"Book more sustainably" - that was the message German airline Lufthansa marketed to travelers when it launched its new "Green Fares" in February. Lufthansa's new program gives passengers the option to spend a little more money to purportedly reduce the climate impacts of their flights. But climate advocates were quick to criticize the program as another case of greenwashing in aviation.

Almost every major airline has an offsets-based program. Passengers are sold the idea that their share of carbon emitted during a flight can be essentially canceled out by paying to support programs that theoretically reduce an equivalent amount of emissions. Currently, the aviation industry is estimated to be responsible for around 2.5 percent of global carbon dioxide emissions.