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Airlines hit hard by rising fuel costs have adopted many of the same countermeasures that drivers hit with higher fuel costs use: fewer trips and more efficient vehicles. The main difference is scale. An individual plane uses a lot more fuel to get from here to there than does any single motor vehicle, even though the U.S. motor vehicle fleet as a whole consumes about twice as much fossil fuel as do U.S. airlines.

A Washington, D.C.-based research group, The International Council on Clean Transportation (ICCT), published on Wednesday a study that ranks 15 U.S.-based airlines on the fuel-efficiency of their operations in 2010. The research methodology took into account the fact that airplanes burn fuel to provide both mobility (moving passengers over a distance) and access (servicing multiple cities with multiple flights).

Based on the ICCT research, here are 2010’s top five airlines in fuel efficiency:

1. Alaska Air Group Inc. (NYSE: ALK) — 1.11
2. Spirit Airlines Inc. (NASDAQ: SAVE) — 1.09
3. Hawaiian Holdings Inc. (NASDAQ: HA) — 1.09
4. Continental Airlines — 1.07
5. Southwest Airlines Co. (NYSE: LUV) — 1.06

A score of 1.0 reflects the industry average. United Airlines, now merged with Continental as United Continental Holdings Inc (NYSE: UAL), scored 1.0 in the ICCT rankings.

Here are the scores for the five least efficient carriers:

1. Allegiant Travel Co. (NASDAQ: ALGT) — 0.88
2. American Airlines — 0.89
3. AirTran Airways — 0.94
4. Delta Air Lines Co. (NYSE: DAL) — 0.96
5. Sun Country Airlines — 0.97

Here are a few of the ICCT’s observations:

• Alaska, Spirit, and Hawaiian are “relatively small carriers serving geographically limited markets.”
• Continental was the most fuel efficient of the legacy (those established before deregulation).
• Many of the worst performers were or were subsequently involved in mergers.
• The most profitable airlines during the period 2009 to 2011, Allegiant , was the least efficient.

Globally aviation greenhouse gas emissions are expected to increase by 4% annually through 2050. If that estimate comes true, aviation will account for 15% of man-made CO2 emissions by that year.