The aviation industry is under pressure to do something about the immense amount of carbon emissions its engines generate. The output is accelerating many times faster than from sectors such as energy and agriculture, contributing to global warming. Paul Eremenko, a 41-old former chief technology officer for Airbus SE and United Technologies Corp., thinks he has a solution: hydrogen-powered airplanes. He well understands, of course, that his idea has a bit of a public relations problem right from the start. Hydrogen power and flight conjures up visions of the flaming Hindenburg airship.