U.S. airlines saw fuel efficiency increase by 3 percent between 2016 and 2018, but the entire industry’s carbon emissions actually rose by more than 7 percent in that time, according to a report from the International Council on Clean Transportation.

The emissions increase is due to air travel growth. The report said airlines can still improve fuel efficiency by purchasing newer planes and filling more seats on flights.

“We are heading off an emissions cliff right now. This is becoming even more urgent,” said report co-author Dan Rutherford.

Air travel accounts for 2.5 percent of global carbon emissions. When the report came out, airline industry officials defended their efforts at shrinking their carbon footprint.

“The fact is that the U.S. airline industry is a green economic engine,” said Airlines for America spokesman Carter Yang. “When you look at the bigger picture, the U.S. carriers transported 42 per cent more passengers and cargo in 2018 than in 2000, and we did it with just a 3 percent increase in total emissions.”

The report listed Frontier as the most fuel-efficient U.S. airline and JetBlue as the least efficient.