Airlines get a bad environmental rap for their fuel-burning emissions, but several are taking steps to reduce their mile-high carbon footprints.

Which airlines get the highest marks for reducing their negative impact on earth? Greenopia, an online directory of eco-friendly businesses and organizations, has compiled its fourth annual scorecard of the greenest American and European fleets, helping ethical travellers do the least amount of harm to the skies. This week, the guide gave BBC Travel a sneak preview of the rankings, and the complete report will be available on on 8 May.

Virgin Airlines tops the US list for the fourth year. Greenopia reports that Virgin earned the “greenest” honours -- that’s four “leafs” -- by a considerable margin. Airlines are rated on six criteria: fuel conservation practices; alternative fuel research; recycling programs; use of organic, local and fair trade food; green design of on-the-ground buildings; and carbon offsets. Virgin, with a young, efficient fleet of airliners, does well in most of them. Greenopia calls it a leader in green food, recycling, biofuel research and carbon offsets for passengers.

Alaska Airlines came in second with three leafs, scoring well in recycling, building design and carbon efficiency. Greenopia’s one complaint: Alaska Airlines doesn’t have a carbon offset program. (Alaska does get top billing for major carriers, as Virgin isn’t considered a major carrier.) Third place went to United, which was also the most improved since last year’s scorecard. While that was mostly due to United’s acquiring of eco-friendly Continental (which came in second last year), the airline also made progress in alternative fuel research, eco food options and offsets.

The rest of the US rankings are: Jet Blue, Delta, US Airways, Southwest, Air Canada and American Airlines, with the last two companies getting just one “leaf” apiece (Greenopia includes Air Canada in its US list).

But Greenopia’s Douglas Mazeffa, the group’s research director, explained that even the airlines at the bottom of the list had made strides in environmental stewardship. “Every single company is making at least a light green effort,” he said. “In the cases of Air Canada and American, it's not that they do a poor job necessarily, it's just that they are not doing as much as the other major airlines -- in terms of reporting, carbon offsets, food options, building design, etc -- and/or that their fleets are simply less efficient.”

In fact, Mazeffa said that airlines overall have been “the most rapidly improving industry of the ones we study.” Greenopia researches everything from cars and electronics to manufacturers of beer and pet food. Airlines can improve their fuel conservation by using single engine taxiing and electric-powered vehicles on the ground. Planes can also fly on biofuels.

As for the European airlines, Air France took the “greenest” title by a wide margin thanks to being the most carbon efficient and having a strong commitment to biofuels, recycling and a carbon offset program. Coming in second, in a four-way tie for second place, were Lufthansa, British Airways, Virgin Atlantic and EasyJet.

European airlines were better than US companies in some areas (offering carbon offsets and investing in biofuels) and weaker in others ( environmental reporting of carbon emissions and recycling rates). Greenopia gathered its data from airlines’ annual reports and other public documents.

Lori Robertson writes the Ethical Traveller column for BBC Travel. You can send ethical dilemmas to