Clean takeoffs

"Our ranking is directed at all fliers, including private travelers as well as businesspeople," said Atmosfair's head Dietrich Brockhagen. The intent of the service is to allow travelers to zero in on airlines that damage the environment as little as possible.

This is the third time that Atmosfair has issued its climate-based rankings. The nonprofit organization is out to convince plane travelers to donate to environmental protection projects. Atmosfair even offers users a tool on its website to determine the emissions for a given flight, then provides a way to pay a corresponding "compensation." The idea is to appeal to people's conscience and offer a voluntary way of offsetting travel-related climate damage.

For those who just want to reduce their carbon footprint without any compensation payments, Tunisair Express may be the best choice. The small North African airline has a fleet of just four airplanes, however, and its routes are limited to Tunisia, Malta, Italy and Libya. In Atmosfair's ranking, Tunisair Express took first place.

'Apples and oranges'

Some German airlines are also among those near the top of the list. The charter airlines TUIfly and Condor landed at spots two and six, respectively. Air Berlin comes in at 12th. But Lufthansa, Germany's largest airline, doesn't fare so well, placing at 67.

"In our view, the study compares apples and oranges," said Lufthansa spokeswoman Sandra Kraft, arguing that one cannot compare charter airlines like TUIfly or Condor with a large commercial airline like Lufthansa.

One reason that charter airlines come out on top is the fact that their flights are often booked to capacity. The companies tend to focus on routes that are especially well traveled - in part only during the months when those legs are most popular.

Climate-friendly luxury

Furthermore, charter planes often include very little room between seats. Comfort zones that one would find in other planes' first or business class sections are generally unavailable. That means many more passengers fit into a single plane, which improves its climate rating.

Dietrich Brockhagen rejects the argument that charter and commercial airlines are too dissimilar to compare. "What interests us is what the customer wants to eat. Whether that's an apple or an orange, we'll offer both to him," Brockhagen said, adding that for many journeys, customers can choose freely between flying with either of the two types of airlines.

Efficient engines

Along with issues concerning capacity, which helped push Tunisair Express to the top, other criteria also help improve an airline's climate rating. The models of plane sent into the sky also play an important role. Just like with cars, there are more and less fuel efficient models.

Lufthansa made its fleet significantly more climate friendly this year and last, according to spokeswoman Kraft. She says that the data used in the current Atmosfair ranking stems from 2011, making it somewhat outdated.

Apart from the planes used, frequent short journeys prove especially harmful to the climate because the energy used during liftoff is very high. Whether the airlines in the ranking focus on offering short, medium or long routes did not play a role for Atmosfair. The most efficient providers were determined for each of the three distance categories.

Discounters left aside

One group of airlines is only included tangentially in the Atmosfair comparison. Those dubbed low cost airlines are evaluated separately from other airlines and simply ranked in groups. Dietrich Brockhagen says one reason for this is the high subsidies that allow low cost providers like Ryanair or Easyjet to sell their tickets at such low prices. The process creates demand and air traffic that would otherwise not exist.

One thing both discounters and big commercial airlines have in common is that their ticket prices determine how full flights are. The boundaries between low cost and conventional airlines have also grown increasingly blurry.

Those who want to maximize the climate friendliness of their flights must be prepared to do without some comfort. Only when a plane is tightly packed is flight efficiency maximized - reason enough for at least some travelers to stick to the ground.