If you burned a litre of petrol on the way to work, consider this: it took 23.5 tonnes of ancient, buried plants to produce. That's the equivalent of 16,200 square metres of wheat, roots and stalks included. So says new research that aims to raise awareness about the need to change our energy-consumption habits.

The long, slow process that converts plant matter into oil is very inefficient, says ecologist Jeff Dukes of the Carnegie Institution of Washington, Stanford, who did the calculations. Less than one part in 10,000 of the organic matter becomes oil.

"So much carbon is lost back to the atmosphere through decomposition, it's only the residues that are turned into fossil fuels," says Dukes. Writing in the journal Climatic Change, he warns that less than a tenth of the carbon in plants buried in peat bogs was turned into coal. In 1997, he points out, we burned fossil fuels equivalent to more than 400 times the amount of plant matter produced on Earth in the same year.