Q: What is the ethanol mandate?

A: In 2007, Congress passed a law requiring oil companies to blend corn ethanol into the gasoline supply. That requirement started at 9 billion gallons and has risen each year since. This year, it's about 13 billion gallons. Barack Obama, who was running for president against a crowded field of Democrats in the Iowa caucuses, was a champion for this law. And with oil prices high and imports climbing, President George W. Bush signed it. Politicians in both parties predicted homegrown corn would help reduce greenhouse gas emissions that contribute to global warming and help wean the country off foreign sources of oil.

Q: Sounds straightforward. Green energy, right?

A: Not really. From the beginning, environmentalists were skeptical of this program. Even people on Obama's environmental team had doubts. Ethanol comes with all kinds of environmental baggage, like the fertilizer used to grow corn and the coal and natural gas that fuel ethanol plants. The ethanol policy also helped send corn prices rising, which encouraged farmers to plant more corn, in some cases on native prairies and land set aside for conserving habitat.

In Des Moines, Iowa, the water utility was strained to the brink this summer as it tried to remove the nitrogen fertilizer residue from drinking water to keep it within safe standards. Some of that fertilizer runs into the nation's rivers, worsening a dead zone in the Gulf of Mexico that can no longer support marine life. Farmers also planted on hilly, erodible soil, which encouraged erosion and the loss of topsoil.

Q: But ethanol helps reduce global warming?

A: That was the intent. But sometimes farmers create cropland by plowing over grassland. Grassland keeps carbon dioxide locked up in the soil. Plow over it, and that gas is released. It can take decades to make up for that loss.

Under Obama, more than 5 million acres of land that had been set aside as conservation land was transformed from grass field back into farmland. That's the size of Yosemite, Yellowstone and Everglades National Parks combined. At least another 1.2 million acres of prairieland in the Great Plains have been plowed over for corn. Many environmentalists and scientists now question whether the ethanol mandate will ever accomplish its primary environmental goal: reducing greenhouse gases.

Q: The ethanol industry disputes those numbers and says no virgin land has been lost. What gives?

A: The government only started tracking this in 2012. The USDA concluded 38,000 acres were lost that year. And farmers in the Dakotas told AP reporters that they were plowing into pristine prairieland. So it's not a question of whether it's happening. The question is on what scale. The government has made it impossible to determine that number precisely. So the AP used the only method available to estimate it: Government crop data collected by satellite. The AP identified tracts of land that were cornfields in 2012 and had been grassland in 2006. The AP then excluded land lost from the Conservation Reserve Program to prevent double counting. The AP vetted this methodology with an independent scientist at South Dakota State University who has published peer-review research on land conversion using the same satellite data.

Q: This must have been factored into the equation when the government wrote this policy, right?

A: Quite the opposite. Scientists and environmental groups warned this might happen but the government and powerful agriculture companies argued it wouldn't. By law, the Environmental Protection Agency was supposed to study whether air and water quality have suffered because of the ethanol policy. That never happened.

Q: We're talking about corn. Like corn you eat in the summer?

A: Not really. That's sweet corn. And it actually makes up only a small percentage of the corn grown in America. Historically, the overwhelming majority of corn grown in America goes toward livestock feed. In the mid-to-late 1990s, about 75 percent of corn went to livestock. Following the ethanol mandate, more corn has been planted and more of it is going to fuel. According to the Department of Agriculture, which has been tracking this for decades, 2010 was the first year on record in which more corn went to fuel than to livestock feed. That was true again in 2011 and 2012. This year, about 43 percent of corn went to fuel and 45 percent to livestock feed.

The ethanol industry argues those figures are misleading because the distillation process leaves behind a residual byproduct that can be used for livestock feed. That byproduct is not measured in the official government data. But however you run the numbers, it's true that more corn is going toward ethanol.

Q: You don't hear a lot about this.

A: Maybe because green is definitely a hot color: Green energy, green jobs, the green economy. But the truth is all energy has costs associated with it.

Everyone knows the environmental cost of oil and natural gas. But the Obama administration rarely acknowledges that its green initiatives have costs, too. And the government allows green companies to do not-so-green things. For wind power, that means the government looks the other way as turbines kill eagles in violation of federal law. For ethanol, the government accepts the environmental consequences in hopes the industry will develop cleaner next-generation biofuels.

Q: So bottom line, is ethanol better for the environment than oil?

A: It depends how you define "better." Burning ethanol releases less carbon dioxide into the atmosphere than burning gasoline. There's no big worry about ethanol spills that might pollute the ocean or kill wildlife. And some ethanol replaced MTBE, the toxic gasoline additive that was terrible for water pollution.

But when you factor in land conversion _ and the erosion, pollution and greenhouse gases that come with it _ ethanol doesn't look as good. Independent scientists say it's tough to make a case for ethanol as long as farmers are plowing over virgin prairie and conservation land. That's why ethanol industry executives don't always factor those effects into their calculations when they say their product is far cleaner than oil.

Trying to determine the effects of one policy in an interconnected global economy is hard enough. Figuring out the environmental effects of that policy is even more complicated. So scientists usually rely on economic and environmental models to estimate the effects. Some studies make ethanol look terrible. Some studies make it look great.

It all depends on what the model looks like and how much land conversion you assume. Bob Dinneen, president of the Renewable Fuels Association, told the AP that "the science is most certainly not there" to model land conversion at all.

Even some of the research being promoted by the ethanol industry isn't cut and dried. For instance, the industry points to a Dutch study that found that urban sprawl internationally was responsible for greater grassland loss than biofuels. But that study, which wasn't peer-reviewed, contains this line, which the ethanol industry does not emphasize: "In the USA, biofuel expansion is the dominant cause of agricultural land use loss."

Q: Then why does the government keep this going?

A: The administration could waive the ethanol requirements and Congress could rewrite the law. That hasn't happened in part because of politics. Abandoning this policy would put the corn-friendly Obama administration on the side of Big Oil, which hates the ethanol mandate. Agriculture interests wield a lot of clout in Congress. And then there are the Iowa caucuses, which can obliterate the dreams of presidential hopefuls who are seen as anti-farmer.

But the administration knows the ethanol mandate hasn't lived up to its environmental promises. Today, when officials talk about it, they are more likely to cast it as an economic lifeline for rural America, not as a green-energy plan.