A number of technologies designed to reduce the environmental impact of flying have been researched, tested and implemented. However, compared with greener cars, where the technologies are proved and the carbon saving huge, the potential for eco-friendly flying looks rather limited. There will be some further gains in engine efficiency over the coming decades, and larger planes with more seats will allow slightly lower emissions per passenger. But there is nothing in the pipeline with the transformative potential of the electric car.

The problem is that electric motors can’t produce enough power to get a plane off the ground, so the only alternative to regular kerosene-based aviation fuels are special kinds of biofuels. These aren’t an ideal solution, since biofuels can be environmentally problematic in themselves, and anyhow it would take a huge chunk of the world’s arable land to grow enough crops to fuel all the world’s planes. (A back-of-the-envelope calculation suggests it might require as much as a fifth of all cropland.)