An unpublished study by the U.S. Transportation Department, European air traffic manager Eurocontrol, Manchester Metropolitan University and the technology firm QinetiQnet said total emissions are set to reach between 1.2 billion and 1.5 billion tons annually by 2025, The Independent reported Tuesday.

The report, presented at a conference last year in Barcelona, Spain, was uncovered by the Aviation Environment Federation.

"Growth of CO2 emissions on this scale will comfortably outstrip any gains made by improved technology and ensure aviation is an even larger contributor to global warming by 2025 than previously thought," spokesman Jeff Gazzard told the British newspaper. "Governments must take action to put a cap on air transport's unrestrained growth."

The International Air Transport Association there has been a 70 percent improvement in fuel efficiency in the last four decades, the newspaper said.