As energy sources wane, the price of oil rises, and climate change becomes a global issue, nations have begun to evaluate alternative energy sources. The goal of alternative energy is to reduce greenhouse gas production, while providing an oil-free source of fuel. One such potential alternative energy source is biofuel, or fuels made from recently dead biological material, such as plants. Currently biofuels are produced from crops such as corn, sugarcane, soybeans, and palms. Ethanol produced from these crops can be used as fuel in place of gasoline. This is clearly an advantageous substitution for the Earth, right? Well in fact, exactly how Earth-friendly biofuels are has recently been called into question. It turns out that the manner in which biofuels are produced has a big impact on how “green” they really are.
The Carbon Debt
Two recent studies in the journal Science question the carbon emissions estimates of biofuels, claiming that current estimates do not address the environmental cost of land conversion and its impact on the environment. Carbon dioxide is the most abundant greenhouse gas in the atmosphere, and carbon emissions are simply the amount of carbon dioxide released into the atmosphere due to man-made processes. Previous studies on biofuels generally compared carbon emissions produced as a byproduct of biofuel production versus those produced in the production of oil. The total carbon emission for these processes is calculated by adding up how much carbon is released into the atmosphere at each production step: mining crude oil (or growing crop plants), refining the resulting fuel, and burning the fuel in a vehicle. Biofuels offer lower emissions compared to oil primarily because growing crops that will be converted to biofuels removes carbon dioxide from the atmosphere.
However, the high demand for biofuels has resulted in land formerly used to grow crops to be converted to land to grow biofuels. When this happens, farmers begin to clear additional land in order to keep up with food demands. This can be problematic because soils and plants are the two of the largest biologically active stores of carbon, meaning that they absorb carbon dioxide from the air and store it. Thus, when native habitats are converted to croplands, large amounts of carbon dioxide are released as a result of burning to clear the land and the decomposition of plant roots and branches as they decay after land clearing. To make matters worse, the forests and native plants no longer exist to absorb carbon dioxide from the atmosphere.
In order to incorporate the effects of land clearing into the evaluation of biofuels, two research groups, from Princeton and the Nature Conservancy in Minneapolis, decided to investigate how land conversion impacts carbon emissions estimates for biofuels. In these studies, the researchers calculated “carbon debt,” a measure of the carbon dioxide released into the atmosphere due to land clearing. In order to calculate carbon debts, each group estimated how much land would likely be converted to cropland in the U.S., Brazil, and Southeast Asia using land conversion statistics from current biofuel production and resulting land conversion in each country. Different geographical regions would convert different types of land, and clearing of different ecosystems would result in different levels of carbon emissions. These estimates allowed the researchers to understand how much carbon would be released into the atmosphere due to land conversion for biofuels. They were also interested in how these numbers compared to carbon emissions incurred using petroleum. They found that converting ecosystems results in large carbon debts. For example, in the U.S., converting central grasslands to produce ethanol from corn would require a 93-year carbon debt. This means that it would take 93 years before corn-based ethanol became more environmentally friendly than petroleum. With the exception of sugarcane biofuels, which can have a carbon debt of 17 years, land conversion to grow all of the crops studied for biofuels would actually emit more greenhouse gases for decades to centuries compared to petroleum use.
Are Biofuels a Lost Cause?
Not all the news is bad for biofuel enthusiasts. Both studies suggest growing biofuels in specific environments as a potential way to escape large carbon debts caused by land conversion. Using abandoned agricultural land to grow native perennials for biofuel production reduces carbon emissions. Converting waste, such as that from the waste parts of crops, does not require the use of additional cropland, and therefore does not have a carbon debt. Focusing on making biofuels that do not require land clearing will reduce carbon debts and make biofuels more environmentally-friendly. It will also bypass the growing conflict between producing biofuels and maintaining an adequate food supply. Another way to lower the environmental cost of biofuels is to develop technologies that will make biofuel production and use more efficient.
Any estimate of carbon emissions from biofuels must include all carbon costs, in order to make accurate, environmentally-friendly policy decisions about alternative energy sources. Produced in an appropriate way, biofuels have the potential to be a clean energy source that can help with the current energy crisis and hopefully provide a sustainable energy source for years to come.
–Johanna Kowalko, Harvard Medical School
For More Information:
National Public Radio’s Science Friday’s coverage of these findings:
< http://www.sciencefriday.com/program/archives/200802081 >
Science Daily’s description of another study identifying differences in the environmental costs of different biofuels:
< http://www.sciencedaily.com/releases/2008/01/080103144404.htm >
National Public Radio’s Science Friday interviews researchers developing innovative ways to harvest energy from plants:
< http://www.sciencefriday.com/program/archives/200804112 >
Primary Literature:
Fargione J, Hill J, Tilman D, Polasky S, Hawthorne P. Land clearing and the biofuel carbon debt. Science. 319, 1235-1238 (2008).
Searchinger T, Heimlich R, Houghton RA, Dong F, Eloibeid A, Fabiosa J, Tokgoz S, Heyes D, Yu TH. Use of U.S. croplands for biofuels increases greenhouse gases through emissions from land-use change. Science. 319, 1238-1240 (2008).