Dear Professor Gallagher,
Thank you for your letter of 16 December to Keith Smith, copied to myself and our coauthors. I am replying on behalf of all of us.
We welcome the opportunity to have our correspondence published, but we consider it important that your letter of 16 December and this one are included.
We welcome the conclusions in your review, that biofuel development should be slowed down because of concerns such as competition with food production for agricultural land, and greenhouse gas emissions associated with land use change - we are essentially on the same side of the argument as you, in this regard. However, that makes it all the more regrettable that the additional problem of the global warming associated with nitrous oxide emissions has been so inappropriately treated.
The detailed series of points we made in our initial response, concerning what we saw as the inadequacy of the conclusions produced on your behalf by North Energy, have never been answered. The general statement made by them, and reiterated here in your letter of 16 December, that our paper "does not represent a reliable method for quantifying N2O emissions as
part of a life cycle analysis", is no substitute for a proper response to our detailed points. You talk about this being "a legitimate area of difference between scientists which can and should be resolved". One can only say in reply that if a reviewer for a reputable journal had made the comments on our work that North Energy did (which were then used virtually unchanged in your Review), and then we sent the editor the responses we sent to you, in that situation we would have had every expectation that the referee, or another one, would be expected to deal with the points raised in detail, not brushed aside.
The central argument contained in our paper - which has been accepted by the peer-review process - is that if new reactive nitrogen enters the terrestrial biosphere, as when nitrogenous fertilizer is applied to a biofuel (or any other) crop, then on average 3-5% of that N will appear in the atmosphere as N2O. This is the only available explanation of the observed increase in the global atmospheric concentration of N2O that has accompanied large-scale fertilizer N use, beginning in the 20th century. The emission of N2O by biological processes resulting from natural (pre-fertiliser era) inputs to the terrestrial biosphere had virtually the same emission factor, and this greatly strengthens the argument that the recent anthropogenic N input to the biosphere is the cause of the additional present-day emissions. If we are wrong about this, then some other adequate explanation needs to be found for this appearance of additional N2O in the atmosphere. Any life cycle analysis, in our view, needs to take account of this 3-5% emission factor. To focus on direct emissions from the crop field, using IPCC default values (and even to add indirect emissions to the total, again with IPCC default values) is to ignore the wide uncertainty ranges that all these default values have (a subject that was discussed in Appendix A of our final peer-reviewed paper). As we state, the sum of the bottom-up emissions derived by the IPCC methodology is not inconsistent with the value we derived via our top-down analysis.
This conclusion of ours has now been strengthened by a new publication by Del Grosso et al. in Eos, Transactions of the American Geophysical Union 89, No. 51, p. 539, 16 Dec 2008. They say:
".....The N2O emission from agricultural systems in the United States for 2005, obtained using the bottom-up approaches, is 0.9 teragram N (Table 1), which is within the range of 0.8-1.4 teragrams N based on the top-down approach [i.e. of Crutzen et al. (2008)].
"Using the IPCC [2006] methodology, we found that approximately 5.8 teragrams of N from N2O currently are emitted from agricultural systems at the global scale (Table 1). This is close to the middle of the range (4.2- 7.0 teragrams) based on the top-down approach. We conclude that on sufficiently large scales, top-down and bottom-up approaches used to calculate N2O emissions from agricultural systems yield similar estimates. We emphasize that the emission factor for the global top-down approach (3-5% of N inputs from symbiotic N fixation and synthetic fertilizer production) cannot be compared directly with the emissions factors used in the IPCC [2006] method because the methods consider different sources of N inputs."
A copy of this new paper accompanies this letter.
Finally, when one includes the 3-5% factor into LCAs, then the outcome is not very different from those in our ACP paper, as we have shown in the recently submitted paper (Mosier et al. in review) in which we discuss the likely impact of agricultural fertilizer N use on the global N2O budget as discussed by Crutzen et al. (2008). In this paper we examine the impact of full N2O accounting in biofuel net greenhouse gas emissions, using the life cycle analyses from Farrell et al. (2006) (EBAMM), Liska et al. (2008) (BESS), for corn-based ethanol production, and Smith et al. (2006) (BGGC) for wheat-based ethanol. Based on the EBAMM analyses, net GHG emissions from the average US corn-belt, corn-ethanol production do not fulfill the requirement for 20% reduction for new facility renewable fuel production in the US Energy Independence and Security Act (EISA, 2007). Using the EBAMM default estimate of N2O conversion of 1.5% of N input, a net GHG saving of 17 % was estimated. When the Crutzen et al. (2008) N2O conversion range (3-5% of new N input) is applied, net greenhouse gas (GHG) savings fall to 2 and -18%. In all cases tested, the more optimistic BESS estimates of net GHG emissions indicate that corn-ethanol decreases GHG emissions by an amount above EISA requirements. When an N2O emission factor of 1.8% of N input is used corn-ethanol GHG savings are by 50-70% compared to conventional gasoline. Using a 3-5% N2O production coefficient decreases the GHG savings to 20-40%. For wheat-ethanol the overall impact of higher N2O emission estimates is that emission reduction decreases from 17-54% when the model default N2O conversion factors are used in the net GHG calculation to 2-44% when a 3% N2O conversion
factor is used and decreases further from -36 to 26% when the 5% N2O conversion factor is used.
References:
Crutzen, P.J., A.R. Mosier, K.A. Smith and W. Winiwarter. 2008. N2O release from agrobiofuel production negates global warming reduction by replacing fossil fuels. Atmos. Chem. Phys. 8:389-395.
EISA. 2007. Energy Independence and Security Act. TITLE II-ENERGY SECURITY THROUGH INCREASED PRODUCTION OF BIOFUELS. Signed December 19, 2007 by President G.W. Bush. 310 pp.
Farrell, A.E., R.J. Plevin, B.T. Turned, A.D Jones, M. O'Hare and D.A. Kammen. 2006. Ethanol can contribute to energy and environmental goals. Science. 311:506-508
Liska, A.J., H.S. Yang, V. Bremer, D.T. Walters, G. Erickson, T. Klopfenstein, D. Kenney, P. Tracy, R. Koelsch, K.G. Cassman. 2008. BESS: Biofuel Energy Systems Simulator; Life-Cycle Energy and Emissions Analysis Model for Corn-Ethanol Biofuel. vers.2008.3.0. www.bess.unl.edu. University of Nebraska-Lincoln.
Smith, T.C., D R Kindred, J. M. Brosnan, R. M. Weightman, M. Shepherd, and R. Sylvester-Bradley. 2006. Wheat as a feedstock for alcohol production (HGCA) The Home-Grown Cereals Authority, Research Review No. 61, London, UK. 89 pp.
Mosier, A.R., P.J. Crutzen, K.A. Smith and W. Winiwarter. (In Review). Nitrous oxide's impact on net greenhouse gas savings from biofuels: Life cycle analysis comparison. Submitted to the International Journal of Technology and Globalization (31 October, 2008).
Yours sincerely
Prof. dr. P. J. Crutzen
Last Modified: 09 Feb 2009