We have passed a tipping point in the search for "carbon-neutral" energy sources, leading to an explosion of interest in the use of plant-derived ethanol and biodiesel as a replacement for fossil fuels. Almost overnight, ethanol went from an additive to make engines run better to a potential solution for global warming.
An extraordinary diversity of plant-derived fuels (biomass energy) have been proposed, from traditional crops like corn, soybeans, and sugar cane, to new crops like switchgrass and jojoba that can be grown on less productive lands, to the harvesting of native species from forests and grasslands. Even algal blooms have received attention.
At first glance, all of them offer the potential for a carbon-neutral fuel to power cars, trucks, and planes. The CO2 emitted when the biofuel is burned is, in principle, taken back out of the atmosphere when the next crop is grown. That much is indisputable. Unfortunately, that is not the end of the story in the calculation of the net benefits (if any) from biofuels.
Engineers and ecological economists were the first to raise the caution flags. It turns out that a great deal of energy is needed to convert plant biomass into liquid biofuel. When direct production costs are considered, in many cases the fossil fuel energy required to make a gallon of ethanol exceeds the energy content of that gallon of ethanol.
An even more profound problem has been identified. Using existing technology, there simply isn't enough land on the planet to produce the plant biomass needed to meet our current appetite for liquid fuels. A recent analysis concluded that even if we devoted the entire U.S. crop of corn and soybeans to biofuels, we would only meet 12% of our demand for gasoline, and 6% for diesel.
Biofuels allow us to turn sunlight into a liquid fuel, but the process is inefficient. Sunlight is a very diffuse resource; during photosynthesis plants only capture a small percentage of the sun's energy as biomass. Converting that plant energy into biofuel is, in turn, energy intensive. There is an enormous research effort underway to find ways to improve the conversion process, but there are fundamental physical and biochemical limits to the biofuel energy we can squeeze from an acre of land.
Early concerns about the rush to produce biofuels focused on the ecological consequences of intensified agriculture. Increased pesticide and fertilizer use will degrade water resources. At the same time, habitat will be lost as farmers pulled fallow land out of reserves and put it into intensive crop production.
Those concerns appear to be well justified. In response to a suite of economic subsidies from both states and Congress, ethanol (and to a lesser extent, biodiesel) production has skyrocketed in the U.S. over the past two years. So much so that there are now bottlenecks in the means to distribute it from the Midwest to the rest of the country.
But the most recent and potentially most critical warning flags about biofuels are a lesson in unintended ecological consequences. Diverting crops from food to fuel has increased crop prices. Combined with almost inexhaustible demand for liquid fuels, this has led to a dramatic rise in the clearing of new land for agriculture around the world. But cutting down a native forest or plowing a native grassland to plant a crop can release an enormous amount of carbon from trees and soils into the atmosphere.
In a series of papers recently published in Science, researchers show that converting natural ecosystems to cropland can release from 10 to over 400 times as much CO2 as the potential annual savings from biofuels produced on those lands.
In the long run, biofuels may serve as a useful source of renewable and carbon-neutral fuel, but the new studies show that getting there is not going to be easy. If we're not careful, policies designed to increase production of biofuels could actually make global warming much worse.
None of the technologies or policies currently under consideration could produce enough biofuels to come close to meeting current demand. The obvious conclusion is that reducing demand through conservation and fuel efficiency is going to be just as important as increasing the supply of biofuels.