Airlines

The most efficient (in terms of carbon dioxide emissions) airlines in the world are listed here below. A is the highest class. Thus currently the best choice is to fly with an airline in class B. (Airlines in class D to F are not listed). The efficiency differs between distances so below the overall information is listed the most efficient large airlines for short, medium and long flight distances respectively.

OVERALL

Efficiency class A: none

Efficiency class B: Tunisair Express, TUIfly, MASwings, Monarch Airlines, SunExpress, Okay Airways

Efficiency class C: Air Transat, TAM Linhas Aereas, Air New Zealand Link, Pegasus Airlines, Meridiana Fly, KLM- Royal Dutch Airlines, Onur Air, Japan Airlines, Thomson Airways, Srilankan Airlines, Air Berlin, China United Airlines, Emirates, Alaska Airlines, Jet Airways (India), Condor Flugdienst, Aegean Airlines, EVA Airways, Corsaor, Thai Airways International, S7 Airlines, XL Airways France, Air Italy, Corendon Airlines, Avianca, Beijing Captial Airlines, Vietnam Airlines, Icelandair, US Airways, Aeroflot Russian Airlines, Horizon Air, Turkish Airlines, Air Europa, Shenzhen Airlines, Quatar Airways, Sichuan Airlines, Air Mauritius, Asiana Airlines

sea level, small island, greenhouse gas emissions
Small, flat islands are the most vulnerable to sea level rise caused by climate change - some are already submerged.

SHORT flights (up to 800km distance)

Efficiency class A: none
Efficiency class B: TUIfly, Tunisair Exp., Okay Airways, MASwings
Efficiency class C: TAM Linhas Aereas, Air Berlin, Turkish Airlines, Korean Air, Emirates

MEDIUM flights (up to 3800km distance)

Efficiency class A: none
Efficiency class B: TUIfly, Monarch SunExpres, TAM Linhas Aereas
Efficiency class C: Emirates, US Airways, United Airlines, Air France, Delta Airlines

LONG flights (over 3800km distance)
Efficiency class A: none
Efficiency class B: TUIfly, Monarch
Efficiency class C: Air Canada, Emirates, China Eastern Airlines, LAN Airlines, US Airways

Source: Atmosfair Airlineindex 2014