In the worldwide push to cut greenhouse gas emissions, transport fuels present one of the thorniest obstacles.

The International Energy Agency, the rich countries’ think-tank, estimates that almost a quarter of the world’s energy-related carbon dioxide emissions are generated by transport. The global spread of personal mobility – with the number of cars on the road expected to triple to 2bn over the next four decades – and the rise in national and international travel and trade means those emissions are set to more than double by 2050, unless something is done to stop them.