For more than a decade, the US has blended ethanol with gasoline to reduce the overall carbon footprint from fossil-fueled cars and trucks. But a new study says the practice may not be meeting its goals. In fact, burning corn ethanol – the main source in the US – can be worse for the climate than just burning gasoline.
Corn drove demand for land and fertilizer far more than previous estimates had suggested. Together, the extra land and fertilizer have increased ethanol’s carbon footprint to the point where lifecycle greenhouse gas emissions — from seed to tank — were higher than those of gasoline. Some researchers have predicted this could happen, but the new paper offers a comprehensive and retrospective look at policy outcomes in the real world.
Proponents have long argued that corn-based ethanol boosts farm income while providing a domestic source of renewable liquid fuel, while critics have said its status as a carbon-reducing gasoline additive rests on questionable accounting. Based on the new study, both sides could be right.
Ethanol as a fuel has long been controversial in the United States. It was added to gasoline nationwide in 2006, and the amount has been increased in the years since under the Federal Renewable Fuel Standard (RFS), a key element of the bipartisan Energy Policy Act of 2005 signed into law by President George W. Bush. Today, most gasoline sold in the United States contains 10 percent ethanol and about a third of the corn harvest in the country is used to produce the fuel. While other sources would qualify, including cellulosic ethanol, “the majority of RFS biofuel production comes from conventional corn ethanol,” the study authors pointed out.
Researchers looked at the problem from a variety of angles, including examining how much additional land was needed to grow corn, how much additional fertilizer was used, how the combination of the two affected water quality, and how crop prices changed in response to the RFS changed . The team examined these changes between 2008 — the year after the law mandating the standard was passed — and 2016.
When the law was first implemented, corn ethanol barely qualified for inclusion in the RFS program. In the first regulatory impact assessment, a slight decrease in emissions from land-use change was projected for the USA, while a significant increase was expected for other countries. (The international surge was expected due to shifts in supply and demand in the global crops market.)
“To comply with GHG guidelines [greenhouse gas] Reduction targets require the RFS to use conventional renewable fuels to achieve full lifecycle GHG savings of at least 20% compared to gasoline,” the researchers wrote. “At adoption, the policy’s regulatory analysis predicted that life-cycle emissions from corn ethanol production would just surpass the 20 percent threshold by 2022, even if emissions were eliminated [new farmland] were included.”
However, the new study says emissions from land-use change and fertilizer use in the U.S., as a result of new crop acreage being planted and old arable land being set aside, have risen nearly 50 percent above estimates from the agency analysis, which allowed corn ethanol to barely grow to qualify.
The researchers found that in response to the new demand, farmers planted an additional 2.8 million hectares (6.9 million acres) for corn that would otherwise have been left alone, an 8.7 percent increase. Because corn needs a lot of nutrients to grow, fertilizer use increased by 3–8 percent.
The extra land put under the plow released a significant amount of carbon, enough to reverse the rating of corn ethanol from a CO2 negative fuel to a CO2 emitting fuel. The largest decline occurred as new arable land released carbon stored in soil and vegetation, including roots of living plants. Farmers were also less likely to enter a field under the Conservation Reserve Program, which pays farmers to grow perennial vegetation on unused farmland.
After the fertilizer was applied, it released a significant amount of nitrous oxide, a potent greenhouse gas that warms the atmosphere 300 times more than the same amount of carbon dioxide over 100 years. The researchers’ estimates of the fertilizer’s carbon load are also likely low because the authors didn’t calculate how much additional pollution the manufacturing process released or to what extent degraded water quality in downstream bodies released more greenhouse gases.
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