July 13, 2006
Source: Proceedings of the
National Academies of Sciences (PNAS) USA,
10.1073/pnas.0604600103
http://www.pnas.org/cgi/content/abstract/0604600103v1
Environmental, economic, and
energetic costs and benefits of biodiesel and ethanol biofuels
Jason 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 55057
Contributed by David Tilman, June 2, 2006
ABSTRACT
Negative environmental
consequences of fossil fuels and concerns about petroleum
supplies have spurred the search for renewable
transportation biofuels. To be a viable alternative,
a biofuel should provide a net energy gain, have
environmental benefits, be economically competitive,
and be producible in large quantities without reducing food
supplies. We use these criteria to evaluate, through
life-cycle accounting, ethanol from corn grain and
biodiesel from soybeans. Ethanol yields 25% more
energy than the energy 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 pesticide
pollutants, respectively, per net energy gain.
Relative to the fossil fuels they displace,
greenhouse gas emissions are reduced 12% by the
production and combustion of ethanol and 41% by biodiesel.
Biodiesel also releases less air pollutants per net
energy gain than ethanol. These advantages of
biodiesel over ethanol come from lower agricultural
inputs and more efficient conversion of feedstocks to fuel.
Neither biofuel can replace much petroleum without
impacting food supplies. Even dedicating all U.S.
corn and soybean production to biofuels would meet
only 12% of gasoline demand and 6% of diesel demand.
Until recent increases in petroleum prices, high
production costs made biofuels unprofitable without subsidies.
Biodiesel provides sufficient environmental advantages to
merit subsidy. Transportation biofuels such as
synfuel hydrocarbons or cellulosic ethanol, if
produced from low-input biomass grown on
agriculturally marginal land or from waste biomass, could
provide much greater supplies and environmental benefits
than food-based biofuels.
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