Generally, airlines across the globe had been able to lower their harmful carbon dioxide emissions, Atmosfair's 2013 Airline Index showed Monday.

The group's barometer calculated CO2 output for specific routes taking into account aircraft types, engines, the use of aerodynamic wingtips, seating capacities and actual utilization volumes. Atmosfair insisted the index was based on figures provided not by the carriers themselves, but by specialized international organizations and data services.

The new ranking showed 14 out of the tested 180 airlines reached the second-best category, notably efficiency class B, while no carrier made it into class A.

Better, but not good enough

Leading the table is Tunisair Express of Tunisia, followed by TUIfly, a charter operator from Germany. China's Juneyao Airlines ranked 7th, securing the best position among the largest network carriers.

"It's interesting to see how quickly the Asian carriers pick up," Atmosfair CEO Dietrich Brockhagen said in a statement. He also pointed out that fuel consumption differed vastly among airlines for the same route.

Brockhagen noted the best results were achieved by airlines which had recently modernized their fleet, employing more fuel-efficient aircraft such as the Boeing 777 or the Airbus 330.

But while airlines were able to lower CO2 emissions per passenger and kilometer by about 1 percent last year, the simultaneous increase in air traffic saw total emissions rise by 5 percent, Atmosfair stated. It added carriers were thus not on a path compatible with the goal of not letting global temperatures rise by more than two degrees Celsius.