It doesn’t look like much: a white steel box the size of a small garden shed, a fan sticking out of one side, sitting in the middle of a stark volcanic lava field.

But it might just be one of the keys to humankind’s survival.

The box is a carbon dioxide collector. It sucks in air and filters out the greenhouse gas. Then the machine sends the gas to be pumped 2,000 feet underground into basalt bedrock. There, the gas turns into rock itself, locked away safely for hundreds of thousands of years.

The collector is testing one of the buzziest new technologies in the quest to save the world from potentially catastrophic global warming, direct air capture. Climate scientists have concluded that to avert disaster, reducing greenhouse gas emissions alone will not be enough. On top of that effort, one way or another, we will have to actually remove CO2 from the atmosphere too.

As global leaders meet Tuesday in Paris on the second anniversary of the Paris climate agreement, the world is waking up to a reality embedded in that accord: Of the 116 scenarios that the Intergovernmental Panel on Climate Change maps out as offering a decent chance of keeping temperature rises at manageable levels by the end of the century, 101 of them rely on “negative emissions” technology.

“If we are serious about the target we absolutely need these technologies,” says Dr. Niall MacDowell, a clean energy expert at Imperial College, London. “In their absence it is impossible under any circumstances” to prevent temperatures from rising more than two degrees C above pre-industrial levels by the year 2100.

Ideas for cleaning up the atmosphere range from planting more forests that would act as carbon sinks, to setting up vast arrays of the kind of CO2 collector now in pilot-project use outside Reykjavik. Skeptics scoff that these schemes are too expensive, or untested, or likely to take up land that farmers need to grow food. Others worry that they distract from the main challenge – reducing global emissions in the first place.

But even the doubters agree that “this does not mean that we should give up. We still have to explore these avenues,” as Glen Peters, a climate scientist at Center for International Climate Research (CICERO) in Oslo, puts it. “But we are probably not going to roll these technologies out easily,” he cautions.

Out of thin air

The difficulty hasn’t deterred the dreamers. Bill Gates and Richard Branson are among the wealthy entrepreneurs who are funding research into negative emissions technologies, known as NETs. And Climeworks, the Swiss company behind the CO2 collector in Iceland, is already making such technology a reality.

Climeworks has been sucking CO2 out of the air since July at an experimental installation near Zurich that is powered by heat from a waste incineration plant. They sell it to a nearby greenhouse, where it acts as a gaseous fertilizer, making tomatoes grow bigger and faster. The company is also selling its CO2 to manufacturers of fizzy drinks.

Melanie Stetson Freeman/Staff CarbFix is testing one of the buzziest new technologies in the quest to save the world from potentially catastrophic global warming, direct air capture.

Christoph Gebald, a co-founder of the firm, has his eyes set on a much bigger prize; Climeworks has sold some of its CO2 collectors to Audi, the German auto manufacturer, which is exploring ways of making synthetic fuel from carbon dioxide and hydrogen.

Synthetic fuels “are a very big market opportunity for us,” says Mr. Gebald. “Just serving the aviation business will take gigatons of CO2 from direct air capture.”

Equally lucrative could be a scheme to bury CO2 and sell carbon credits to environmentally conscious corporations that want to erase their carbon footprint.

With this in mind, Climeworks has hooked up with CarbFix, a project that since 2011 has been pumping the CO2 emissions from Reykjavik Energy’s geothermal plant underground.

Scientists discovered, to their surprise, that the gas – dissolved at high pressure in water – mineralizes into solid rock within a year or two, not the thousands of years they had expected, quickly filling the pores in Iceland’s basalt substrate.

The single Climeworks extractor installed next to CarbFix’s pump can process only 50 metric tons of CO2 from the atmosphere each year – about the same as an average American family’s greenhouse gas emissions.

But the contraption is nonetheless the world’s first negative emissions plant, and by mid-2019 Gebald plans to set up an array of collectors big enough to capture 3,000 metric tons of CO2.

Selling carbon credits is a niche market for the time being, he acknowledges. It costs $600 to capture a metric ton of CO2 at the moment (and another $25 to pump and store it underground). At those prices the only viable customers might be low-emission, high profit companies such as banks and insurance companies.

As his company improves its technology and scales up its operations, though, the price of CO2 from direct air capture will come down to about $200 a metric ton within five years, Gebald predicts, and to $100 a metric ton by mid-century.

That would make it commercially very viable, he says. International logistics companies relying heavily on airplanes, for example, might want to go carbon neutral by buying credits from Climeworks. More ambitious firms could use them to go carbon negative.

“We are not a silver bullet,” Gebalt cautions. Big polluters such as fossil fuel power plants, cement factories, or fertilizer manufacturers, he says, would find his solution much too expensive. But since NETs are central to almost all the positive climate change scenarios that scientists have come up with, he says, his company’s work is essential. “If you want two degrees, you need Climeworks.”

... Including the carbon sink

The technology involved in sucking CO2 out of thin air and isolating it is still in its infancy, and questions remain about how easily the method could be scaled up enough to make an impact on climate change.

A negative emissions technology that is by no means in its infancy, however, is photosynthesis, nature’s own way of absorbing CO2 from the atmosphere, which has been operating for eons. Many scientists, seeking an easy, cheap, and tested method of mitigating climate change, are drawn to the simple option of planting forests and letting the trees act as a carbon sink.

But would that be so simple? Where might such giant forests be planted? Would they displace farmland? Could enough potential foresters be found and trained to look after them? Could we be sure that the forests would be maintained long enough to do the job assigned to them?

A variation on forestation, which would both absorb CO2 and generate power, is “bioenergy with carbon capture and storage” (BECCS). This would involve planting large areas of the globe with biomass and then burning it in power plants to create electricity. The plants’ smokestacks would be fitted with equipment to scrub the smoke clean of CO2, which could then be stored underground.

A recent scientific review of different negative emissions technologies found BECCS to hold the greatest potential of any technology for cleaning up the atmosphere. It suggested that under ideal circumstances BECCS could absorb as much as 12 gigatons of CO2 a year – as much as the world needs to be sucking up by 2100 – but that it might take two India’s worth of land to grow the necessary amount of biomass.

This raises some difficult questions. How easily could a farmer be persuaded to stop growing food and start cultivating elephant grass or willow trees? And as the world’s population grows, where would the food to feed more hungry mouths come from? Equally important, how would huge plantations of biomass crops impact biodiversity, a cardinal ecological principle?

Carbon capture and storage uses existing technology, and is especially apt for dirty industries such as cement or fossil fuel power generation. “Carbon scrubbers” can be affixed to power plant stacks to filter CO2 before it enters the atmosphere.

Carbon capture and storage doesn't remove emissions already in the atmosphere (so not officially a negative emissions technology), but it could play a big future role in reducing emissions. In fact, though, only 22 large CCS projects exist worldwide today, according to Noah Deich, founder of the Center for Carbon Removal, a think tank based in Oakland, Calif.

That is because CCS installations add to manufacturers’ operating costs, and they have no incentive to use them other than the sense of being good corporate citizens, which has not yet proved sufficient motivation.

Melanie Stetson Freeman/Staff Bergur Sigfusson and his team at CarbFix were surprised to find that CO2 dissolved at high pressure in water mineralizes into solid rock within a year or two.

Make 'em pay

It will only become worth companies’ while to reduce their emissions when carbon has a price, either on a cap-and-trade market or through a carbon tax, say activists. And the same is true of negative emissions technologies. Some environmentally conscious corporations might pay Climeworks to capture and bury CO2 on their behalf because it is good for their images and their souls, but they are few and far between.

“To make direct air capture viable on a large scale we will need carbon pricing, through a tax or a market,” says Valentin Gutknecht, Climeworks’s marketing manager.

As yet, there is little sign of either ramping up, though most energy experts expect that a price will eventually be put on carbon, and that once in place it will rise.

Negative emissions technologies also need investment now if they are to have any hope of making a difference in time. Though philanthropists are financing moonshots, governments are still not paying much attention. In October the US Department of Energy announced grants worth $26 million to explore carbon-capture technologies – 10 times less than its fossil energy research and development budget for 2018.

A 2015 US Academy of Science report urged more federal funding to improve CO2 removal technologies, warning that “current technologies would take decades to achieve moderate results.”

That kind of caution and uncertainty suggest that the world would be unwise to bet too heavily on CO2 removal to solve its climate problem, especially if that meant easing up on efforts to reduce greenhouse gas emissions in the first place.

The danger, says Nils Markusson, an environmental technology researcher at Lancaster University in Britain, is that “if you think there will be a solution to a risk, it encourages you to take more risk,” – what is known as “moral hazard.”

“Negative emissions include a variety of technologies not in routine use,” Dr. Markusson points out. “A lot of them might fail, and if we had already assumed they would be online by a certain date, counting on them and basing decisions on that, there is a risk that we would have burned our bridges.”

A lot of climate modeling “is based on expectations that we will be using technologies that are not yet tested,” Markusson adds. “That is very scary.”

“The world works differently from models,” points out Dr. Peters of CICERO. “In practice, much of this technology will not happen.”

For Mr. Deich, whose think tank is promoting all kinds of negative emissions technologies, this is an argument to try everything, even while we reduce CO2 emissions at a faster rate than we are managing now. “We need as many shots at goal as possible,” he says.

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The technical challenges facing negative emissions technologies are enormous, Deich acknowledges, and the financial costs could be huge. But in the context of a $6 trillion a year energy market, the money to solve technical issues could be found, he insists.

The biggest question, he argues, is political. “Will we be able to communicate to governments the value of providing this service?” he wonders. “It is not a function of technical resources; it’s a matter of political will. And that is very difficult to predict.”