As fuel costs drop for the nation's airlines, so too will the incentive to fly more efficiently.
That was the warning from a nonprofit environmental group that tracks fuel efficiency and air pollution among the world's airlines.
The Washington-based group, the International Council on Clean Transportation, issued a report last week noting that fuel efficiency among U.S. airlines remained flat from 2012 to 2013, largely because the delivery of new planes has dropped off and the percentage of seats filled on planes has flattened out.
Air travel generated 3% of U.S. carbon dioxide emissions in 2013, the report said. Jet fuel costs have dropped 20% over the last year, according to federal data.
The report named low-cost carriers Spirit, Frontier and Alaska as the most fuel-efficient domestic carriers in 2013, while American Airlines fell to the bottom of the ranking.
American Airlines responded, saying it has taken in 99 new planes this year, while retiring 26 older MD-80 jets in 2014, among other changes.
"We are in the midst of a significant fleet renewal," said American spokesman Matt Miller.