Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 7 4 2007 10.5194/acpd-7-11191-2007 http://www.atmos-chem-phys-discuss.net/7/11191/2007/ http://www.atmos-chem-phys-discuss.net/7/11191/2007/acpd-7-11191-2007.html http://www.atmos-chem-phys-discuss.net/7/11191/2007/acpd-7-11191-2007.pdf 11191 11205 2007-08-01 N₂O release from agro-biofuel production negates global warming reduction by replacing fossil fuels P. J. Crutzen crutzen@mpch-mainz.mpg.de A. R. Mosier K. A. Smith W. Winiwarter Max Planck Institute for Chemistry, Department of Atmospheric Chemistry, Mainz, Germany Scripps Institution of Oceanography, University of California, La Jolla, USA International Institute for Applied Systems Analysis (IIASA), Laxenburg, Austria Mount Pleasant, SC, USA School of Geosciences, University of Edinburgh, Edinburgh, UK Austrian Research Centers – ARC, Vienna, Austria The relationship, on a global basis, between the amount of N fixed by chemical, biological or atmospheric processes entering the terrestrial biosphere, and the total emission of nitrous oxide (N₂O), has been re-examined, using known global atmospheric removal rates and concentration growth of N₂O as a proxy for overall emissions. The relationship, in both the pre-industrial period and in recent times, after taking into account the large-scale changes in synthetic N fertiliser production and deforestation, is consistent, showing an overall conversion factor of 3–5%. This factor is covered only in part by the ~1% of "direct" emissions from agricultural crop lands estimated by IPCC (2006), or the "indirect" emissions cited therein. This means that the extra N₂O entering the atmosphere as a result of using N to produce crops for biofuels will also be correspondingly greater than that estimated just on the basis of IPCC (2006). When the extra N₂O emission from biofuel production is calculated in "CO₂-equivalent" global warming terms, and compared with the quasi-cooling effect of "saving" emissions of fossil fuel derived CO₂, the outcome is that the production of commonly used biofuels, such as biodiesel from rapeseed and bioethanol from corn (maize), can contribute as much or more to global warming by N₂O emissions than cooling by fossil fuel savings. Crops with less N demand, such as grasses and woody coppice species have more favourable climate impacts. This analysis only considers the conversion of biomass to biofuel. 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