A billion acres of farmland around the world have been abandoned and could now be used to grow biofuel crops, a new study suggests.

One of the criticisms of biofuels such as ethanol from corn or rice is that the crops eat into land that could be used to grow food, which is increasingly in short supply globally, causing frustration and hunger that have led to protests and riots. The alternative of clearing forests to grow biofuel crops is unacceptable to many.

Yet somewhere between 1 billion and 1.2 billion acres of agricultural land is lying fallow, the study finds. That compares to about 3.8 billion acres that are currently in use.

The researchers caution, however, that biofuels will be no magic bullet to resolving possible energy crises in the future.

"Our results showed that if you used all these abandoned agricultural lands, you might obtain up to 8 percent of current energy needs," said Elliott Campbell, a postdoctoral fellow in biology at Stanford University and lead author of the report. "So this result is basically showing us that biofuels could be a meaningful, but a small portion of our total energy future."

The study, based on satellite imagery and historical maps, is detailed today in the online edition of the journal Environmental Science &Technology. It was funded by the Carnegie Institution and the Global Climate and Energy Project at Stanford.

Land has fallen out of agricultural production for a variety of reasons. In some instances, new technologies or infrastructure have made land with better soil available, as when farmers in the eastern United States left their farms for the richer prairie soils of the Midwest. Elsewhere, soil erosion or depleted soil nutrients have forced farmers away from plots that could still support other crops such as switchgrass that could be used for biofuel.

"These abandoned agricultural lands are distributed throughout the world, in places with a variety of different climates," Campbell said. "So the type of plant species that might give you the most biomass per year would probably depend on the local climate."