Next time you fill up at the petrol station, ponder this figure - it took over 23 tonnes of plants to produce each and every litre of petrol you pump into your tank.
Jeff Dukes, from the Carnegie Institution of Washington, came up with this astonishing figure while studying how efficiently fossil fuels store sunshine as energy. "Fossil fuels developed from ancient deposits of organic material, and thus can be thought of as a vast store of solar energy," Dukes says. Plants use photosynthesis to turn the sun's energy into carbon, which is then converted into gas, oil or coal (known in the coal industry as buried sunshine). Over millions of years the plant matter, trapped in peat swamps or as sediments on the sea floor or lake beds, is converted by heat and pressure to form fossil fuels.
It turns out that this process is a very inefficient one, as Duke discovered when he used existing data to estimate how much carbon was lost at each stage. Only 9% of the carbon in the original plants makes it to coal, while just a tiny proportion - 1/10,750 - remains in oil or gas.