It might seem counterintuitive at first, but some scientists argue that widespread biofuel production is a negative-sum game: Producing enough biodiesel or ethanol to replace one gallon of petroleum fuel, they argue, requires the energy equivalent to several gallons' worth of petroleum fuel [source: Pimentel].
To put it another way, think about a field of corn being grown for ethanol. It may produce 100 gallons of the fuel out of one season's crop. But if the tractors that tend the field burn 75 gallons of fuel in the season, the truck to transport the corn to a processor burns 20 gallons on the trip, and the processor uses the energy of 40 gallons of fuel to run its distillation equipment, is the ethanol produced really an environmentally friendly, low-emission fuel? Add other resource costs into the equation, such as the gallons of fresh water needed to grow the plants and the amount of fertilizer needed to keep them healthy, and it becomes even harder to equate biofuel with real energy and carbon emission savings.
A 2005 study suggested that, using current farming and production technology, it takes anywhere from 27 to 118 percent more energy to produce a gallon of biodiesel than the energy it contains [source: Pimentel]. While technology may eventually narrow those ratios, the input-output energy ratio of modern biofuel production is a major drawback to its widespread use.