Enlarge (credit: Petmal / Getty Images) As the world races to decarbonize everything from the electricity grid to industry, it faces particular problems with transportation—which alone is responsible for about a quarter of our planet’s energy-related greenhouse gas emissions. The fuels for transport need to be not just green, cheap, and powerful, but also lightweight and safe enough to be carried around. Fossil fuels—mainly gasoline and diesel—have been extraordinarily effective at powering a diverse range of mobile machines.