United Airlines planes are seen on platform at the Newark Liberty International Airport in New Jersey Thomson Reuters Eco-activists often indict air travel as a carbon-spewing threat to the global climate.

Worldwide, commercial jets pour an estimated 700 million tons of carbon dioxide into the atmosphere each year, leading to headlines like this one in The New York Times not long ago: “Your Biggest Carbon Sin May Be Air Travel.”

But it turns out that air travel’s notoriety as a most-wanted carbon criminal is a bum rap.

Passenger jets are in fact significantly more efficient than automobiles, if you measure on a passenger-mile basis.

And, if you’re worried about your personal carbon footprint, getting on a plane from New York to LA may actually put less carbon into the air than driving to work tomorrow.

Big picture or small

There are two ways to look at carbon emissions from aircraft running on standard jet fuel.

For the systemic big picture, we can consider industry-wide fuel usage and total passenger-miles, calculating each passenger’s share of the overall CO2 burden.

Or we can take the personal-carbon-footprint point of view: How much carbon will be added to the atmosphere if I take this flight, compared to staying home?

Either way you look at it, the passenger jet blows away the automobile in terms of efficiency and CO2 emissions per mile.

Rapid aviation progress

It hasn’t always been so.

Back in 1970, according to a recent report from the University of Michigan, planes were twice as energy-hungry as cars.

Looking at U.S. passenger-miles flown and total fuel burned, the report concluded that it took 10,185 BTUs of energy to move one person one mile in an airplane.

(A BTU is a British Thermal Unit, the amount of energy to heat one pound of water by one degree F.)

For light vehicles (cars, sport utilities, pick-up trucks, and vans), the corresponding 1970 figure was only 5,067 BTUs per passenger-mile, just half the planes’ number.

Cars are lined up for sale at a Ford dealer in Lakewood Thomson Reuters Score one for the cars.

Since then, automotive fuel economy has improved steadily, from an average of 13.5 mpg to 21.5 mpg in 2010.

However, each mile those cars cover moves fewer people nowadays. In 1970, the average car trip included 1.9 people.

By 2010, thanks to more single-occupancy commuting, the average load factor per trip had fallen to about 1.4.

With the lower load factor eating away at the higher mpg ratings, overall automotive energy intensity improved a mere 17 percent between 1970 and 2010, to 4,218 BTU’s per passenger-mile.

Jets improve dramatically



Planes, on the other hand, have made huge strides in both fuel efficiency and passenger load factors since 1970.

The noisy, thirsty turbojet engines of yore have been replaced by fuel-efficient high-bypass turbofans, while improved aerodynamics and carbon-fiber structures have made jets slicker and lighter.

The latest Boeing 787 burns less than half the fuel per mile of the 60s-era turbojet-powered 707.

Moreover, the airlines have drastically increased their load factors, as any long-time frequent flier can attest (and lament).

In the pre-deregulation days of 1970, jets typically flew around barely half full. By 2002, the load factor was up to 70 percent; now it's well into cattle-car territory at 83 percent.

Overall, the combination of better technology and more crowded planes has ratcheted down the energy intensity of air travel by an impressive 74 percent, to only 2,691 BTUs per passenger-mile.

That’s 57 percent better than your average automobile.

Advantage: planes.

Speed bonus

Commercial jets’ superior efficiency is particularly astonishing given that the planes travel 10 times faster than cars.

The laws of physics typically exact a harsh energy penalty for going fast: The energy required to overcome air resistance goes up with the square of the speed.

Thus it takes four times the energy to fly twice as fast through air. All else being equal, to go 10 times faster theoretically takes 100 times more energy.

And yet, commercial jets, leveraging their huge size and the low-drag environment away from the ground, go 10 times faster than cars on less energy per passenger-mile.

Amazing.

Can any car match an airplane?

If you want to match the efficiency of air travel on the ground, you’ll need to drive a car that gets about 33 miles per gallon—assuming you’re taking the statistically prescribed additional 0.4 person along for the ride.

If you’re driving alone, you’ll need a car that gets about 47 mpg. That all but rules out any gasoline-powered car other than the Toyota Prius.

Or you could stick with your average 21-mpg car and simply carry along an additional 1.2 passengers.

Better yet, drive an electric car, which is typically three to four times more efficient than a gas car. Even as a solo commuter, you’ll easily trounce the most efficient commercial jet in a Green Cred face-off.

The seven-seat Tesla Model X, in fact, may be the most energy-efficient production passenger-carrying vehicle in the world. With an EPA efficiency rating of 92MPGe, the X has the potential to achieve a staggering 644 passenger-miles per gallon equivalent.

That’s six times better than a state-of-the-art 787 with every seat full.

I can’t think of another passenger vehicle in production that can even come close to the Model X for carbon saintliness.

Any suggestions from readers?

Your personal carbon footprint

BTUs and passenger-miles aside, how much extra fuel will be burned (and extra CO2 emitted) if you decide to take that vacation flight to the Virgin Islands instead of staying home?

As a budding carbon criminal, how guilty should you feel? Let’s do the numbers.

Assume a Boeing 757 flying from New York to St. Thomas.

The fuel consumption of the 757, like all aircraft, depends partly on the total weight of the plane, including fuel and passengers.

The heavier the plane, the more fuel burned. (That’s where your extra body weight, plus your third and fourth suitcases, come in.)

According to the 757 flight manual, at a speed of Mach 0.79 and 35,000 feet, the plane will burn 6,938 pounds of fuel per hour at a flight weight of 195,000 lbs.

At a heavier weight of 205,000 pounds, the manual says that fuel consumption climbs to 7,132 pounds per hour.

So 10,000 pounds of extra weight causes the plane to burn 194 more pounds of fuel per hour.

Flickr / James Emery As a 175-pounder with 25 pounds of luggage, your 200 lbs amount to 1/50th of that extra 10,000-pound burden.

Your pro-rated share of the extra fuel burned: 1/50th of 194 pounds, or 3.9 pounds per hour.

Your flight to St. Thomas will take about 3 hours. Call it 12 extra pounds of fuel burned. Throw in an extra 2 pounds for taxi, climb, and descent.

Grand total of extra fuel burned because you were on the flight: 14 pounds. Or a little over 2 gallons.

Carbon criminal? Hardly. You’ll probably use more fuel than that commuting to work tomorrow.

Go ahead, take the vacation. Lying on the beach instead of driving to work may even reduce your carbon footprint for the week, round-trip flight included.

Too easy

The problem with airline travel is not that it’s inherently carbon-intensive.

The problem is that commercial jets, so astoundingly fast and efficient, make it cheap and easy to travel very long distances.

And so we use them to do just that.

For most people, a year’s worth of travel by car covers 10,000 to 15,000 miles. A round trip to Europe by air could eat up that same distance—very efficiently, of course—over a long weekend.

Maybe that New York Times headline should have read “Your Biggest Carbon Sin May Be Travel Itself.”