Letters

Biofuels and the Environment

Science  17 Aug 2007:
Vol. 317, Issue 5840, pp. 897b-898b
DOI: 10.1126/science.317.5840.897b

In his Editorial “The biofuels conundrum” (27 April, p. 515), Donald Kennedy describes how biofuels, although at first glance a boon for the environment, have hidden costs that could prove environmentally and socially disastrous. His solution is “to abandon this cluttered arena” and to invest in research in plant physiology to overcome biomass recalcitrance for cellulosic conversion. There are several problems with this view. First, it is unlikely that a single approach will suit all circumstances. For example, gasification techniques seem efficient and promising for some feedstocks, but not all. Second, cellulosic conversion demands uniform feedstocks, which translates into high-input and environmentally destructive biofuel monocultures. Such monocultures are unlikely to be sustainable. Third, given the evolutionarily conserved structure of cell walls, it is possible that fooling with it would lead to crops that are prone to structural failure or, more likely, sensitivity to fungal pathogens.

A high-diversity, orchid-rich meadow in the White Carpathian Mountains, which has been mowed almost every year for half a millennium.

CREDIT: MICHAEL W. PALMER

More research into plant physiology may help solve some of these problems, but perhaps some of our money is better spent supporting plant ecology. We already know that diverse grasslands can outperform monocultures in biofuels production (1). We also know that such grasslands are being lost or degraded worldwide because of a lack of active management. Learning how to use such grasslands sustainably for biofuels will not only reduce greenhouse gas emissions but also promote biodiversity. But we must first move away from a crop-based mentality and rely on expertise from a much broader scientific base than is currently being considered.

Reference

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Donald Kennedy's Editorial “The biofuels conundrum” (27 April, p. 515) is timely and needed. However, we have two concerns. The first is that the suggested use of corn stover as a source for ethanol fuel has serious environmental implications. U.S. agriculture is currently losing topsoil 10 times faster than sustainability (1). Removing corn stover and/or leaving the soil unprotected will intensify soil erosion 10-fold or more (2). Without the protection of crop residues, soil loss may increase as much as 100-fold (3). Increasing soil erosion also intensifies the global warming problem and other problems (4, 5).

Another concern is the fact that green plants collect little solar energy, an average of only 0.1% per year (6). Photovoltaics, in contrast, collect 10 to 20% of the solar energy or 100 to 200 times the rate of green plants (6).

Crops, forestry, and other green plants collect a total of 53 exajoules of solar energy per year from sunlight (7). However, Americans consume more than twice this amount of fossil fuel energy each year (8). Some suggest that ethanol produced from corn and cellulosic biomass could replace 30% of the oil used in the United States (9). Yet the 20% of the U.S. corn crop now converted into 5 billion gallons of ethanol replaces 1% of U.S. petroleum consumption (6). Ethanol yield from sugar is better, as documented in Brazil (Kennedy points this out), but the environmental, economic, and social costs are enormous. Soil erosion associated with sugarcane is greater than any other crop grown in Brazil (10).

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Response

I'm happy that Pimentel and Palmer both see the problems with biofuels, and I welcome their departure from that “cluttered space.” Pimentel may be right about corn stover; I paired it with wood chips merely to illustrate a distinction and not to advocate its use. By all means, let's focus on other cellulose sources. Palmer is, in my view, too discouraged about the capacity of renewed emphasis on plant physiology and biochemistry to produce some solutions in this area. But he's dead right about ecology! Work as promising as Tilman's certainly deserves support.

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