Building a big enough NET: Here’s how we can pull CO2 out of the air

Ryan Duncombe, Beam Project

Photo by Marcin Jozwiak on Unsplash

Current concentrations of CO2 in the atmosphere are 420 parts per million, and unless that number declines to 350 parts per million, we stand little chance of keeping global warming under the 2ºC target set by the Paris Agreement, let alone the 1.5ºC target that would mitigate many of the catastrophic effects inevitable with 2 degrees of warming.

There are only two ways to reverse this number, which is increasing by about 2.5 ppm per year. One is to reduce the amount of carbon emitted into the atmosphere, and let the atmosphere naturally reduce its CO2 concentration, which it does by about 1 ppm per year. The second way is to actively remove carbon from the atmosphere with what are called Negative Emissions Technologies (NETs).

There are many proposed NETs, most of which build on the ways the earth already absorbs carbon from the atmosphere through its oceans, forests, and soils (these methods collectively place the natural lifespan of CO2 in our atmosphere at roughly 100 years). There is one technology, though, that many have called a silver bullet — direct air capture, or DAC. Also known as carbon capture, DAC uses human-made materials to actively draw carbon dioxide out of the atmosphere in the effort to reduce global warming. It sounds perfect — we put too much CO2 in the atmosphere? Suck it back out and the problem is solved! But, as with most “silver bullet” technologies, it isn’t quite so simple.

What is direct air capture?

To explain why DAC is no panacea (besides the simple explanation that there is no panacea when it comes to climate change!), we first have to distinguish it from the term “carbon capture” more broadly. Carbon capture includes many already active technologies used to extract CO2 from the exhaust streams of power plants or industrial plants.

There are currently about 60 carbon capture devices in use around the world right now, capturing 127 million tonnes of CO2 per year, with a further 40 projects in various stages of construction. These devices use the same general concepts as DAC, but trap exhaust CO2 from factory emissions that reach concentrations of more than 100,000 parts per million. This is orders of magnitude more concentrated than the 400 ppm in the atmosphere, making this process far more efficient and leaving a lot more room for error.

While that increase in efficiency may sound like a good thing, the crucial factor, however, is that these facilities do not count as negative emissions technologies, as they are merely reducing the existing emissions from factories. If scaled up fully, they could reduce emissions by 14%. While this would be a staggering and important reduction, it also highlights the need for a technology like direct air capture, which can remove carbon from all emissions sources, and is truly a negative emissions technology. Carbon capture tech is limited to addressing stationary sources of carbon, such as power plants, manufacturing plants, and factories. Distributed emissions, such as those from cars, trucks, airplanes, ships, and animals can likely never be addressed via carbon capture.

True DAC is still in development

Developing technology for true direct air capture is not as simple as applying current carbon capture tech to the open atmosphere, as the difference between capturing CO2 at 100,000 ppm and at 400 ppm requires a dramatically different approach to reach a sufficient level of efficiency. To better understand why, it will help to go into a little more detail about the actual technologies involved. While the idea of DAC has been around for years, this technology is still developing, and there are two broad approaches that hold the most promise.

The first type of DAC tech uses water-based solutions containing various hydroxide compounds. These compounds, such as sodium hydroxide, calcium hydroxide, or potassium hydroxide, contain a base metal (the sodium, calcium, or potassium) bound to a hydroxide group, which is only an oxygen and a hydrogen. The base metal in each compound possesses a natural affinity for CO2, so under the right conditions it can exchange its hydroxide group for CO2. It’s a bit like dissolving salt in water. And, similarly to the salt-in-water analogy, it can be quite difficult to reverse the process, and this process needs to be reversible to be feasible. Not only is reversal necessary to regenerate the starting compounds for repeated use, but the CO2 needs to be released to be properly stored or utilized in other ways (we’ll get to downstream uses of CO2 in a minute). Unfortunately, releasing CO2 from these compounds requires temperatures of more than 800ºC, which means a ton of energy is needed. For a project whose sole purpose is to lower the carbon in the atmosphere, this means that energy would have to be completely renewably sourced to make DAC worthwhile.

The second promising DAC technique uses amine-based filters, or sponges, to directly capture CO2 from the air. These require much lower temperatures than the hydroxide-based DACs: only around 80–120ºC (around the temperature at which water boils). This means they can use waste heat from other industrial practices at no cost, in either monetary or carbon currency. However, the need to regenerate, replace, and dispose of amine filters could ultimately mean higher operational costs and environmental concerns from the chemicals used.

Among the other, less developed DAC options, one high-upside piece of tech worth mentioning is called Bioenergy with Carbon Capture and Sequestration (BCCS). BCCS uses trees (the original carbon capture machines) to form carbon sinks, which can then be burned to produce energy. The carbon emitted from burning can captured for downstream use. This strategy has high upside for both efficiency and cost, but the land use requirements could be prohibitive, and have prevented its development so far.

How much does it cost?

It’s impossible to discuss the costs and benefits of direct air capture technology without discussing the very real financial cost of the process. Carbon capture from factory emissions can cost as little as $36 a ton. Direct air capture, however, is still estimated to cost between $100 and $300 a ton, a cripplingly expensive price that remains the biggest barrier to DAC adoption. This is approximately 1–2 orders of magnitude above the trading price of CO2, so for DAC to become feasible, it’d have to get down to the neighborhood of $30-$60/ton. While the estimated costs of DAC are expected to get there, it won’t be without a decade or two of continued investment in research and development.

Additionally, the current high price of carbon means that none of the DAC facilities in operation are directly burying their carbon — instead, they’re monetizing it. Carbon is valuable — its value is what got us in this mess in the first place, and its why DAC facilities sell their captured carbon for various uses. A New York company called Global Thermostat sells their CO2 to Coca-Cola for use in their carbonated beverages. Swiss company Climeworks uses their greenhouse gas to increase crop yields in actual greenhouses. And Carbon Engineering, a DAC company from British Columbia, is pioneering the use of DAC carbon in synthetic fuels.

Unfortunately, “using” DAC carbon means that the captured carbon will eventually make its way back into the atmosphere. This means that, right now, DAC is only being used as a carbon neutral technology, not a carbon negative one. No one will pay to bury carbon when it has no value in the ground (except to, you know, save the world).

The US is testing out new tax incentives for carbon capture and sequestration (CCS), or burying captured carbon, but right now there’s nothing on the scale that will scratch the surface of what’s needed to prevent climate catastrophe, and countries are doing a poor job working together to tackle the issue. It’s a classic tragedy of the commons — a given country might be interested in DAC if it could reduce the carbon concentration only for itself, but if the cost is felt alone and the benefit is worldwide, no one wants to foot the bill.

Meanwhile, over 5,000 fossil fuel companies have received at least $3 billion from COVID-related US aid in just the last few months. Add that to the roughly $20 billion in subsidies they receive each year and tell me again we “can’t afford” to fix climate change. At the current $100/tonne of CO2 removed by DAC, that money could pay to remove 230 million tonnes of CO2 from the atmosphere every year (or about 0.1 ppm per year — it may not sound like a lot, but that could be almost 4% of the way to a carbon neutral world already!). So, to close the loop, the point is that we could easily afford to pay for this if our priorities were where they should be.

Low carbon fuels are the most likely way of financing DAC

The good news is there appears to be a way to sell the carbon from DAC in such a way as to make it marketable before the end-times motivate world governments to fund it. Unfortunately, the most immediately useful and incentivized use of captured CO2 is…in fuels. Yes, this means that that carbon will be re-emitted to the atmosphere. However, while the carbon would be emitted, it wouldn’t increase carbon in the atmosphere, since atmospheric carbon would be its source. Fuels made from captured CO2 would enable a closed carbon loop, which would still be a significant improvement over the current situation.

Importantly, the concentration of carbon in modern synthetic fuels is much, much lower than in fossil fuels, and even lower than that of biofuels like corn ethanol and biodiesel. Synthetic carbon fuels can reach as low as 10% the concentration of carbon per unit of energy released as fossil fuels, and about 15% that of biodiesels. They also require about 1% the land use of other biofuels. This means that despite the carbon neutral use of captured CO2, the coverage it can provide in replacing fossil fuels regarding energy and land use can be immense.

Synthetic fuels are made by using electricity (renewably sourced, please) to combine CO2 with hydrogen. Carbon Engineering is capable of making very low carbon fuels using this method and is even targeting commercialization of their “air to fuels” product in 2021. While the financial cost is of air to fuels (A2F) is still higher than available biofuels, the carbon content of is much lower, so in a market that values carbon content, A2F could have a significant advantage.

That’s why Carbon Engineering is targeting California, whose Low Carbon Fuel Standard decreases the allowable carbon content of energy producers each year. Carbon credits under the program currently trade for about $135 per tonne, meaning A2F has a real shot of being competitive. If it can compete, it’ll be the first real shot for DAC-produced carbon to find a meaningful foothold in a market and could bring significant opportunity for expansion and innovation in the technology of synthetic fuels.

What about true sequestration?

Once DAC technology is off the ground, either because we’ve successfully incentivized its financing, or climate change has progressed enough that we’ve reached desperation, we can start truly sequestering that carbon. This fascinating article uses a thought experiment to show the scale of true sequestration needed. The volume of carbon needing sequestering would be huge — enough to build 138,000 carbon pyramids the size of the Great Pyramid at Giza. That would mean building 3.8 pyramids a day for one hundred years, and the resulting structures would cover an area ten times the size of New York City.

Now, obviously we aren’t going to build hundreds of thousands of pyramids, but it does highlight the scale of the problem, and injecting it back into the ground instead isn’t trivial. The CO2 would have to be transformed into a more secure form to prevent leaking, and those forms take up more space than the oil its replacing, meaning someplace else would still be needed for storing the extra carbon (back to the pyramids idea, perhaps?). So even if carbon capture and sequestration technology was ready tomorrow, it’s not clear where or how the carbon would actually be stored.


While the ultimate goal remains carbon sequestration, synthetic fuels or other uses for DAC carbon are an important development step for the technology. Ideally, they would allow for further development, increased scale, and capability of direct air capture tech, so when the time comes and the world finally places a value on sequestered carbon, it will be available. There’s a lot that needs to go right for DAC to provide a meaningful reduction in atmospheric CO2, so it’s far from the miracle technology some fossil fuels companies are promising. But it will undoubtedly be a crucial component, and the International Energy Agency recently announced that carbon capture and storage will be “critical” for meeting climate targets. There remain significant hurdles financially, and it’s not clear if they can or will be rectified, even in California’s market. Carbon-free energy will be required to operate it, and it’s not clear how available that will be, either. Despite these factors, it remains clear that DAC has significant potential to aid in the fight to return to 350 ppm, and with more research and continued effort, the downsides could be minimized. And the potential upside? Well, the (carbon-free) sky's the limit.

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