For the seventh year in a row, Alaska Airlines was the most fuel-efficient airline among U.S. carriers, according to a study from the International Council on Clean Transportation (ICCT).

Why it matters: Naya Olmer, the study's author, said Alaska Airlines "burns about 13% less fuel than the industry average, it's a profitable airline, and it's done this for seven years running...So, it's possible." The report also notes that aviation accounts for around 2.5% of global CO2 emissions, and U.S. carriers make up 30% of that.

• But, but, but: As U.S. carriers saw a 10% spike in overall revenue passenger miles between 2014 and 2016, energy effienciy fell by the wayside and CO2 emissions jumped by 7%.
• The other leaders in fuel-efficiency include Frontier and Spirit, which use "cleaner fleets, higher load factors, denser seating configuartions," and more.
• Virgin America was the least fuel-efficient U.S. carrier in 2016; Jet Blue came second-to-last.

One more thing: Alaska Airlines bought Virgin American last year. Dan Rutherford, aviation director at the ICCT, told Axios that "a back-of-the-envelope suggests that Alaska would have lost its fuel efficiency advantage over Frontier, Spirit, and Southwest in 2016 if the merger had gone ahead then. But Alaska's future efficiency will depend on how it actually operates Virgin's leased A320s, and also for how long."