TilmanBiofuelsScience09

TilmanBiofuelsScience09 - POLICYFORUM ENERGY Benecial...

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17 JULY 2009 VOL 325 SCIENCE www.sciencemag.org 270 POLICY FORUM R ecent analyses of the energy and greenhouse-gas performance of alter- native biofuels have ignited a con- troversy that may be best resolved by apply- ing two simple principles. In a world seek- ing solutions to its energy, environmental, and food challenges, society cannot afford to miss out on the global greenhouse-gas emis- sion reductions and the local environmental and societal benefi ts when biofuels are done right. However, society also cannot accept the undesirable impacts of biofuels done wrong. Biofuels done right can be produced in sub- stantial quantities ( 1 ). However, they must be derived from feedstocks produced with much lower life-cycle greenhouse-gas emissions than traditional fossil fuels and with little or no competition with food production (see fi gure, below). Feedstocks in this category include, but may not be limited to, the following: 1) Perennial plants grown on degraded lands abandoned from agricultural use. Use of such lands minimizes competition with food crops. This also minimizes the poten- tial for direct and indirect land-clearing asso- ciated with biofuel expansion, as well as the resultant creation of long-term carbon debt and biodiversity loss. Moreover, if managed properly, use of degraded lands for biofuels could increase wildlife habitat, improve water quality, and increase carbon sequestration in soils ( 1 3 ). The key to carbon gains is to use land that initially is not storing large quanti- ties of carbon in soils or vegetation and yet is capable of producing an abundant bio- mass crop ( 4 , 5 ). Some initial analyses on the global potential of degraded lands suggest that they could meet meaningful amounts of current global demand for liquid transporta- tion fuels ( 5 7 ). 2) Crop residues. Crop residues such as corn stover and straw from rice and wheat are produced in abundance. They are rich in elements (C, N, and P) essen- tial for maintaining soil fertility and carbon stores, and they help minimize soil erosion. Recent research suggests that it is to the ben- efit of farmers to leave substantial quanti- ties of crop residues on the land ( 8 ), but that, nonetheless, even conservative removal rates can provide a sustainable biomass resource about as large as that from dedicated peren- nial crops grown on degraded lands ( 1 ). 3) Sustainably harvested wood and forest residues. Another abundant feedstock is resi- dues from forestry operations, which include slash (branches, but not leaves or needles) that currently is left in place, unused resi- dues from mill and pulp operations, and for- est “thinnings” removed to reduce fire risk or to allow select trees to attain merchant- able sizes more quickly ( 9 , 10 ). 4) Double crops and mixed cropping systems. Double crops grown between the summer growing seasons of conventional row crops and harvested for biofuel pro- duction before row crops are planted in the spring are representative of a class of land- use options with potential to produce bio- fuel feedstocks without decreasing food
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