From the heavy whiff of kerosene to the roar of jet engines and the criss-crossing of contrails, the recent return of commercial flights to our skies is an all-too-familiar attack on the senses. How aircraft will look, sound and even smell in another 30 years is altogether less certain. Despite accounting for just over 2% of the world’s greenhouse gas emissions, the aviation industry — almost exclusively dependent on fossil fuel — is seen as the poster child for global warming.