Our addiction to fossil fuels is unrelenting, which means that the skies will continue to fill with carbon dioxide for the foreseeable future. Efforts to scale back coal and oil consumption are underway, but once the trigger has been pulled, it’s hard to put the bullet back into the chamber: conventional wisdom states that the CO2 up there isn’t going anywhere.

Some scientists, however, beg to differ. They imagine a future wherein colossal network of machines scrub the sky of CO2. It sounds science fictional, but prototypes of these machines exist, and recently, the idea received a flurry of media attention when a team of researchers announced they might be scaled up more cheaply than we thought.

Whether or not that’s true is now a multi-trillion dollar question. Like other so-called geoengineering schemes, this one makes big promises that will require major investments before it can be realized. And whether or not the technology can be scaled up, it’s no panacea for climate change.

At its core, though, the idea is alluring for its simplicity.

“It’s a waste management problem,” Klaus Lackner, the Director of the Center for Negative Carbon Emissions (CNCE) at Arizona State University, told Earther. “It’s about picking up the litter we have left in the street.”

Cleaning up the thin blue line above fast enough to, say, meet the goals of the Paris Agreement, would require nothing less than quickly mass-producing devices that can capture all of the human-emitted CO2 and stop it escaping back into the atmosphere. It’s a daunting task, but at this very moment, there are field tests and pilot schemes for carbon-sucking machines taking place across the world. All of them utilize a technique known as Direct Air Capture, or DAC.

DAC machines are essentially artificial trees that rely on the fact that CO2 isn’t immune to the magic of chemistry. All you need is a chamber that lets air enter, and a substance that reacts with the CO2 to form a stable compound, typically a type of carbonate.

Carbon Engineering (CE), the British Columbia-based research institute behind the hyped new paper, has a pilot scheme up and running that continuously captures CO2 in a contraption about the size of a small one-floor house. First, it uses a liquid hydroxide solution to turn the greenhouse gas into a dissolved carbonate salt. A “pellet reactor” then forces small bits of calcium carbonate to precipitate out of solution, which is then dried and heated, releasing the CO2 and creating calcium oxide. The CO2 is then compressed and stored, while the oxide, once rehydrated by steam, is sent back to the pellet reactor, forming calcium carbonate anew and closing the loop.

Climeworks, a Zurich-based engineering company that’s also building small-scale prototypes of DAC units, opts for a slightly different design. It uses porous filters coated in CO2-scrubbing amines, chemical relatives of ammonia, the compound that gives cat pee its characteristic odor.

Allen Wright, the Executive Director of CNCE, told Earther that because these CO2-absorbing materials tend to be liquid, energy is required to pump them around a column of air in order to effectively scrub it of CO2. CNCE, however, is testing out a solid scrubbing system that can be simply placed in a tower, where the passive motion of the air does the work for it.

“The sorbent we’ve developed over the years has a strong affinity for CO2 when it’s dry, but all you have to do to remove the CO2 it is to make it wet—either using a flood of water or mist. That’s it.” Wright explained. “All you need to do to re-dry it is place it back in the air stream.”

Wet or dry, DAC plants still need electricity to power their carbon capture processes. Clearly, it defeats the purpose of cleaning up the atmosphere to attach these plants to high-carbon grids, which is why most proponents cite solar, wind, or nuclear power as the way to go. CE’s pilot plant relies on hydroelectricity. (Its design also partly relies on natural gas, although the company says the machine’s configuration avoids any accidental emissions of CO2 generated on-site.)

At present, all pilot schemes are small. Climeworks operates the most extensive operation to date, and it removes just 1,000 tons of CO2 per year, equivalent to around 60 average Americans’ yearly carbon footprints.

But there’s some evidence that these machines are scalable. CE’s recent carbon capture study, which is based partly based on the company’s pilot scheme, suggests that in theory it’s feasible to design a plant that captures one million tons of CO2 per year—equal to the emissions of 250,000 average cars. It’s been previously suggested that such a plant would take up one square kilometer of space.

The commercialized cost of running such a DAC plant remains very difficult to ascertain. A recent National Research Council study put the cost at $60 per ton CO2 for capture only, to $1,000 per ton of CO2 for both capture and reuse. An influential 2011 study by the American Physical Society puts it at around $600, and pretty much dismissed the scheme as too expensive. This new CE paper puts the cost at between $94 and $232 per ton, citing a more efficient design.

“Sure, the cost is probably still a little high—but it can come down,” Lackner said, recalling how the car engine got cheap through ruthless mass production and that solar panels now cost around one percent of what they did in the 1960s.

But we can’t just scrub the sky of CO2—what do we do with all the CO2 we’ve captured with our machines? If we want to permanently reverse climate change, then we’ll have no choice but to bury plenty of it.

One option would be to store the offending greenhouse gas back in the place from whence it came—the Earth itself. CarbFix, an internationally funded scheme based in Iceland, is currently working to demonstrate this technology, which takes waste CO2 from a geothermal plant, dilutes it in water, and pipes it underground into basaltic rock where it interacts with calcium, magnesium, and iron to transform into stable calcareous rocks.

There are other possibilities too. Why not, for example, stick the CO2 back in the oil and gas reservoirs that we’ve already depleted? You’ve also got deep saline aquifers, those that aren’t contributing to the world’s drinkable water supply. Noting that the Icelandic system isn’t widely applicable, CE’s founder and Acting Chief Scientist David Keith told Earther that “saline formations are more likely long-term large-scale locations for storage.”

In theory, leakage shouldn’t be an issue if the storage sites are well selected and managed. An IPCC report concluded that more than 99 percent of the sequestered CO2 likely to remain down there for at least 1,000 years. The latest study to investigate this, published earlier this week, found that there’s a 50-50 chance that 98 percent of the sequestered CO2 will stay in the subsurface for 10,000 years. Even under a scenario where such sites are poorly regulated, 78 percent of CO2 was expected to stay down there, according to the study’s models.

There’s also the possibility that we could do other things aside from just storing CO2. An oft-discussed idea would be turning it into a carbon neutral hydrocarbon fuel—something that CE, for one, is able to do already on a very small scale. At this stage, though, that’s putting the cart before the horse: first and foremost comes the colossal drawdown of atmospheric CO2.

DAC is arguably the more optimistic cousin to the technological Pandora’s Box that is solar radiation management, which aims to blanket the sky with reflective dust and shield us from the Sun. That might slow down global warming, but it’s covering up a pervasive problem with a brand-new one, complete with unpredictable and potentially catastrophic outcomes. The worst outcome of DAC is that it doesn’t work, or that it becomes a distraction from other important climate mitigation efforts.

That said, even its staunchest proponents wouldn’t call DAC a deus ex machina solution. When discussing how many CE plants it would take to cancel out the world’s 2016 carbon footprint, Keith said: “It doesn’t make sense to think about capturing all the world’s CO2 with a single technology.”

Especially when there remain major hurdles and uncertainties. Despite the rapidly-lowering cost estimates for ensnaring atmospheric CO2, the cost of constructing a full-size DAC plant remains unknown. Companies like CNCE and CE likely won’t be able to build full-sized DAC plants by themselves—they may need massive investments from outside sources.

The same goes for burying the carbon, which Vox’s David Roberts compared to “lighting money on fire.”

“In the free enterprise, capitalistic world we live in, it won’t be invested in until they can think of a way to make it profitable,” Allen said. That’s perhaps why so much fuss is made about the promise of carbon neutral fuel: to lure in the energy industry looking to its future profit margins. It shouldn’t be surprising that one of CE’s investors is Norman Murray Edwards, an oil sands financier.

There’s also the sheer scale of DAC required to capture a meaningful fraction of the world’s emissions, which reached 36.2 billion tons of CO2-equivalent in 2016. Assuming all the CO2 captured by DAC systems is permanently stored, we would need roughly 36,200 of the plants described by CE to make the planet carbon neutral each year, should no other climate mitigation action be taken. And other mitigation action is important.

Some DAC critics worry we may too much faith in a technology that has yet to prove itself on a large scale, one that ultimately may wind up being a climate mitigation side-show. Michael Norton, the environment program director at the European Academies Science Advisory Council (EASAC), told Earther that the IPCC models that say negative emissions technologies (NETs) are required to meet the Paris Agreement targets are merely that: models, based on optimistic speculations that the tech will be readily available in the near-future.

Norton was the co-author of a 2018 EASAC report that looked at what various types of NETs, including DAC, natural carbon capture via reforestation, and even biomass-driven capture-and-burial schemes may offer to the world. The report does not dismiss NETs out of hand, but it points out serious issues with them, including that based on current data, DAC won’t make a significant difference to the climate fast enough.

DAC clearly isn’t a silver bullet to climate change, but there may come a point when the world decides these machines are necessary. Lackner suspects that as soon as one major player adopts DAC, others will fall into line. There will be major governance issues to work out, and at some point, adding more CO2 into the atmosphere may be need to be outlawed unless companies can prove they’re taking the same amount back out of the sky.

Embracing DAC—any serious climate mitigation effort, really—will also require a significant shift in public opinion. “We spend half our effort talking to people and explaining why the removal of CO2 from the atmosphere is an important thing to think about,” Wright said. “There are a lot of people that don’t want to have to consider changing their lifestyle or paying more for products because of someone’s concern about some gas in the atmosphere.”

Saying that, Wright said that he’s “optimistic in a funny kind of way.”

“Humans are very good at adapting to different problems, and we’re really good at building stuff,” he said.

Robin George Andrews is a volcanologist turned science writer with a penchant for extravagant tales, from stellar streams to climate change

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