"We find that ethanol can, if it is made correctly, contribute significantly to both energy and environmental goals. However, the current way of producing ethanol with corn probably only meets energy goals," said Alexander Farrell at the University of California Berkeley.

Farrell and colleagues looked at six studies used to argue for and against the development of ethanol as an energy source.

"One of the main purposes is to explain why the studies found in the literature have such divergent results," Farrell said in a telephone interview.

"Some of the studies use what appear to be obsolete data or data whose quality cannot be verified," Farrell added.

Currently, ethanol is not a significant source of fuel, but is blended into gasoline in some states. Environmentalists hope it could be developed as a cleaner source of fuel than oil or gas.

"The 3.4 billion gallons (15.5 billion liters) of ethanol blended into gasoline in 2004 amounted to about 2 percent of all gasoline sold by volume and 1.3 percent of its energy content," the researchers wrote.

Farrell said it was possible to put ethanol in a car and run it, but making ethanol using current technology is expensive and contributes to pollution and greenhouse gases.

"(The environmental cost) comes entirely from making fertilizer, running the tractors over the farm and operating the biorefinery," Farrell said.

Better methods now being investigated would use the woody parts of plants, using what is known as cellulosic technology to break down the tough fibers.

"Ethanol can be, if it's made the right way with cellulosic technology, a really good fuel for the United States," said Farrell, an assistant professor of energy and resources.

"At the moment, cellulosic technology is just too expensive. If that changes -- and the technology is developing rapidly -- then we might see cellulosic technology enter the commercial market within five years."

Writing in the same journal, scientists from Imperial College London, Georgia Tech and Oak Ridge National Laboratory in Tennessee said they had teamed up to find ways to make a facility to do that.

Their facility would make a range of fuels, foods, chemicals, animal feeds, materials, heat and power using what is known as biomass -- a collection of renewable plant matter and biological material such as trees, grasses and agricultural crops.

"We're looking at a future for biomass where we use the entire plant and produce a range of different materials from it," Charlotte Williams of Imperial's Department of Chemistry said in a statement.

"Before we freeze in the dark, we must prepare to make the transition from nonrenewable carbon resources to renewable bioresources," her team wrote.

An oil industry expert said it was possible.

"Credible studies show that with plausible technology developments, biofuels could supply some 30 percent of global demand in an environmentally responsible manner without affecting food production," Steven Koonin, chief scientist for BP in London, wrote in a commentary.