Researchers argue crop-based biofuels only show GHG savings because of LCA accounting flaw; the need for “additional” biomass
11 June 2012
In an invited opinion paper published in the journal GCB Bioenergy, Kevin Smith from the University of Edinburgh and Tim Searchinger from Princeton University argue that current Life Cycle Analysis (LCA) models indicate that crop-based biofuels generate greenhouse gas (GHG) savings compared with fossil fuels only because the models ignore the emissions of CO2 from vehicles burning the biofuels without determining if the biomass is “additional”.
Additional biomass is biomass from additional plant growth or biomass that would decompose rapidly if not used for bioenergy. The models also underestimate the ultimate emissions of N2O from nitrogen fertilizer use, the authors suggest.
In a world that needs to produce more food while reducing emissions from both land-use change and nitrous oxide, it would be rather surprising to discover benefits from biofuels that use much of the world’s best cropland and crops with high nitrogen demand.
...The proper focus of biofuels policy should be on the generation of additional biomass from waste feedstocks, or high-yielding bioenergy crops with low nitrogen demand on land that is capable of generating such yields but now produces little biomass for humans, carbon sequestration or nature.—Smith and Searchinger
LCA models for biofuels include calculations of the GHG emissions—including CO2and N2O—from the manufacture of the fertilizers and pesticides used in crop production; from fossil fuel used in the transport of fertilizer to the farm, farming operations and transport of the crop to the biofuel refinery; and from the refining process. These LCAs do not count the CO2 emitted by combusting biomass, and they nearly always assume a very small conversion rate of nitrogen to N2O, the authors note, posing the question as to whether these estimated reductions in emissions are real.
Where then do LCAs find the GHG reductions? They do so in critical part by ignoring the carbon emitted as CO2 from the exhaust pipes of vehicles that use biofuels, as well as the CO2 emitted by fermentation. In the case of ethanol, that CO2 is equal to 107 g MJ-1, which by itself is roughly 25% more than the entire lifecycle emissions of petrol according to typical estimates. The key question is the extent to which ignoring these very real releases of carbon is legitimate.
To the advocates of biofuels, this credit is self-evident because the growth of the grain or other plants used for the biofuel absorbs the same amount of carbon as that emitted by refining and burning the fuel. By this theory, plant growth ‘offsets’ the emissions from combustion.
That can be true, but a critical requirement for any offset is that it be additional. Biofuels cannot claim a credit for plant growth that would occur anyway. Certainly, biofuels save emissions from burning fossil fuels, but if all they do is substitute emissions from burning biomass, that does not by itself provide any benefit unless the biomass is better than the fossil fuel; and for it to be better in an emissions-related sense, the biomass must be ‘additional’. Biofuels can only reduce CO2 in the atmosphere if their generation results either in more CO2 absorbed from the atmosphere, or less CO2 emitted to it in some other way.—Smith and Searchinger
The authors provide several examples of ways to obtain greenhouse gas benefits from biofuels:
Plant otherwise fallow land for biofuel crops. The added growth absorbs additional CO2, and thereby offsets the emissions from combustion.
Planting new forests (putting leakage aside), and storing more carbon above ground. Another legitimate offset occurs if biofuels use forest residues that would otherwise decompose and give off their carbon to the air, and the offset results from the reduction in carbon emitted from the forest floor.
However, when biofuels take feedstocks from a central storage of crops that farmers would grow anyway, they do not provide any offset through direct additional plant growth. In this case, the LCAs that automatically ignore the CO2 emitted by vehicle exhaust pipes are not only incomplete, they have no direct justification for doing so. And what happens when farmers clear forests, savannahs or grasslands to plant biofuel crops? The biofuel crop absorbs CO2, yet does so at the expense of losing carbon stored in vegetation and soils, and perhaps losing ongoing carbon sequestration as for- ests continue to grow. The problem is not that biofuels reduce GHG emissions, and land-use change increases them; the problem more accurately in such a case is that biofuels result in no positive land use or other market-based change that leads to greenhouse gas reductions in the first place.—Smith and Searchinger
The overestimation of bioenergy LCAs becomes increasingly magnified when the omission of CO2 is combined with the underestimation of nitrogen emissions from fertilizer application. According to lead author Keith Smith, “Emissions of N2O from the soil make a large contribution to the global warming associated with crop production because each kilogram of N2O emitted to the atmosphere has about the same effect as 300kg of CO2.” He notes that several current LCAs underestimate the percentage of nitrogen fertilizer application that is actually emitted to the atmosphere as a GHG. The authors claim that the observed increase in atmospheric N2O shows that this percentage is in reality nearly double the values used in the LCAs, which greatly changes their outcome.
Smith, K. A. and Searchinger, T. D. (2012), Crop-based biofuels and associated environmental concerns. GCB Bioenergy. doi: 10.1111/j.1757-1707.2012.01182.x
Timothy D Searchinger (2010) Biofuels and the need for additional carbon. Environ. Res. Lett. 5 024007 (10pp) doi: 10.1088/1748-9326/5/2/024007
Helmut Haberl, Detlef Sprinz, Marc Bonazountas, Pierluigi Cocco, Yves Desaubies, Mogens Henze, Ole Hertel, Richard K. Johnson, Ulrike Kastrup, Pierre Laconte, Eckart Lange, Peter Novak, Jouni Paavola, Anette Reenberg, Sybille van den Hove, Theo Vermeire, Peter Wadhams, Timothy Searchinger (2012) Correcting a fundamental error in greenhouse gas accounting related to bioenergy, Energy Policy, Volume 45, June 2012, Pages 18-23, doi: 10.1016/j.enpol.2012.02.051
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