Last year, the world's airlines pledged to achieve net-zero emissions by 2050, and whether that actually happens or not, over the coming decades aviation will experience more innovation than it has since the jet engine in the 1940s, transforming the way planes are powered and the way they look. Christopher de Bellaigue goes to the frontiers of the new technologies, from a startup in Iceland that shows what it takes to truly capture carbon, a California firm using hydrogen tanks to power their planes, to an airship called the Flying Whale. This is the story of the search for a way to fly green.