Is removing CO2 from the atmosphere realistic?

Climeworks's direct air capture technology on the roof of a waste incineration plant in Zurich. (Keystone/Walter Bieri)

Efforts to capture carbon in the air as a way to combat global warming are gaining momentum, after a group of chemists announced mid-December that they had developed a new method to transform CO2 into jet fuel. But while the idea looks good on paper, can these "artificial trees" truly have an impact on the climate?

Why it matters. Numerous start-ups have ventured into direct air capture (DAC) such as Climeworks, a Swiss firm founded in 2009. These companies are hoping to capitalise on the rise of carbon tax policies to expand their technologies and revenues, with the promise of contributing in an efficient way to the Paris climate agreement. But for these technologies to help us stay below a 2°C rise in global warming, it would require an input of more than half of current global energy consumption.

How it works. Plants, algae and plankton help to regulate the climate by pumping CO2 into the air and thereby securing the carbon it contains. The artificial tree process, presented in 1999 by American Professor Klaus Lackner from Arizona State University, aims to imitate this photosynthesis.

  • The technology is based on powerful fans that suck in air to bring it into contact with an alkaline solution chosen for its affinity with CO2.

  • After the CO2 is dissolved in the liquid, the solution is heated at a high temperature at around 300°C, releasing pure carbon dioxide. The latter is then collected in a tank, after being pressurised.

  • Other processes, such as the one used by Climeworks, bring the CO2 into contact with an adsorbent filter which is then heated at around 100°C to extract the CO2.

  • The captured carbon dioxide can then be sequestered underground at great depths or transformed into syngas, a mixture of carbon monoxide from the CO2 and hydrogen—extracted from the water by electrolysis—which can then be transformed into fuels, or compounds for the chemical industry, or to gasify beverages, as Climeworks does in partnership with Valser. Through this process, fuels produced from atmospheric CO2 would virtually be climate-neutral, since their combustion would only release CO2 that has previously been extracted from the atmosphere.

Does it really work? Indeed, it does. And numerous industrial prototypes are thriving all over the world, such as Climeworks’s facilities in Switzerland and Iceland, or the mechanical trees of the Irish company Silicon Kingdom Holdings, which uses the technology developed by Lackner's team.

Carbon Engineering, founded by Canadian physicist David Keith, is building a plant capable of capturing 1 million tonnes of CO2 per year. EDF's British subsidiary announced on 23 November its plans to test several processes of DAC and hydrogen production by using the heat released by the Sizewell C EPR nuclear reactor that it expects to build in southeast England. That is if, the EPR reactor gets the go-ahead from the UK government.

Profitability. On its part, Climeworks is offering the public the opportunity to participate in its research effort through a subscription (from 7.50 to 53 francs per month), allowing 85 to 600 kg of CO2 to be captured annually. This puts the cost per tonne of CO2 at around 1000 francs, compared to a price of around 32 francs on the European emissions market. Daniel Egger, Climeworks’s marketing manager, explains:

“The price we offer is very close to our cost price. But we are counting on economies of scale to reach 180 francs per tonne by 2030 and 90 francs per tonne later on.”

This was one of Lackner's initial goal:

“At this price, that would mean an additional cost of 20 cents per gallon [editor’s note: 4 cents per litre], which is quite bearable. But we hope to be able to make it three times cheaper.”

Olivier Boucher, climatologist at the Pierre-Simon Laplace Institute (IPSL) in Paris, and author of a report on CO2 capture in 2019, elaborates:

“Some technologies are already well mastered. One of them has been used for years on board of the International Space Station to filter the CO2 emitted by astronauts. But it is still too early to know whether it can be relied upon on a large enough scale to have an impact on the climate.”

What happens to the CO2? As in the case of experimental capture facilities in coal, gas or biomass power plants, the captured CO2 can then be transported to suitable locations to be buried deep underground. In that event, direct capture would help reduce the amount of CO2 in the atmosphere. But other approaches focus on the production of carbon-neutral fuels. For Lackner, it is too late to choose.

“Sequestration is very expensive, and it is difficult to find suitable sites. And meanwhile, the amount of CO2 in the air continues to rise. I think that CO2 should be used instead to make the fuels and products we need. Especially since we know that in the long run, they will not be much more expensive than they are today.”

Climate policy researcher Laurent Drouet from the European Institute on Economics and the Environment in Milan remains sceptical.

“I find it hard to believe in the effectiveness of what some people present as a carbon cycle, namely capturing CO2, making fuels that emit this CO2 and starting over again to infinity.”

Too good to be true? While the technology seems to be well-advanced and appealing, the scale of the climate problem linked to CO2 emissions should not be underestimated. Drouet underlines:

“We release about 40bn tonnes (Gt) of CO2 per year into the atmosphere. To keep the temperature rise below +2°C, compared to 1850, we would need to capture and sequester about 30 Gt of CO2 per year. This represents a significant amount of energy consumption.”

Studies that he co-authored in 2019 show that to achieve this goal it would require an input of more than half of current global energy consumption. Lackner does not dispute this assessment, but stresses that this excess energy consumption could come from renewable energy sources.

“It will not be free, but it is undoubtedly the only way to remain within the objectives of the Paris agreements,” he says.

Daniel Egger, marketing director of Climeworks:

"Today we use waste heat, for example from a waste incinerator in Switzerland, or a geothermal power plant in Iceland. But it is clear that this will not be enough on a large scale.

We will have to resort to massive use of wind and solar power in the most favourable regions of the world. This would make no sense in Switzerland, but it is possible to do so in very isolated but windy or sunny locations, especially if we convert this CO2 into liquid fuels that are easy to transport.”

Does direct capture from the air threaten water resources? “We do not use any water for CO2 extraction, other than the one we capture from the air at the same time,” he says. “For processes that do use water, facilities can be built near the sea,” Lackner adds.

Is it possible on a large scale? For DAC advocates, it is entirely feasible. Lackner highlights:

“Around 100 million installations would be needed, which would be built or replaced at a rate of 10 million units per year. Millions of containers and tens of millions of cars are manufactured every year. This shows that a large-scale switch to direct capture is entirely possible.”

That is as long as there is enough material available. Egger explains: “We have, for example, evaluated the amount of aluminium required for large-scale use of DAC. It’s not insignificant, but it would still be much less than what the transport industry uses. Our life-cycle analysis shows that materials are not a barrier.” According to Lackner, DAC would also lead to a 10 per cent increase in global plastic consumption, “but the material would be produced by processing the captured CO2”.

Drouet does not share this optimism about the resources needed to capture CO2 directly from the air.

“Today the chemicals used for capture are by-products of the chemical industry. More installations would redirect the entire chemical industry towards producing the necessary compounds! In addition, the considerable amount of energy needed would require more wind turbines and solar panels, which use a great amount of resources.

And all this would have to be done for installations that would produce nothing more than this climate service? All this makes me doubt that a scenario to combat global warming based on direct capture is possible.”

The next step. So what are we waiting for? For Lackner, it's primarily a matter of political decision.

“We must continue our efforts to reduce our emissions, and couple them with CO2 capture, for example to power aircraft or ships, whose emissions are increasing every year. For these unavoidable modes of transport, liquid fuels are more efficient than hydrogen or electricity. It's all well and good to plant trees to sequester carbon, but our technologies are a thousand times more efficient for the same amount of space.”

For Boucher, research should continue.

“It's a card to be played, especially as it would be an interesting way of storing wind and solar energy in the form of fuel, and therefore of compensating for their intermittency (Editor's note: when production fluctuates for example between day and night and because of weather conditions). There is no miracle solution to global warming. We will only be effective if we mobilise all the solutions we have at our disposal.”

Drouet concludes:

“What is certain is that there is no natural incentive to invest in this technology as there is in the automobile industry or other industries whose products are sought after by consumers. The only incentive may come from a high carbon price, and in that case, it will first turn into a cash machine for future actors of the industry.”

Translated from French to English by Michelle Langrand


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