A new study, led by John DeCicco, Research Professor at the University of Michigan, investigates whether biofuels are indeed carbon neutral. Since the 1973 oil embargo, U.S. energy policy has sought to replace petroleum-based transportation fuels with alternatives. One current prominent option is using biofuels, such as ethanol in place of gasoline and biodiesel instead of ordinary diesel. The new study is available here.
Transportation generates one-fourth of U.S. greenhouse gas emissions, so addressing this sector’s impact is crucial for climate protection, the study said. Many scientists view biofuels as inherently carbon-neutral: they assume the carbon dioxide (CO2) plants absorb from the air as they grow completely offsets, or “neutralizes,” the CO2 emitted when fuels made from plants burn. Many years of computer modeling based on this assumption, including work supported by the U.S. Department of Energy, concluded that using biofuels to replace gasoline significantly reduced CO2 emissions from transportation.
The new study examined crop data to evaluate whether enough CO2 was absorbed on farmland to balance out the CO2 emitted when biofuels are burned. The study found that once all the emissions associated with growing feedstock crops and manufacturing biofuel are factored in, biofuels actually increase CO2 emissions rather than reducing them.
Federal and state policies have subsidized corn ethanol since the 1970s, but biofuels gained support as a tool for promoting energy independence and reducing oil imports after the September 11, 2001 attacks. In 2005 Congress enacted the renewable fuel standard, which required fuel refiners to blend 7.5 billion gallons of ethanol into gasoline by 2012.
In 2007, Congress dramatically expanded the RFS program with support from some major environmental groups. The new standard more than tripled annual U.S. renewable fuel consumption, which rose from 4.1 billion gallons in 2005 to 15.4 billion gallons in 2015.
The study examined data from 2005-2013 during this sharp increase in renewable fuel use. Rather than assuming that producing and using biofuels was carbon-neutral, the study compared the amount of CO2 absorbed on cropland to the quantity emitted during biofuel production and consumption.
Existing crop growth already takes large amounts of CO2 out of the atmosphere. The empirical question in the study is whether biofuel production increases the rate of CO2 uptake enough to fully offset CO2 emissions produced when corn is fermented into ethanol and when biofuels are burned.
Most of the crops that went into biofuels during this period were already being cultivated; the main change was that farmers sold more of their harvest to biofuel makers and less for food and animal feed. Some farmers expanded corn and soybean production or switched to these commodities from less profitable crops.
But as long as growing conditions remain constant, corn plants take CO2 out of the atmosphere at the same rate regardless of how the corn is used. Therefore, to properly evaluate biofuels, one must evaluate CO2 uptake on all cropland as crop growth is the CO2 “sponge” that takes carbon out of the atmosphere.
When the study performed such an evaluation, it was found that from 2005 through 2013, cumulative carbon uptake on U.S. farmland increased by 49 teragrams (a teragram is one million metric tons). Planted areas of most other field crops declined during this period, so this increased CO2 uptake can be largely attributed to crops grown for biofuels.
Over the same period, however, CO2 emissions from fermenting and burning biofuels increased by 132 teragrams. Therefore, the greater carbon uptake associated with crop growth offset only 37 percent of biofuel-related CO2 emissions from 2005 through 2013. In other words, biofuels are far from inherently carbon-neutral.
Growing more corn and soybeans has opened the CO2 uptake “drain” a bit more, mostly by displacing other crops. That’s especially true for corn, whose high yields remove carbon from the atmosphere at a rate of two tons per acre, faster than most other crops.
Nevertheless, expanding production of corn and soybeans for biofuels increased CO2 uptake only enough to offset 37 percent of the CO2 directly tied to biofuel use. Moreover, it was far from enough to offset other GHG emissions during biofuel production from sources including fertilizer use, farm operations and fuel refining. Additionally, when farmers convert grasslands, wetlands and other habitats that store large quantities of carbon into cropland, very large CO2 releases occur.
The study concluded that in the United States, to date, renewable fuels actually are more harmful to climate than gasoline. It is still urgent to mitigate CO2 from oil, which is the largest source of CO2 emissions in the United States and the second-largest globally after coal. But the study affirms that, as a cure for climate change, biofuels are “worse than the disease.”
