To the Editor

Loarie et al.1 are to be congratulated in highlighting the need for a holistic approach when assessing the net climate impact of biofuels, including that of albedo change. However, their statement that “...expanding sugar cane into existing crop and pasture land has a direct local cooling effect that reinforces the indirect climate benefits of this land-use option” risks underplaying the importance of non-carbon dioxide emissions from sugar-cane cultivation.

Sugar-cane production is nitrogen-intensive, and the relatively high temperatures and soil water contents in the tropics — where most of it is cultivated — can enhance denitrification rates, boosting emissions of the greenhouse-gas nitrous oxide (N2O)2,3. Recent studies examining N2O emissions from sugar-cane production in Australia have reported very high N2O emission factors (the proportion of nitrogen applied as fertilizer that is emitted as N2O), predicting that an emission factor of 3–5% is common to sugar-cane cultivation across a wide range of environments in Australia3.

At present, the global coverage of field measurements of N2O fluxes from biofuel cultivation is not sufficient to allow reliable bottom-up estimation of fluxes for all regions, land-management practices and crops. It may well be that the Intergovernmental Panel on Climate Change's4 default N2O emission factor of 1% for emissions arising directly from nitrogen addition is applicable to sugar cane in Brazil, but the uncertainty inherent in this figure is high (±300%). If an emission factor of 3–5% is more representative for these systems, the N2O 'penalty' in terms of the net climate-forcing impact of sugar-cane production may be substantial5. Wherever climate change mitigation is the primary objective of biofuel cultivation, the fullest possible picture in terms of net direct and indirect climate forcing is required.