Corn is king in the U.S. Midwest, and few companies in the field are larger than Archer Daniels Midland. The agribusiness giant runs a corn processing plant in Decatur, Ill. that turns tons of golden kernels into thousands of liters of alcohol, food chemicals, even syrup. All that industrial fermentation produces millions of tons of carbon dioxide—each liter of ethanol generates nearly 800 grams of CO2—the primary greenhouse gas contributing to global warming and the key ingredient in the photosynthesis that made those corn kernels possible.

That is why ADM has partnered with the U.S. Department of Energy to bury some of that CO2 in a saltwater aquifer that fills porous rock 7,000 feet below the facility. If all the brewery's CO2 could be stored in this way, the ethanol could become a biofuel to burn, one that actually reduces the amount of CO2 in the air and that  seems to be one of the last hopes on offer to keep global warming below 2 degrees Celsius.

"The world depends on removing large amounts of CO2 from the atmosphere in order to bring concentrations well below 450 ppm in 2100," said economist Ottmar Edenhoffer, when laying out the U.N. Intergovernmental Panel on Climate Change's views on how to combat climate change in 2014. "The fact that agriculture provides options to remove CO2 from the atmosphere means this sector has a unique role."

This is the pairing of biology and technology that a number of prominent organizations such as the IPCC and DOE rely on to provide any hope that civilization will begin to combat climate change. Even the United Nations Environment Program's "Emissions Gap" report now focuses on pulling pollution out of the atmosphere in the future to provide hope that remaining below 2 C of warming is possible. That's because concentrations of carbon dioxide have reached 0.04 percent of the atmosphere. CO2 molecules will trap heat for centuries or more unless drawn back out of the air. Fortunately, plankton and plants have been doing that for eons.

"Biomass pulls CO2 from the air via photosynthesis," says Noah Deich, executive director of The Center for Carbon Removal, "and the capture part prevents that CO2 from escaping back into the atmosphere."

This bio-tech has its own acronym: BECCS, for bio-energy-carbon-capture-and-storage. It's a pairing of the technology that could be employed to clean up coal-fired power plants with green plants serving as fuel after sucking in CO2. The IPCC relies on this unproven technology to remove some 160 billion metric tons of carbon over the next century in many of its scenarios for the future. And yet this critical technology found itself confined to small rooms constructed out of plywood buried within a labyrinth at the latest iteration of climate talks in Paris, a handful of enthusiasts, both old and new, talking to each other about roads not taken and humility while zero megawatts of plant-fired power with CO2 capture and storage exist in the world today.

Carbon capture and storage on its own is the main hope—outside of time machines—for dealing with greenhouse gas pollution from fossil fuels. China alone could soon burn 4 billion metric tons of coal per year, none of it with CCS, making goals to keep climate change below an additional 1 C of global warming difficult. Without CCS, the IEA estimates that reducing global pollution would cost $2 trillion more by 2050, and yet few besides Southern Company, U.S. taxpayers and customers of Mississippi Power are paying for it. "This is a place designed for the future," says Lee Youngblood, a spokesman for Mississippi Power, which is building the first coal-fired power plant with CCS in the U.S. "We plan for 40 years into the future."

But it is this association with coal, or even saving this biggest of climate villains, that may be holding back development of the technology. Storing the captured CO2 deep underground—a dimly understood realm where fracking may have poisoned the public well of goodwill—has yet to be tackled outside the oil industry. Where storage has come up, in Germany and the Netherlands, for example, it has been stopped due to safety concerns, and the cost of the power plants with CCS doesn't help. "It's big, big money, it's a long-term commitment, all of that scares the horses," says physicist Graeme Sweeney, chairman of an advisory council for the E.U. on CCS, reflecting on the U.K.'s recent decision to drop financial support for CCS paired in part with biofuels at the White Rose project in Yorkshire.

In fact, outside of Decatur, this pairing of plants and CCS is not faring well. Europe has increased the amount of wood it burns to produce electricity, but the resulting CO2 is not captured or buried. Similarly, the hundreds of ethanol breweries across the U.S. Midwest often hold CO2 instead of just venting it, but then sell it for reuse, obviating any climate benefit. And no one knows where all the plants will come from to do BECCS. "We need a lot of biomass for this and we can't go cutting down rainforests," Deich says.

There are also questions about exactly how much carbon—if any—is saved if a tree is cut down and burned in a power plant. That's because CO2 is added to the atmosphere not just by burning the tree but also through the process of cutting it down and transporting it to the power plant, as well as from the forest now missing a tree. To become CO2-negative requires replanting copses of multiple additional trees to account for the emissions from that one tree cut down, a process that can take several years or more to achieve any CO2 drawdown. Worse, if an old forest is cut down and not replanted, instead replaced with crops like, say, cotton, CO2 pollution can end up being twice as much from burning the trees than even from burning coal and releasing fossil carbon. This problem can become even bigger for biofuels like corn ethanol that emit greenhouse gases at every step, from laughing gas emanating from corn fields after fertilization to the CO2 from the fermentation of kernels into ethanol. "The idea of net negative emissions by 2050 is not credible right now," says Guido Schmidt-Traub, executive director of the Sustainable Development Solutions Network, which is working on plans to eliminate CO2 in national economies.  

Finding enough usable land to plant the required biomass is also an issue. Much of the world's arable land is already committed to producing food, feed or fiber, let alone fuel. One potential alternative is to grow seaweed but that remains untested for the most part. "The onus is on industry to do this,” says physicist Myles Allen of the University of Oxford. “This is not a public good, it's a waste disposal problem. There should be an implicit obligation on companies to take back CO2 in any product they deliver."

Instead, the U.S. brews ethanol from corn across the Midwest, exacerbating climate change pollution, while felling forests in the Southeast and shipping the resulting wood pellets overseas. Even the ADM plant in Decatur, Ill. only captures a portion of the CO2 made by its fermentation, roughly a million tons injected deep underground so far and maybe 5 million tons more in future years. The rest ends up in the air, exacerbating global warming. "I don't think there's any question we're going to overshoot," Sweeney notes. "We're going to have to take CO2 out of the atmosphere. We're going to have to make this work."