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Biofuels could boost global warming, finds study


21 September 2007

Growing and burning many biofuels may actually raise rather than lower greenhouse gas emissions, a new study led by Nobel prize-winning chemist Paul Crutzen has shown. The findings come in the wake of a recent OECD report, which warned nations not to rush headlong into growing energy crops because they cause food shortages and damage biodiversity.

Crutzen and colleagues have calculated that growing some of the most commonly used biofuel crops releases around twice the amount of the potent greenhouse gas nitrous oxide (N2O) than previously thought - wiping out any benefits from not using fossil fuels and, worse, probably contributing to global warming. The work appears in Atmospheric Chemistry and Physics and is currently subject to open review. 

'The significance of it is that the supposed benefits of biofuel are even more disputable than had been thought hitherto,' Keith Smith, a co-author on the paper from the University of Edinburgh, told Chemistry World. 'What we are saying is that [growing many biofuels] is probably of no benefit and in fact is actually making the climate issue worse.'

"What we are saying is that growing biofuels is probably of no benefit and in fact is actually making the climate issue worse"
- Keith Smith

Crutzen, famous for his work on nitrogen oxides and the ozone layer, declined to comment before the paper is officially published. But the paper suggests that microbes convert much more of the nitrogen in fertiliser to N2O than previously thought - 3 to 5 per cent or twice the widely accepted figure of 2 per cent used by the International Panel on Climate Change (IPCC).

For rapeseed biodiesel, which accounts for about 80 per cent of the biofuel production in Europe, the relative warming due to N2O emissions is estimated at 1 to 1.7 times larger than the quasi-cooling effect due to saved fossil CO2 emissions. For corn bioethanol, dominant in the US, the figure is 0.9 to 1.5. Only cane sugar bioethanol - with a relative warming of 0.5 to 0.9 - looks like a viable alternative to conventional fuels.

Some previous estimates had suggested that biofuels could cut greenhouse gas emissions by up to 40 per cent.2

Global picture 

The IPCC's N2O conversion factor is derived using data from plant experiments. But Crutzen takes a different approach, using atmospheric measurements and ice core data to calculate the total amount of N2O in the atmosphere. He then subtracts the level of N2O in pre-industrial times - before fertilizers were available - to take account of N2O from natural processes such as leguminous plants growing in forests, lightning, and burn offs. 

Assuming the rest of the N2O is attributable to newly-fixed nitrogen from fertilizer use, and knowing the amount of fertilizer applied globally, he can calculate the contribution of fertilizers to N2O levels. 

The results may well trigger a rethink by the IPCC, says Smith. 'Should we go along the road of adding up the experimental evidence for each of the processes or are we better off using the global numbers?' 

Critical reception

But other experts are critical of Crutzen's approach. Simon Donner, a nitrogen researcher based at Princeton University, US, says the method is elegant but there is little evidence to show the N2O yield from fertilized plants is really as high as 3-5 per cent. Crutzen's basic assumption, that  pre-industrial N2O emissions are the same as natural N2O emissions, is 'probably wrong', says Donner.

One reason he gives is that farmers plant crops in places that have nitrogen rich soils anyway. 'It is possible we are indirectly increasing the "natural" source of N2O by drawing down the soil nitrogen in the world's agricultural regions,' he explains. 

Others dispute the values chosen by Crutzen to calculate his budget. Stefan Rauh, an agricultural scientist at the Instituteof Agricultural Economics and Farm Management in Munich, Germany, says some of the rates for converting crops into biofuel should be higher. 'If you use the other factors you get a little net climate cooling,' he said. 

Meanwhile, a report prepared by the OECD for a recent Round Table on Sustainable Development questions the benefits of first generation biofuels and concludes that governments should scrap mandatory targets. 

Richard Doornbosch, the report's author, says both the report and Crutzen's work highlights the importance of establishing correct full life-cycle assessments for biofuels. 'Without them, government policies can't distinguish between one biofuel and another - risking making problems worse,' said Doornbosch. 

Zoe Corbyn 

 

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References

1 PJ Crutzen et al, Atmos. Chem. Phys. Discuss., 2007, 7, 11191
2 J Hill et al, Proc. Natl. Acad. Sci. USA, 2006, 103, 11206 (DOI: 10.1073/pnas.0604600103) 

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