Although touted as the future of solar power, high-efficiency multiple-junction solar cells owe their performance to the rare metal indium, and there is only a 10 year supply of it left.

While fuel cells are still the most effective way to turn natural gas into electricity, these mostly rely on expensive platinum to catalyse the reaction, and if the 500 million vehicles in use today were fitted with fuel cells, all the world’s platinum would be exhausted within 15 years.

Carbon nanotubes and biofuels are two possible solutions, but carbon nanotubes are many years away from commercial use, and biofuels are limited by the world’s arable land.