Can our climate be saved by vacuuming carbon out of the skies

Imagine: A switch is flicked and, in a heartbeat, every process spewing deadly pollution into the heavens is replaced with something clean and sustainable. Sadly, even then, the Earth would still tip towards being uninhabitable thanks to all of the carbon we’ve already dumped up there. If we as a species are to survive then all of that junk needs to be pulled back to Earth, and fast. Proponents of Direct Air Capture believe it’s a vital weapon to accomplish that task; its critics say it’s so inefficient that we’d be better off trying anything else first.

Direct Air Capture

Put simply, Direct Air Capture (DAC) is the practice of removing CO2 from the atmosphere by pulling air through a mechanical or chemical filter. Air is typically drawn through a DAC system via one or more fans, while filtering is done with a solid (known as a sorbent) or with a liquid (known as a solvent). Once captured, heat or electricity is applied to the filter material to remove the CO2, both to re-use the filter and get the CO2 ready to move on. It’s this last stage that’s often the most energy-intensive, and therefore costly, part of the process. Given the amount of air that will need to be cleaned (all of it) for this to work, DAC needs to be as energy efficient as possible.

The most cost-effective way to do this is by capping the smokestacks of a carbon-intensive process, like a factory or fossil fuel power plant to prevent more CO2 release. But that does nothing to reduce the excess CO2 already in the atmosphere. That’s why some scientists and entrepreneurs are inclined to gamble on DAC plants in free air to scrub the heavens clean.

The NOAA explains that in 1960, humanity was pumping out 11 billion tons of carbon dioxide into the air each year. Half a century later, and that figure now stands closer to 40 billion, which is why emissions-reduction work is so vital. But even if we did manage to reduce all of our new emissions to zero, we’d still have to address the 950 gigatons or so of CO2 lurking in the atmosphere already. At the time of writing, the CO2 in the atmosphere as recorded by the NOAA’s Global Monitoring Lab at Mauna Loa is 422.38ppm. The scientific consensus is any figure over 350ppm will spell catastrophic doom for humanity and the state of the planet more generally.

This June, the University of Oxford published research saying that if we want to limit warming to just 1.5 degrees (which would be catastrophic), humanity will need to extract between seven and nine billion tons of carbon dioxide out of the air each year by 2050. The COP28 declaration supports signatory nations throwing their weight behind carbon capture technologies. The Intergovernmental Panel on Climate Change (IPCC) says there is no viable pathway to averting climate change unless large volumes of CO2 are pulled from the air. This has been the status quo for a while: In 2017, a coalition of prominent scientists led by Professor Jim Hansen said it was imperative that humanity began mass-removing atmospheric CO2.

What to do with all the CO2

Once DAC has sucked the unwanted carbon out of the air, it needs to be put somewhere. One option, The British Geological Survey explains, is to easily and affordably convert CO2 to its supercritical form, which behaves like a runny liquid. This liquid can then be stored underground after being injected into porous rocks, with old oil fields and coal seams appearing to be ideal places. The oil and gas industry actually uses this approach to boost production in existing fields, as the liquid CO2 fills up the space, pushing more oil toward the extraction site. But the International Energy Agency’s (IEA) briefing paper on Direct Air Capture suggests more than half of all atmospheric CO2 emissions recovered will need to be sequestered.

Obviously, getting more fossil fuels out of the ground to burn does not do very much for the climate, and ideally the governments of the world would just invest in effective carbon capture to prevent us from boiling to death. Fortunately for humanity’s fixation on market solutions, recycling some of the non-sequestered CO2 could become an industry unto itself.

CO2 can also be turned into synthetic fuels in traditional combustion engines. Air travel is the most obvious example, especially given that the size and weight of batteries make it nearly impossible to build an electric jumbo jet. Recovered CO2 can also be used as the base for common non-fuel products including construction materials, in chemical and agricultural products, not to mention putting the fizz in our drinks.

Holocene is one of many companies looking to turn CO2 extraction into a viable, long term business by selling carbon removal credits to big businesses. Its approach is to pull air through water which has been embedded with an amnio acid that binds to CO2. The water and CO2 mix is then combined with guanidine, which turns the CO2 into a solid that can be easily filtered out, allowing the amino acid water to be reused. The solid CO2 is then heated to a low temperature, which separates the guanidine from gaseous CO2, ready for use or sequestration. Holocene believes a reusable solvent (and reusable chemical treatment) combined with the low-temperature heat makes its approach far more cost-effective than that of its rivals.

Mission Zero is also looking to develop a low-cost way of procuring large quantities of CO2 from the atmosphere. It draws air into its hardware and then applies a water-based solvent. But rather than treating this mix chemically, it uses electrodialysis and an ion exchange process to purify the liquid and extract the CO2. From there, the liquid can be reused and the CO2, again, can either be buried underground or, turned into viable products. The company says that its electro-chemical process is similarly far more cost and energy-efficient than many of the other companies operating in this space.

Given the commercial sensitivities involved, it’s not easy to get a real handle on how much it costs to extract CO2 from the atmosphere using DAC in open air. Depending on where you look, the figure can be as much as $600 per ton, but a more common figure is between the $300 and $400 mark. For years, the received wisdom has been that DAC needs to reach a cost of $100 per ton in order to become economically viable.

Earlier this year, a German climate-focused VC firm, Extantia Capital went digging into the source of that $100 shibboleth and traced it back to a paper from early DAC firm Carbon Engineering in 2018 when it published a paper projecting its long-term cost would fall to as little as $94 per ton. Suddenly, the phrase “less than $100 per ton” became the benchmark to which all other DAC companies were held. But, as Extantia’s Torben Schreiter wrote, that figure was also pegged to 2016 dollar prices, so it hasn’t grown with inflation. In 2023, the World Economic Forum said the cost of Direct Air Capture had to fall “below $200 per ton” before it would be widely adopted.

It doesn’t matter if your aims are environmental or industrial, we know the volume of CO2 that needs to be extracted from the atmosphere is significant. For that to be viable, the cost of extraction needs to fall by a significant degree. A more mature metric would be that pricing falls in line with, or below, the perpetually in-flux cost of carbon dioxide as a commodity.

“All these DAC approaches use a bunch of energy,” said Holocene’s CEO Keeton Ross. Ross says it’s the cost of this energy that is keeping the price of Direct Air Capture higher than it needs to be. He believes heat-based systems (like Holocene’s) will likely win out in the end because heat can come from any number of affordable sources. These claims of being able to cut the costs of DAC were compelling enough that in September Google invested in Holocene and pledged to buy carbon credits from it in future.

Dr. Nicholas Chadwick, CEO of Mission Zero, told Engadget his company is targeting around $350 per ton by 2026, but that figure is “dependent on a specific price of electricity.” That price, he believes, is “substantially better than what’s available in the commodity market,” making it a no-brainer for industries that are reliant on CO2 to start buying from Mission Zero.

Roadblocks

The obvious objection to Direct Air Capture is that while there’s a lot of carbon dioxide in the atmosphere, it’s still a relatively small proportion of the whole. I’ve heard the process described as panning for gold in the ocean, and the energy costs alone will make it unfeasible on the scale necessary. In 2022, the Institute for Energy Economics and Financial Analysis bluntly claimed the process “simply won’t work.” Part of the objection was that it can be (and is) used for enhanced oil recovery, but also that when DAC facilities are up and running, they’re often far less effective at capturing CO2 than initially promised.

In 2023, a piece published by the Bulletin of Atomic Scientists expressed outrage that the US Department of Energy invested $600 million in one such project. Its authors said the energy costs required to filter that much air to extract just 0.04 percent of its total are far in excess of other, already less expensive ways to reduce emissions, and that there won’t be any dramatic improvement in the physics and chemistry that will make Direct Air Capture dramatically more efficient. They said, bluntly, “It’s just dumb to build today something that we won’t need for 50 years, if ever.”

Chadwick said a lot of the criticisms around DAC center on its technical feasibility, which he says is the wrong point. “There are tons of industrial processes where the thermodynamics are terrible, look at ammonia,” he said, “it took years and years to get the yields to where they are right now.” What drove those otherwise inefficient processes was the “economic imperative for it in the marketplace,” he said. “When someone proves they can do [Direct Air Capture] for $200 a ton, all of these arguments go away.”

Both Chadwick and Ross spoke about the importance of scale to help accelerate the still quite nascent industry. In 2023, Carbon Engineering, 1PointFive and Occidental broke ground on the Stratos plant in Texas that, when completed, is expected to suck 500,000 tons of CO2 out of the air per year. Both are optimistic, however, that the projects that are currency under construction will help engineers solve those questions. It’s a long, long way to go before we get to the billions of tons experts believe we’ll need to be extracting to have a hope of survival.