The report, which is an annual update to an earlier report (earlier post), also investigates changes in fuel efficiency since 2010, both for individual airlines and the industry as a whole.

Dr. Daniel Rutherford and Irene Kwan linked the slowing industry improvement rate in recent years to a lack of new, more-efficient aircraft types, the time lag between new aircraft delivery and penetration into the in-use fleet, and diminishing gains from increasing load factors.

Alaska, Spirit, and Frontier tied as the most fuel-efficient domestic carriers in 2013. Alaska and Spirit have consistently led the performance ranking since ICCT’s original baseline analysis of 2010 data. Frontier jumped ahead of Southwest Airlines due to a large (+10%) one-year improvement.

The ICCT researchers also found that the fuel efficiency gap between the most and least efficient airlines widened slightly to 27% in 2013. Allegiant improved its fuel efficiency in 2013 by adding second-hand Boeing 757-200, A320 and A319 aircraft to its older MD-80s fleet starting in 2011, while American’s fuel efficiency declined by about 1.5% from 2012 to 2013.

Domestic flights in the US account for about 24% of global CO₂ emissions from commercial aircraft, and are expected to grow an average of 1% per year over the next 20 years, increasing annual emissions from 116 million metric tons (MMT) in 2014 to about 143 MMT CO₂ by 2034.

There continues to be no clear correlation between airline profitability and efficiency, though all 13 major US domestic carriers were profitable in 2013.