The largest airlines in the world managed to decrease their CO2 emissions by about 1 percent within a year calculated per passenger and kilometer, the Bonn-based environmental pressure group Atmosfair said in its 2014 Airline Index on Monday.

But it added the big picture was less encouraging, with total CO2 output by the air traffic industry having risen by some 3 percent simply because of rapidly growing flight volumes.

Atmosfair's index compared the greenhouse gas emissions of 193 airlines worldwide, evaluating their CO2 efficiency. With 31.2 million flights in total, the barometer thus covered over 90 percent of global air traffic.

Unreachable targets?

"The steady rise of carbon emissions shows that nations of the world must double their efforts at the upcoming climate conference in Lima," Atmosfair Managing Director Dietrich Brockhagen said in a statement, warning that the aviation industry remained within the global CO2 growth rate of 3 percent annually and with it far from limiting global warming to a maximum increase of 2 percent a year.

The progress made by individual airlines was mainly achieved by fleet modernization and retrofitting aircraft with aerodynamic winglets.

Among the large scheduled international carriers, China's Okay Airways led the ranking with a modern fleet and high occupancy ahead of Canadian Air Transat and Brazil's TAM Linhas Aereas.

Tuifly performed best among the German carriers, followed by Air Berlin and Condor.