hour burns a lot of energy. Adding a few more people doesn’t require that much more. As a result, a plane with 200 people aboard will be much more efficient on a per-person basis than a plane with 100 people aboard.

For decades, airlines seemed to be indifferent as to how full their planes were. In fact, that’s one of the reasons they found it so hard to make money. In 1970, it was not uncommon for airplanes to fly with half their seats empty. In 2002, the load factor for domestic flights was 70.37, meaning about 30 percent of all seats were unfilled. That’s untenable. As airlines have become more ruthless—cutting routes and flights, overbooking existing flights—the load factor has risen virtually every year. In 2013, the domestic load factor was 83.47, an increase of nearly 20 percent from 2002. (Since international flights are slightly less crowded, the combined load factor for domestic and international flights was slightly less impressive in 2013: 82.78 percent.) The number of passenger miles flown in 2013 was up 37 percent from 2002. But the number of flights, seen here, has risen only 6 percent in those years.

Of course, that’s not to say that aviation-related emissions don’t contribute to global warming. In fact, a 2009 study in the journal Atmospheric Environment suggests that the releasing emissions higher in the atmosphere can be more damaging than release them at ground level.

Here’s the irony: At the same time that air travel has become far more efficient in the absence of higher government standards, car travel has actually become less efficient even though federal standards have become tougher. Those changes have much more to do with the way we use the equipment than with the equipment itself. In 1970, the typical car journey saw 1.9 people in a vehicle. But by 2010, the load factor per trip had fallen to 1.38. “The load factor has gone up in aviation and has gone down in driving,” as Sivak noted.

In fact, it’s the way people in America drive—the vehicles they choose to drive and the people they choose to drive with—that helps keep airplanes competitive. Writing in these pages earlier this year, Slate contributor Eric Holthaus said that he had foresworn flying because of its disproportionate climactic impact. In his piece describing why he took a 28-hour bus ride from Wisconsin to Atlanta, he noted that the amount of emissions associated with driving that distance in a Honda Civic (1,386 pounds of carbon dioxide) was roughly equal to the amount of emissions he would have caused in flying that distance (1,587). Note, of course, that the 2012 Honda Civic gets 39 miles per gallon on the highway, where most of that driving would take place—far above average. Sivak’s study points out that the average light-duty vehicle on the road in the U.S. gets about 21.5 miles per gallon. His study suggests that vehicles would have to get more than 33.8 miles per gallon in order to be less fuel-intensive than air travel. Holthaus crossed that threshold by incurring a huge personal inconvenience, but most drivers don’t come close.

*Correction, March 26, 2015: This article originally misstated that in terms of BTUs per passenger mile, flying was 43 percent better than driving in 2010. It was 43 percent better. (Return.)