Flying is one of those things... If you look at it strictly from an environmental point of view, it's burns a lot of fossil fuels and should be avoided. But if you look at it from other angles, it can be quite good, allowing people to see distant corners of our planet, discover other cultures, reunite with family and friends, etc. Ideally, someday we'll be able to keep the positives and get rid of the negatives, probably with radically more efficient planes (like these) powered by next-gen aviation biofuels.
In the meantime, not all airlines are created equal when it comes to fuel efficiency, mostly because of the composition and age of their fleets and their routes. To be clear, this could only mean that certain airlines are lucky because they have 'easier' routes to serve or have just updated their fleet while another airline has an upgrade cycle coming soon (with even better planes than those who have already upgraded!). I don't look at this as a 100% apples to apples comparison, just an interesting look at the variability in the industry.
Atop the ranking of 15 airlines for 2010 is Alaska Air Group (ALK), which flies mostly newer Boeing (BA) 737 jets and Bombardier (BBD/B:CN)Q400 turboprops at Horizon Air, its regional operation. The airline’s network also is largely composed of flights to and from its hubs in Seattle and Portland, Ore. That limits the “circuity” of its flying, one of the metrics in the ranking released Tuesday by the International Council on Clean Transportation, a nonprofit environmental consulting firm funded by Climate Works and the Hewlett Foundation. (source)
The fuel efficiency gap between the best and worst airlines flying US domestic routes was 26%, according to the study. That's pretty significant!
At the bottom of the industry ranking is Allegiant Travel (ALGT), the ultra-low-cost airline that flies primarily 1980s-era McDonnell-Douglas MD-80s, one of the least fuel-efficient planes in airline fleets and a model that is rapidly being retired. A heavy mix of MD-80s also weighed on the rankings for American (AAMRQ), next-to-last at 14th, and Delta Air Lines (DAL), 11th. The former Continental Airlines, which merged with United (UAL) in 2011, was the top network airline for fuel efficiency in 2010, in fourth place.(source)