Federal ethanol subsidies aimed at slowing climate change instead helped trigger the annual release of 30 million tons of greenhouse gases as farmers cleared land to plant more crops for production of the renewable fuel, UW-Madison researchers said Wednesday.

The National Wildlife Foundation sponsored the research and called it the first comprehensive estimate of carbon released into the atmosphere by cropland expansion.

UW-Madison graduate students Seth Spawn and Tyler Lark presented their findings at the wildlife federation’s America’s Grasslands Conference in Texas. The study will be submitted to a peer-reviewed journal within weeks, said Holly Gibbs, a professor of geography and co-author.

The researchers spent years examining satellite imagery and high-resolution maps showing the vegetation cleared away and soil types plowed up when land was cleared from 2008 to 2012.

During those four years crop prices were high amid government incentives promoting plant-based ethanol production. Ethanol was seen as an environmentally-friendly way to reduce fossil fuels burned in auto engines.

The UW-Madison researchers also examined U.S. Environmental Protection Agency policies that should have prevented cropland expansion, but didn’t.

“We are formulating ideas for some of the policy changes that could close these loopholes so that the policies could lead to climate benefits instead climate harm,” Gibbs said.

An EPA spokesman said the agency will review the study after publication.

The Renewable Fuels Association, a Washington-based industry group, issued a statement reprising its criticism of a 2015 UW-Madison study estimating 7 million acres were converted to cropland from 2008 to 2012. RFA spokeswoman Rachel Gantz said satellite data is unreliable compared to government reports showing shrinking acreage.

The Wisconsin Corn Growers Association and the Wisconsin Biofuels Association didn’t respond to requests for comment Wednesday.

Gibbs said the researchers have met with groups like the corn growers and acknowledged that while the data isn’t perfect it is the best available.

Estimated emissions

The new study estimated carbon emissions based on acreage of grasslands, wetlands and wooded parcels that were gobbled up. Most was grassland.

Typically, the grass would be cut and fed to cattle or left to decompose, and the carbon it had held would be released into the atmosphere in a matter of years, said Spawn, the lead author of the study.

But most of the release — about 75 percent — came from the soil after it was plowed. Exposure to air sparked the breakdown of organic matter that had been buried. Much of that carbon would be released within a matter of decades, but smaller amounts could linger for a century or more.

Carbon that is held out of the atmosphere in plants and soil is a key factor in climate change research. Spawn said the converted land could recapture carbon if it was replanted with grasses, but it would take decades.

“It’s still not clear in the scientific literature whether ecosystem carbon stocks can be fully restored to their original carbon levels,” Spawn said. “Obviously the most prudent solution would be to cease expansion which would necessitate reflecting on how we use crops and perhaps focusing on increasing yields on existing land.”

On average, it would take 54 years for the carbon emissions from expanding corn acreage to be offset by reductions in greenhouse gases from ethanol production, the study found. The payback time would average 556 years for soybeans used for ethanol.

Woodlands converted

Wisconsin was among six states where farming expanded onto particularly carbon-rich land such as forests and wetlands. The six states contributed 35 percent of the carbon releases, the report states.

The National Wildlife Federation compared the 30 tons of carbon released annually to emissions from six coal-burning power plants.

“These findings should serve as a wake-up call that it’s time to act with purpose and urgency to fix the ethanol mandate and confront climate change to protect our health, environment, economy, and wildlife,” said Collin O’Mara, federation president and chief executive officer. “Delay will make the problems worse and more costly to solve. We have solutions. It is time to enact them.”

The federation wants Congress to decrease the reliance on corn ethanol and encourage more sustainable fuels, restore damaged wildlife habitat, ban cultivation of noxious or invasive plants as biofuels, and enforce laws that are supposed to prevent conversion of wildlife habitat into corn crops.

A bipartisan group of 64 members of Congress recently wrote a letter to the EPA calling for changes because of “pitfalls and costs” of the ethanol program.

A 2005 law approved by a bipartisan Congress created incentives for ethanol production — called the Renewable Fuel Standard — to reduce reliance on imported oil and address climate change while creating a huge new market for farmers. The U.S. ethanol industry has grown with an estimated annual $16 billion in wholesale sales, and the EPA keeps increasing the amount required in gasoline.

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