No matter where you’re flying or what airline you’re flying, there’s no way around it — flying is a fuel-intensive way to travel. As I wrote Friday, it’s nearly impossible for aviation to be made climate-friendly, so the sector plans to meet emission targets by establishing a scheme for purchasing carbon offsets sometime in the future.

In the meantime, aviation industry leaders (who don’t dispute the science of global warming) are concentrating on energy efficiency, in part because fuel is an airline’s number one cost. But not every airline is doing it as well as others.

The International Council on Clean Transportation (ICCT), an independent nonprofit funded by private foundations and entities such as the UE and World Bank, has been tracking airline fuel efficiency since 2010. Its latest report found no overall net gain in fuel efficiency from 2012 to 2013, and a 27 percent gap between the three most efficient carriers — Alaska, Spirit and Frontier Airlines— and the least efficient one, American Airlines.

The report’s “fuel efficiency score” is a unitless measure calculated using an airline’s revenue passenger miles, the number of airports it serves and its flight frequency per unit of fuel burned. A score of 1.00 is the industry average. The greater the number, the better the efficiency.

CARRIER FUEL EFFICIENCY SCORE, 2013 Alaska 1.11 Spirit 1.11 Frontier 1.11 Southwest 1.07 Hawaiian 1.02 United 1.01 JetBlue Airways 0.99 Delta 0.98 Virgin America 0.97 US Airways 0.97 Sun Country 0.93 Allegiant Air 0.92 American 0.88

Alaska and Spirit Airlines have occupied the top two spots since 2010, and Frontier climbed to number three in 2013, after holding the number five spot for the last two years.

The differences between airlines probably come down to aircraft and operations, says Nancy Young, vice president for environment at Airlines for America, an industry trade group. Airplanes can last several decades, and replacement cycles vary by airline. Efficiency normally rises when an airline purchases new aircraft.

Then there’s operations. Alaska Airlines is based at Sea-Tac, an airport that uses the fuel-saving NextGen navigation system. “The efficiency of the air traffic management system, which the FAA controls, makes a very significant difference on how fuel-efficiently you can fly,” Young says. (American Airlines spokesperson Brianna Jackson declined to comment on the report.)

Despite obvious financial incentives for airlines to cut fuel usage, the study found no direct correlation between profitability and efficiency. Frontier, tied for first in the efficiency rankings, was rated as just barely profitable in 2013, while Spirit and Alaska topped the lists for both efficiency and profitability. The report ranked Allegiant fourth-highest in profitability, but second to last in fuel efficiency.