Many people consider the wider use of biofuels a promising way of reducing the amount of surplus carbon dioxide (CO2) being pumped into the air by the world’s mechanised transport. The theory is that plants such as sugar cane, maize (corn, to Americans), oilseed rape and wheat take up CO2 during their growth, so burning fuels made from them should have no net effect on the amount of that gas in the atmosphere. Biofuels, therefore, should not contribute to global warming.

Theory, though, does not always translate into practice, and just as governments have committed themselves to the greater use of biofuels (see table), questions are being raised about how green this form of energy really is. The latest come from a report produced by a team of scientists working on behalf of the International Council for Science (ICSU), a Paris-based federation of scientific associations from around the world.

The ICSU report concludes that, so far, the production of biofuels has aggravated rather than ameliorated global warming. In particular, it supports some controversial findings published in 2007 by Paul Crutzen of the Max Planck Institute for Chemistry in Mainz, Germany. Dr Crutzen concluded that most analyses had underestimated the importance to global warming of a gas called nitrous oxide (N2O) by a factor of between three and five. The amount of this gas released by farming biofuel crops such as maize and rape probably negates by itself any advantage offered by reduced emissions of CO2.

Although N2O is not common in the Earth’s atmosphere, it is a more potent greenhouse gas than CO2 and it hangs around longer. The upshot is that, over the course of a century, its ability to warm the planet is almost 300 times that of an equivalent mass of CO2. Robert Howarth, a professor of ecology at Cornell University who was involved in writing the ICSU report, said that although the methods used by Dr Crutzen could be criticised, his fundamental conclusions were correct.

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