Corn Can't Solve Our Problem

In Brazil, ethanol made from sugar cane produces about twice as much ethanol per acre as corn. Brazilian ethanol refineries get much of their power from burning cane residue, in effect recycling carbon from the atmosphere. The environmental benefit is large. Sugar-cane ethanol grown on established soils releases 80 percent less greenhouse gases than gasoline.

But that isn't the case for sugar-cane ethanol or soybean biodiesel from Brazil's newly cleared lands, including tropical forests and savannas. Clearing land releases immense amounts of greenhouse gases into the air, because much of the material in the plants and soil is broken down into carbon dioxide.

Plants and soil contain three times more carbon than the atmosphere. The trees and soil of an acre of rainforest -- which, once cleared, is suitable for growing soybeans -- contain about 120 tons of organic carbon. An acre of tropical woodland or savanna, suitable for sugar cane, contains about half this amount. About a fourth of the carbon in an ecosystem is released to the atmosphere as carbon dioxide when trees are clear-cut, brush and branches are burned or rot, and roots decay. Even more is lost during the first 20 to 50 years of farming, as soil carbon decomposes into carbon dioxide and as wood products are burned or decay.

This means that when tropical woodland is cleared to produce sugar cane for ethanol, the greenhouse gas released is about 50 percent greater than what occurs from the production and use of the same amount of gasoline. And that statistic holds for at least two decades.

Simply being "renewable" does not automatically make a fuel better for the atmosphere than the fossil fuel it replaces, nor guarantee that society gains any new energy by its production. The European Union was recently shocked to learn that some of its imported biodiesel, derived from palm trees planted on rain-forest lands, was more than twice as bad for climate warming as petroleum diesel. So much for the "benefits" of that form of biodiesel.

Although current Brazilian ethanol is environmentally friendly, the long-term environmental implications of buying more ethanol and biodiesel from Brazil, a possibility raised recently during President Bush's trip to that country, are cloudy. It could be harmful to both the climate and the preservation of tropical plant and animal species if it involved, directly or indirectly, additional clearing of native ecosystems.

Concerns about the environmental effects of ethanol production are starting to be felt in the United States as well. It appears that American farmers may add 10 million acres of corn this year to meet booming demand for ethanol. Some of this land could come from millions of acres now set aside nationwide for conservation under a government-subsidized program. Those uncultivated acres absorb atmospheric carbon, so farming them and converting the corn into ethanol could release more carbon dioxide into the air than would burning gasoline.

There are biofuel crops that can be grown with much less energy and chemicals than the food crops we currently use for biofuels. And they can be grown on our less fertile land, especially land that has been degraded by farming. This would decrease competition between food and biofuel. The United States has about 60 million acres of such land -- in the Conservation Reserve Program, road edge rights-of-way and abandoned farmlands.

In a 10-year experiment reported in Science magazine in December, we explored how much bioenergy could be produced by 18 different native prairie plant species grown on highly degraded and infertile soil. We planted 172 plots in central Minnesota with various combinations of these species, randomly chosen. We found, on this highly degraded land, that the plots planted with mixtures of many native prairie perennial species yielded 238 percent more bioenergy than those planted with single species. High plant diversity led to high productivity, and little fertilizer or chemical weed or pest killers was required.

The prairie "hay" harvested from these plots can be used to create high-value energy sources. For instance, it can be mixed with coal and burned for electricity generation. It can be "gasified," then chemically combined to make ethanol or synthetic gasoline. Or it can be burned in a turbine engine to make electricity. A technique that is undergoing rapid development involves bioengineering enzymes that digest parts of plants (the cellulose) into sugars that are then fermented into ethanol.

Whether converted into electricity, ethanol or synthetic gasoline, the high-diversity hay from infertile land produced as much or more new usable energy per acre as corn for ethanol on fertile land. And it could be harvested year after year.

Even more surprising were the greenhouse gas benefits. When high-diversity mixtures of native plants are grown on degraded soils, they remove carbon dioxide from the air. Much of this carbon ends up stored in the soil. In essence, mixtures of native plants gradually restore the carbon levels that degraded soils had before being cleared and farmed. This benefit lasts for about a century.

Across the full process of growing high-diversity prairie hay, converting it into an energy source and using that energy, we found a net removal and storage of about a ton and a half of atmospheric carbon dioxide per acre. The net effect is that ethanol or synthetic gasoline produced from this grass on degraded land can provide energy that actually reduces atmospheric levels of carbon dioxide.

When one of these carbon-negative biofuels is mixed with gasoline, the resulting blend releases less carbon dioxide than traditional gasoline.

Biofuels, if used properly, can help us balance our need for food, energy and a habitable and sustainable environment. To help this happen, though, we need a national biofuels policy that favors our best options. We must determine the carbon impacts of each method of making these fuels, then mandate fuel blending that achieves a prescribed greenhouse gas reduction. We have the knowledge and technology to start solving these problems.

tilman@umn.edu; hill0408@umn.edu

David Tilman is an ecologist at the University of Minnesota and a member of the National Academy of Sciences. Jason Hill is a research associate in the Department of Applied Economics at the University of Minnesota.