Over the last few decades the big players in the aviation industry like Boeing have pointed to new technologies like hydrogen fuel cells, composite airframes and biofuels as a route to make air travel sustainable. But a new report by an international coalition of authors says those solutions are either physically impractical or don't provide nearly enough improvements in efficiency to compensate for the rapid growth in air travel. One of the authors of the study is James Higham - he's a professor of tourism studies at Otago University.