Environmental, economic, and energetic costsand benefits of biodiesel and ethanol biofuelsJason Hill*†‡§, Erik Nelson†, David Tilman*§, Stephen Polasky*†, and Douglas Tiffany†Departments of *Ecology, Evolution, and Behavior and†Applied Economics, University of Minnesota, St. Paul, MN 55108; and‡Department of Biology, St.Olaf College, Northfield, MN 55057Contributed by David Tilman, June 2, 2006Negative environmental consequences of fossil fuels and concernsabout petroleum supplies have spurred the search for renewabletransportation biofuels. To be a viable alternative, a biofuel shouldprovide a net energy gain, have environmental benefits, be eco-nomically competitive, and be producible in large quantities with-out reducing food supplies. We use these criteria to evaluate,through life-cycle accounting, ethanol from corn grain and biodie-sel from soybeans. Ethanol yields 25% more energy than theenergy invested in its production, whereas biodiesel yields 93%more. Compared with ethanol, biodiesel releases just 1.0%, 8.3%,and 13% of the agricultural nitrogen, phosphorus, and pesticidepollutants, respectively, per net energy gain. Relative to the fossilfuels they displace, greenhouse gas emissions are reduced 12% bythe production and combustion of ethanol and 41% by biodiesel.Biodiesel also releases less air pollutants per net energy gain thanethanol. These advantages of biodiesel over ethanol come fromlower agricultural inputs and more efficient conversion of feed-stocks to fuel. Neither biofuel can replace much petroleum withoutimpacting food supplies. Even dedicating all U.S. corn and soybeanproduction to biofuels would meet only 12% of gasoline demandand 6% of diesel demand. Until recent increases in petroleumprices, high production costs made biofuels unprofitable withoutsubsidies. Biodiesel provides sufficient environmental advantagesto merit subsidy. Transportation biofuels such as synfuel hydro-carbons or cellulosic ethanol, if produced from low-input biomassgrown on agriculturally marginal land or from waste biomass,could provide much greater supplies and environmental benefitsthan food-based biofuels.corn 兩 soybean 兩 life-cycle accounting 兩 agriculture 兩 fossil fuelHigh energy prices, increasing energy imports, c oncernsabout petroleum supplies, and greater rec ognition of theenvironmental consequences of fossil fuels have driven interestin transportation biofuels. Determining whether alternativefuels provide benefits over the fossil fuels they displace requiresthorough accounting of the direct and indirect inputs andoutputs for their full production and use life c ycles. Here wedeter mine the net societal benefits of corn grain (Zea mays ssp.mays) ethanol and soybean (Glycine max) biodiesel, the t wopredominant U.S. alternative transportation fuels, relative togasoline and diesel, the fossil fuels they displace in the market.We do so by using current, well supported public data on farmyields, commodity and fuel prices, farm energy and agrichemicalinputs, production plant efficiencies, coproduct production,greenhouse gas (GHG) emissions, and other environmentalef fects.To be a v iable substitute for a fossil fuel, an alternative fuelshould not only have superior environmental benefits over thefossil fuel it displaces, be economically competitive with it, andbe producible in sufficient quantities to make a meaningfulimpact on energy demands, but it should also provide a netenergy gain over the energy sources used to produce it. Wetherefore analyze each biofuel industry, including farms andproduction facilities, as though it were an ‘‘island ec onomy’’ thatis a net energy exporter only if the energy value of the biofueland its coproducts exceeds that of all direct and indirect energyinputs (see Tables 1–6 and Supporting Text, which are publishedas supporting information on the PNAS web site).Biofuel production requires energy to grow crops and convertthem to biofuels. We estimate farm energy use for producingc orn and soybeans, including energy use for growing the hybridor variet al seed planted to produce the crop, powering farmmachinery, producing farm machinery and buildings, producingfertilizers and pesticides, and sustaining farmers and their house-holds. We also estimate the energy used in converting crops tobiofuels, including energy use in transporting the crops to biofuelproduction facilities, building and operating biofuel productionfacilities, and sustaining production facility workers and theirhouseholds. Outputs of biofuel production include the biofuelsthemselves and any simultaneously generated coproducts. Forpurposes of energy accounting, we assign the biofuels themselvesan energy content equal to their available energy upon combus-tion. Coproducts, such as distillers’ dry grain with solubles(DDGS) from corn and soybean meal and glycerol from soy-beans, are typically not combusted directly; rather, we assignthem energy equivalent values.ResultsNet Energy Balance (NEB). Despite our use of expansive systemboundaries for energy inputs, our analyses show that both corngrain ethanol and soybean biodiesel production result in positiveNEBs (i.e., biofuel energy content exceeds fossil fuel energyinputs) (Fig. 1; see also Tables 7 and 8, which are published assupporting information on the PNAS web site), which reinforcerecent findings (1–5). Although these earlier reports did notac count for all of the energy inputs included in our analyses,recent advances in crop yields and biofuel production efficien-cies, which are reflected in our analyses, have essentially offsetthe ef fects of the broad boundaries for energy accounting that wehave used. Our results counter the assertion that expandingsystem boundaries to include energetic costs of producing farmmachinery and processing facilities causes negative NEB valuesfor both biofuels (6–8). In short, we find no support for theassertion that either biofuel requires more energy to make thanit yields. However, the NEB for corn grain ethanol is small,providing ⬇25% more energy than required for its production.A lmost all of this NEB is attributable to the energ y credit for itsDDGS coproduct, which is an imal feed, rather than to theethanol itself containing more energy than used in its produc-tion. Corn g rain ethanol has a low NEB because of the highenergy input required to produce corn and to c onvert it intoethanol. In contrast, soybean biodiesel provides ⬇93% moreenergy than is