Biofuels are renewable energy sources derived from plants. They are thought to be a more environmentally friendly energy source than conventional petroleum.
They are thought to offer the possibility of lowering greenhouse gas emissions, because plants absorb carbon dioxide. Their combustion is also thought to produce fewer toxic air pollutants than conventional petroleum.
However, the advantages (or lack thereof) of biofuels over conventional petroleum depends upon the method used to produce them. For example, if they require heavy fertilization or irrigation, are produced in low yield, are difficult to process efficiently, are energy-intensive, take the place of other needed crops, and/or are polluting, biofuels may not be an attractive option.
This question is probed by Jason Hill (University of Minnesota) and coworkers. Focusing on greenhouse gas emissions and fine particulate matter in the United States, they have shown that biofuels can have either higher or lower climate and health costs than conventional petroleum, depending on the combustion methods and plant sources.
Analysis of emission sources, production, and costs.
The scientists chose to focus their efforts on two emissions sources, greenhouse gases and fine particulate matter. All stages of ethanol and gasoline production and combustion (such as processing and distribution) emit these two classes of pollutants.
The scientists considered several different methods of producing ethanol, from either corn or cellulose (prairie grasses). This is because there is currently no one standard method to produce biofuels, and they all produce (or use) different amounts of energy.
Corn-based energy production and usage was gathered from industry-reported data, also taking into consideration possible dramatic future technological improvements. Large-scale celluose-based energy production does not exist to date, necessitating the use of current realistic predictions of energy production and usage.
The scientists consider emissions based on an expansion of fuel utilization in the United States similar that observed from 2006 to 2007. Some comparisons are made on bases that are potentially favorable to ethanol, such as the assumption that soybean agriculture will not be displaced by corn or prairie grass agriculture.
The costs of carbon capture and storage, carbon trading, and enhanced climate change are factored into the scientists' model. Values for these three costs have been previously reported in the scientific literature.
There is a great deal of uncertainty in these three costs, such as the costs arising from an unknown amount of climate change and the ability of humans to adapt to it. However, this uncertainty should have more of an effect on the estimated absolute values of the climate and health costs than on the relative comparison between the energy sources.
The scientists also note that while greenhouse gases mix in the atmosphere globally, fine particulate matter is a regional issue. They therefore factor population density near the emission source (gasoline production in urban areas, biofuel production in rural areas) into their cost estimates.
Cost of greenhouse gas emissions.
Taking all of these issues into consideration, the scientists were able to arrive at a range of estimates of the costs related to greenhouse gas emissions. These estimates were made for gasoline, corn-derived ethanol, and celluose-derived ethanol.
The scientists found that ethanol produced from cellulose results in lower greenhouse gas emissions than either gasoline or ethanol produced from corn. This is due to its higher yield and lower fertilizer input.
The greenhouse gas cost of ethanol produced from corn is roughly the same or more than that from gasoline. However, the cost of ethanol produced from cellulose can be roughly 1/3 that of gasoline (but under some conditions can cost roughly the same).
Cost of fine particulate emissions.
Similarly, the scientists found that ethanol produced from cellulose yields lower fine particulate costs than either gasoline or ethanol produced from corn. Corn-based ethanol is always more costly in this respect than gasoline.
The fine particulate cost of ethanol produced from corn is almost three times that of gasoline. However, the cost of ethanol produced from cellulose is less than half that of gasoline.
There are regional differences in cost as well. Costs increase in the midwest United States, due to increased corn or prairie grass production and utilization.
However, the situation on the west coast improves, as long as cellulose-derived ethanol is produced. This is due to reduced pollutant emissions from refineries, and the excess energy generated that displaces other energy sources, such as coal.
Hard numbers.
The scientists estimated the total climate change and health cost for gasoline, corn-derived ethanol, and cellulose-derived ethanol. For every billion ethanol-equivalent gallons of fuel, the cost is:
Cellulose-derived ethanol, such as from prairie grasses, is obviously beneficial overall. While the absolute numbers have a lot of uncertainty, the relative ranking is solid.
The scientists did not consider other environmental costs, such as habitat loss and soil erosion. This is a much more complex question to address.
Industrial advances may also come into play in the future, changing cost estimates. Additionally, ethanol is not the only possible biofuel.
Other scientists are investigating how to produce molecules from cellulose that are of lower cost and higher energy than ethanol. This would also change cost estimates.
How to act on this information.
Perhaps the most relevant point to make is that biofuel production is not the only method to combat climate change and pollution; mass transit, solar energy, and fuel cells are others. Few people seem to recognize that lowering our energy demand (possibly lowering our perceived standard of living) may be the most effective, and even necessary, approach.
Biofuel production is not the answer to climate problems by itself, and under certain conditions is worse than gasoline. The scientists' estimates reported here should be taken into serious consideration by public and private officials responsible for tackling the problem of global warming on a national and international level.
for more information:
Hill, J.; Polasky, S.; Nelson, E.; Tilman, D.; Huo, H.; Ludwig, L.;
Neumann, J.
Climate change and health costs of air emissions from
biofuels and gasoline.
Proc. Natl. Acad. Sci. USA 2009, 106, 2077-2082.