Finnair was the least polluting of the world’s largest airlines in 2014, according to a new study, which also revealed that airlines had not made a significant reduction in greenhouse gas (GHG) emissions over the previous seven years. Recent mergers helped make U.S. carriers among the largest emitters of carbon dioxide (CO₂).

The study by the University of Warwick in Coventry, UK, examined data drawn from the annual reports of 20 large airlines as well as from the Carbon Disclosure Project, a voluntary carbon data reporting organization based in London. It took into account emissions from burning jet fuel and indirect CO₂ emissions from airline ground operations and services.

Finnair had the smallest “carbon footprint” in 2014, followed by TAP Portugal and Virgin Australia. Finnair, based in Vantaa, Finland, operates a fleet Airbus wide- and narrow-body airliners and Embraer regional jets. It was also the first airline in Europe to operate the Airbus A350 XWB; however, it first took delivery of that type last October. Airbus contends the A350 XWB, powered by Rolls-Royce Trent XWB engines, provides a 25 percent reduction in fuel burn and emissions.

“Finnair performs best due to the age and type of its planes, the routes it flies and the overall number of connections it offers,” said Frederik Dahlmann, of the university’s Warwick Business School, who conducted the study. “Plus it is probably among the most advanced when it comes to accounting for and managing its emissions over time,” he added.

Following its 2013 merger with US Airways, American Airlines had the study’s largest footprint, followed by Delta and United Airlines—the product of a 2010 merger with Continental. Also among the largest emitters were Lufthansa, Air France-KLM and Emirates.

The data available from reporting airlines also revealed that the industry since 2007 has not improved much when it comes to GHG emissions. “The data demonstrates that for most airlines emissions are either growing or stagnant; none are showing a significant reduction in CO₂ emissions,” Dahlmann said. “This is despite many airlines introducing modern and more efficient planes to their fleet. The increasing number of flights, which is set to double by 2030, means cutting emissions is a real problem.”

The study findings comport with analysis by the European Environment Agency, a branch of the European Union, which found tht aviation emissions remained relatively stable at around 150 million metric tons (Mt) of CO₂ per year during the period 2009-2013, the website GreenAir Online reported. In its latest annual assessment, the agency found that 596 commercial and non-commercial operators covered by the EU emissions trading scheme, which applied to intra-European flights, emitted 54.9 million metric tons (Mt) of CO2 in 2014, or just over one-third of EU aviation emissions.

The international aviation sector was not covered in the United Nations Framework Convention on Climate Change accord reached in in Paris in December, which set a goal of limiting global warming to less than 2 degrees Celsius from pre-industrial levels. The International Civil Aviation Organization has set a goal of reducing aviation’s net CO₂ emissions by 50 percent from the 2005 level by 2050. It also recently agreed to a 4 percent cut in CO₂ emissions compared to 2015 levels for new commercial jets delivered after 2028, Dahlmann said.