|Relative warming derived from N2O production for crops, crop residues, and forages used in the production of biofuel. Click to enlarge.|
A new study led by Paul Crutzen, winner of a Nobel Prize in Chemistry in 1995 for work on the formation and decomposition of ozone in the atmosphere, re-examines the total emission of nitrous oxide (N2O) from crop production and concludes that growing and burning many biofuel crops may actually raise, rather than lower, net greenhouse gas emissions.
N2O is a by-product of fixed nitrogen application in agriculture and is a greenhouse gas with a global warming potential (GWP) 296 times larger than an equal mass of CO2.
Crutzen and his colleagues calculated that growing some of the most commonly used biofuel crops releases around twice the amount of N2O than previously thought, thereby wiping out any benefits from not using fossil fuels and potentially contributing to global warming.
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. It does not take into account the use of fossil fuel on the farms and for fertilizer and pesticide production, but it also neglects the production of useful co-products. Both factors partially compensate each other. This needs to be analyzed in a full life cycle assessment.—P. J. Crutzen et al.
The significance of it is that the supposed benefits of biofuels are even more disputable than had been thought hitherto. What we are saying is that [growing many biofuels] is probably of no benefit and in fact is actually making the climate issue worse.—Keith Smith, a co-author on the paper and atmospheric scientist from the University of Edinburgh
The work is currently subject to open review in the journal Atmospheric Chemistry and Physics. Crutzen has declined to comment until that process is completed. The paper suggests that microbes convert much more of the nitrogen in fertilizer to nitrous oxide than previously thought—3 to 5 percent, compared to the widely accepted figure of 2 percent used by the International Panel on Climate Change (IPCC) to calculate the impact of fertilizers on climate change.
For rapeseed biodiesel, which accounts for about 80 percent of the biofuel production in Europe, the relative warming due to nitrous oxide emissions is estimated at 1 to 1.7 times larger than the relative cooling effect due to saved fossil CO2 emissions. For corn bioethanol, dominant in the US, the figure is 0.9 to 1.5. Only sugarcane bioethanol—with a relative warming of 0.5 to 0.9—looks like a better alternative to conventional fuels.
As release of N2O affects climate and stratospheric ozone chemistry by the production of biofuels, much more research on the sources of N2O and the nitrogen cycle is urgently needed...Here we concentrated on the climate effects due only to required N fertilization in biomass production and we have shown that, depending on N content, the use of several agricultural crops for energy production can readily lead to N2O emissions large enough to cause climate warming instead of cooling by “saved fossil CO2”. What we have discussed is one important step in a life cycle analysis, i.e. the emissions of N2O, which must be considered in addition to the fossil fuel input and co-production of useful chemicals in biofuel production.
We have also shown that the replacement of fossil fuels by biofuels may not bring the intended climate cooling due to the accompanying emissions of N2O. There are also other factors to consider in connection with the introduction of biofuels. We have not yet considered the extent to which the high percentage of N-fertilizer which is not taken up by the plants, and the organic nitrogen in the harvested plant material, may stimulate CO2 uptake from the atmosphere; estimates for this effect are very uncertain. We conclude, however, that the relatively large emission of N2O exacerbates the already huge challenge of getting global warming under control.—P. J. Crutzen et al.
P. J. Crutzen, A. R. Mosier, K. A. Smith, and W. Winiwarter. “N2O release from agro-biofuel production negates global warming reduction by replacing fossil fuels” Atmos. Chem. Phys. Discuss., 7, 11191-11205, 2007