Airlines’ greenhouse gas emissions increased more than twice as fast as their improvements in fuel efficiency between 2016 and 2018, according to a new report by the International Council on Clean Transportation.

While aircraft fleets boosted their efficiency by 3% over that period, their emissions grew 7%, The Associated Press reports.

The study concluded that carriers could cut their emissions and fuel consumption by more than 25% by “buying newer planes and filling them with more passengers,” AP says. But report co-author Dan Rutherford said airlines’ operational improvements aren’t moving fast enough to keep up with rising travel volumes. “We are heading off an emissions cliff right now,” he said. “This is becoming even more urgent.” 

With more than 40 new Airbus jets in its fleet, Denver-based Frontier Airlines posted the highest efficiency among the top 11 U.S. carriers, the news agency notes. New York-based JetBlue ranked last, with spokesperson Tamara Young attributing the company’s performance to fewer seats in coach and a larger number of flights in congested areas like New York.

With airline organizations moving at a glacial pace to address their emissions, aviation specialists in the global climate community have been warning for years that the sector could undercut the wider effort to bring greenhouse gas emissions under control. But the U.S. aviation lobby still took issue with ICCT’s findings.

“The fact is that the U.S. airline industry is a green economic engine,” said Airlines for America spokesperson Carter Yang. “When you look at the bigger picture, the U.S. carriers transported 42% more passengers and cargo in 2018 than in 2000, and we did it with just a 3% increase in total emissions.”Still, AP says the ICCT report could contribute to the flight shaming trend that has begun to sweep parts of Europe. “It’s crazy that you can get all this data on the cost of your ticket and amenities on your flight, but you have no information about how carbon-intensive your flight is,” Rutherford said.