N2O is made by bacteria that live in soil and water and, these days, their raw material is often the nitrogen-rich fertiliser that modern farming requires. Since the 1960s the amount of fertiliser used by farmers has increased sixfold, and not all of that extra nitrogen ends up in their crops. Maize, in particular, is described by experts in the field as a “nitrogen-leaky” plant because it has shallow roots and takes up nitrogen for only a few months of the year. This would make maize (which is one of the main sources of biofuel) a particularly bad contributor to global N2O emissions.

But it is not just biofuels that are to blame. The ICSU report suggests N2O emissions in general are probably more important than had been realised. Previous studies, including those by the International Panel on Climate Change (IPCC), a United Nations-appointed body of experts, may have miscalculated their significance-and according to Adrian Williams of Cranfield University, in Britain, even the IPCC’s approach suggests that the global-warming potential of most of Britain’s annual crops is dominated by N2O emissions.

The broader issue, therefore, is the extent to which humanity has hijacked the “nitrogen cycle”, as the passage of that gas into and out of the atmosphere is known, for its own use. Alan Townsend, of the University of Colorado, Boulder, is one of those trying to calculate the extent of this change. What seems certain is that the nitrogen cycle is changing faster and more profoundly than the carbon cycle, which has attracted much more attention.

... contd.