JANESVILLE, Minn. — White steam billows into the sky, a rolling cloud visible for miles from Guardian Energy's plant in southern Minnesota.

You can't see the greenhouse gases pumped out the stacks with the water vapor in the air, but they are there — more than 178,000 metric tons in 2019 alone. Eighteen of Minnesota's 19 corn ethanol plants are among the 100 facilities expelling the most greenhouse gases in the state, according to a Star Tribune analysis of data from the U.S. EPA's Greenhouse Gas Reporting Program.

Combined, the state's corn distilleries produced more than 1.7 million metric tons of the climate poisoning gases in 2019. That is less than 5% of the total from Minnesota's top 100 greenhouse polluters. But the manufacture of what's intended as a green alternative to gasoline produces as much pollution as driving 350,000 cars for a year or burning 900,000 tons of coal.

Producers have made headway shrinking the overall carbon intensity of corn ethanol with better crop yields and more efficient distilleries. Minnesota's total corn ethanol production from 2012 through 2019 rose 25%; the greenhouse gases from the plants rose a much smaller 5%, the EPA data shows.

"It keeps improving," said Ashwin Raman, spokesman for the Minnesota Bio-Fuels Association. "This is something that they all take seriously."

There are conflicting views on how corn ethanol's carbon footprint stacks up against gasoline.

The U.S. Department of Agriculture estimates that corn ethanol's life-cycle greenhouse gases — including what's vented from production facilities — are now about 40% lower than gasoline. California's regulator estimates it's from 25% to 40% lower, depending on the producer. Other research says it's less.

The latest report, published in the Proceedings of the National Academy of Sciences,concludes ethanol's life-cycle greenhouse gases are likely 24% higher than gasoline. Ethanol production and the intensive cropping system that goes with it have taken a serious toll on the environment, from eroding soil and destroying pollinator habitat to polluting drinking water wells and the Mississippi River with nitrate and phosphorous from fertilizer runoff.

"Corn ethanol is not a climate friendly or environment friendly fuel," said the paper's author, University of Wisconsin researcher Tyler Lark.

About 200 semi trucks a day pull into Guardian's operation outside Janesville, a small farm town in Waseca County's gently rolling fields.

It's the heart of Minnesota's ethanol country, and the trucks drop loads of corn from surrounding counties into a pit to feed the distillery. Lines of railcars wait to carry off the ethanol, cattle feed and corn oil for renewable diesel the plant produces.

As plant manager Ben Stover sees it, liquid fuels will always have a place in the U.S. economy, even as the electric vehicle revolution advances.

"I see this as a very key way we are going to get to significant reductions in carbon emissions," said Stover, at the wheel of his Ford F150 truck at the plant. "I think we need better technology than lithium batteries."

In terms of the units of heat required to cook a gallon of ethanol, Guardian Energy's plant is one of the lowest in the industry, Stover said with pride.

The plant's greenhouse gas emissions rose 6% from 2012 to 2019, according to EPA data, but production rose faster — 32% during that time, said Stover's boss Jeanne McCaherty, CEO of Prior Lake-based Guardian Energy Management. That means the greenhouse gases per gallon of ethanol have fallen — progress the industry is trying to build on.

Key to that, McCaherty said, was the new steam heat recovery system it installed about three years ago, lowering its fuel use.

"That was a really big one," said McCaherty. "We work at this constantly."

Other ethanol producers in Minnesota echoed those sentiments in interviews, saying their greenhouse gas emissions may have risen, but production rose faster.

Context is important, said McCaherty. The emissions from all of Minnesota's ethanol plants combined are still about three times lower than what the Flint Hills oil refinery produces in a year, she noted.

McCaherty chairs the Renewable Fuels Association, a national ethanol trade group whose members have pledged to drive their carbon to net zero by 2050. There's a range of strategies, chiefly improving energy efficiency, using more renewable electricity from wind and solar and working with farmers to control emissions by using less fertilizer and other practices.

"There's not going to be one winning answer to this," McCaherty said.

Carbon capture and sequestration is expected to play a major role. Across Minnesota, ethanol producers are weighing two huge carbon dioxide pipelines proposed to cross Minnesota and transport the gas for burial underground.

Not everyone's on board. Poet, a major ethanol maker, said it's not interested in the pipes: "We'd rather sell our CO2 to Budweiser and Coca-Cola and municipal waste water districts than try to inject it back in the ground," said spokesman Michael Walz.

Other strategies are brewing.

Minnesota lawmakers are again considering a new low carbon fuel standard like those in California and Oregon. It would require all fuel suppliers in the state to reduce the life-cycle greenhouse gases from transportation fuel at least 20% by 2035.

Ethanol producers would clearly benefit, but the standard is agnostic on the type of fuel, said Brendan Jordan, vice president at the Great Plains Institute, a Minneapolis think tank promoting the approach. The standard would create a powerful incentive to accelerate the decline in ethanol's carbon intensity, Jordan said.

Fuel suppliers that meet the carbon reduction targets can sell carbon credits; those that don't would have to buy them. The bill, sponsored by Rep. Todd Lippert, DFL-Northfield, and Sen. Dave Senjem, R-Rochester, frames a carbon trading program run by the state Department of Commerce. Last year a proposal for a clean fuels standard passed Minnesota's Democrat-led House, but not the Republican-led Senate.

Such fuel standards, of course, rely on measuring ethanol's life-cycle greenhouse gas emissions for a carbon intensity score, which has proved challenging. One sticking point is accounting for the greenhouse gases associated with land use change — pushing food production to other land in order to grow large amounts of corn for ethanol.

Most of the science "grossly underestimates" the emissions associated with that, said Jason Hill, professor of bioproducts and biosystems engineering at the University of Minnesota.

"Everything associated with the carbon intensity of ethanol is smoke and mirrors," said Hill. "Anything that increases the amount of corn ethanol we use … will be taking us in the wrong direction. What ethanol is is another market for corn."