Biofuels are a growing part of the movement to replace fossil fuels as our primary energy source. The idea is that fuel made from crops like corn, sugarcane, and switchgrass is sustainable and lowers greenhouse gas emissions by reabsorbing CO2 when the next batch is grown. Unfortunately biofuels are not quite the easy solution they seem. Over the past few years experts have identified a number of negative side-effects, including increased food prices and , surprisingly, an overall increase in greenhouse gas emissions.
First, the production of fuel from sources like corn is not (currently) terribly efficient anyway. One gallon of corn ethanol provides less energy than a gallon of gasoline, making it more expensive per unit of energy. Production efficiency is only around 23%, meaning that it costs you 1 unit of energy to get 1.3 units of energy. Even in ideal conditions this process only results in a 22% reduction in CO2 emissions.
In a recent article in Science magazine, Searchinger et al. discuss how the land-use change associated with producing biofuels actually leads to an overall increase in greenhouse gas emissions. The land needed to grow crops for biofuels has to come from somewhere, and this inevitably results in increased CO2 emissions. Converting forest or grassland for biofuel use releases all the carbon stored in that land as biomass. Searchinger et al. estimate that corn ethanol use will indirectly double greenhouse gas emissions. Converting existing cropland has the same effect, and has the added effect of raising food price. This in turn, according to an article by Morton et al. in PNAS, has increased deforestation by farmers responding to the increased food prices and needing to clear more land for food crops. Deforestation also, of course, adds more carbon to the atmosphere. Increased grain prices have also had a detrimental effect on poorer countries, leading to food shortages. In an article in this month's Science magazine, Robertson et al. write that in addition to increasing greenhouse gas emissions, grain-based biofuels (like corn) also lead to increased soil erosion and nutrient loss, decreased air quality, and decreased biodiversity.
So, corn ethanol fuel, the current leading biofuel, is not quite as green as it may appear. This is not to say that biofuels in general do not have promise; they do, but they must be approached with caution. Certain crops (e.g. perennial grasses) and certain methods may have greater potential for reducing greenhouse gases. Careful research will be required to effectively replace fossil fuels with biofuels. For now, I think it's fair to say that, like the hydrogen fuel cell, biofuels create more hype than help. On the other hand, we've got to start somewhere, and biofuels probably should play (and will play) an important role in our energy economy.