Direct Air Capture
This report was last updated in October 2021. It may no longer be accurate, both with respect to the evidence it presents and our assessment of the evidence. We may revise this report in the future, depending on our research capacity and research priorities. Questions and comments are welcome.
Carbon dioxide (CO2) is the most abundant greenhouse gas (GHG) in our atmosphere. To combat the worst effects of climate change, we need to reduce the amount of CO2 that we produce. However, we will also need to remove the CO2 that already exists or will be difficult to abate in the near term. An important avenue for removing CO2 is direct air capture (DAC). This is a process wherein a machine pulls CO2 from the surrounding air and, in many cases, permanently stores that CO2 underground to prevent it from contributing to warming our planet.
Giving Green primarily recommends that individuals direct their donations to organizations pushing for policy change: the more CO2 mitigated, the less we have to remove via CDR. However, for individuals and businesses who prefer their donations to support immediate and certain GHG removal, we recommend one direct air capture firm, Climeworks, which offers a subscription service for individuals to support carbon removal directly from its website.
DAC as a carbon "offset"
DAC is part of the larger suite of carbon dioxide removal (CDR) solutions, both technological and biological, that remove carbon dioxide directly from the atmosphere. CDR is distinct from carbon capture, utilization, and sequestration (CCUS), which generally refers to processes that capture concentrated CO2 at the source of emission, e.g. via a collector placed at the top of a smokestack, and the use of carbon captured in this manner. While a CCUS project reduces emissions after its installation and at the specific facility at which it is installed, at best resulting in a net zero facility, CDR projects do not need to be tied to an emitting facility and, if scaled widely, can result in net negative emissions; technological CDR methods are also referred to as negative emissions technologies (NETs). Thus, CDR has great promise in addressing the CO2 already in the atmosphere, or CO2 that will be emitted in the future from “hard to abate” sectors which are technologically or economically difficult to decarbonize. In the IPCC Special Report on Global Warming of 1.5ºC, released in 2018, “all analysed pathways limiting warming to 1.5ºC with no or limited overshoot use CDR to some extent”.
However, CDR comes with challenges: carbon dioxide is much more dilute in the atmosphere than it is in a smokestack, making it difficult and often expensive to capture: for instance, forestry requires large amounts of land, and DAC requires large amounts of energy. We believe, as do most people working in the CDR space, that GHG mitigation should be our first priority, and that CDR is a necessity to reduce GHG levels beyond what mitigation alone can accomplish and avert the worst impacts of climate change.
This report focuses on DAC projects as opposed to other types of CDR (forestry, soil carbon, enhanced weathering, etc) or other types of CCUS.
DAC projects will typically use the harvested CO2 for commercial purposes or inject it underground with the sole intent of permanently removing it from the atmosphere. Projects that sequester CO2 are sometimes referred to as direct air capture and storage (DACS) or direct air carbon capture and sequestration (DACCS). Some of these projects inject the CO2 under thick layers of rocks to prevent it from leaking out. Others inject it into geological formations that react with the CO2 and turn it into a solid, thereby preventing it from leaking back into the atmosphere.
Giving Green generally prefers carbon removal projects to avoided emissions projects. DAC with sequestration is considered carbon removal. DAC without sequestration has elements of both avoidance and removal. In such projects, the CO2 may be sold as a commodity and replace CO2 that would otherwise have been produced for that commercial purpose, e.g. to provide the bubbles in a can of soda, or may be converted into another usable form, such as “carbon neutral” fuel. We focus primarily on DACS in this report, as causality and additionality claims are more complex when carbon is utilized and not sequestered.
There are a few elements to establishing causality of DACS projects (i.e., that the project directly leads to reduced atmospheric GHGs):
Successful removal of carbon from the atmosphere
Minimal leakage of CO2
Minimal carbon intensity of energy required
Byproducts of sequestration
Successful removal of carbon from the atmosphere: DAC projects are technically difficult. Capturing CO2 from the air requires advanced technology that is in its early stages of development.
Successful sequestration: DAC projects also need to show that they have sequestered the CO2 they captured, generally by injecting it underground.
Minimal leakage of CO2: Even after CO2 has been sequestered, it may can leak back into the atmosphere. Many projects inject CO2 into geological formations from which it is unlikely to leak, such as underneath impermeable rocks. However, this is not foolproof; projects should have a method for tracking and preventing leakage from their sequestration sites.
Minimal carbon intensity of energy required: The process of removing CO2 from the atmosphere and then sequestering it underground is energy intensive and, currently, fairly inefficient. While some amount of investment in this process should happen to increase the efficiency of the technology, the energy cost of running the DAC machines, and the carbon associated with producing that energy, must be considered against claimed GHG reductions.
Byproducts of Sequestration: One method of sequestering CO2 is to inject it into oil wells to extract oil that cannot be extracted using normal means. This procedure is known as Enhanced Oil Recovery (EOR), and since EOR is by far the most valuable use for carbon dioxide, much of captured carbon is used for it. Whether or not EOR results in more or fewer emissions is a source of great controversy in the environmental community. If carbon sequestration is part of the production process, it can decrease the carbon footprint of oil. However, carbon capture may increase emissions in the long run by extending oil production beyond what would otherwise be financially tenable. Some reporting alleges that, of tax credits claimed for sequestration in the US under what is known as Section 45Q, 85-90% were used for OER, but only 5% were reported to the EPA for verification of said sequestration. Due to this uncertainty around EOR’s true climate impact, Giving Green does not recommend offsets that use DAC for EOR.
The CCUS market, which is much more mature than the DAC market, does not rely on revenue from carbon credits. Instead, the captured CO2 is primarily resold for other commercial uses (e.g. EOR or carbonated beverages). But DACS projects that sequester the carbon without commercial gain are likely to rely almost entirely on revenue from carbon credits or philanthropy. We see the market for DACS carbon credits as important for encouraging the growth of this industry and in funding specific projects to remove GHGs.
The cost of DACS currently far exceeds the amount of money that comes in through carbon credits, meaning that most funding still comes from private capital looking for commercial uses or for eventual profits from carbon credits. We believe that most DACS projects would not continue in the absence of the ability to sell carbon credits as a whole; however, it is hard to directly tie your specific credit purchase to the viability of a given project. We thus view the additionality of most of these projects as mixed.
DAC projects have large up-front capital costs and large operational costs (such as electricity). DAC projects must keep a steady flow of revenue coming in to pay these operational costs, and therefore additional money from carbon credit purchases theoretically allows them to run the machines for more time. Also, some DAC projects are modular, so additional funds can be used to expand the system to capture more carbon. Since there is a very plausible path from additional offsets to additional carbon removed, most DAC projects score highly on marginal additionally.
When CO2 is captured and successfully sequestered with a low likelihood of leakage, the permanence of this process is high. Projects in this sector have varying levels of permanence. We are skeptical of geologic sequestration and worry that the CO2 will eventually leak into the atmosphere. However, some projects use natural geological processes to convert the CO2 into a rock form. We see these projects as highly permanent.
DAC projects do not tend to offer co-benefits.
Compared to other sectors, the expenses required to avoid or remove emissions through DAC are substantial. A significant part of these costs can be attributed to the relatively young technology that is expensive to engineer and maintain.
We believe that the cost of DAC has the potential to drop sharply over time given enough support (e.g. via the purchase of carbon credits). For many technologies, a decrease in cost is observed as cumulative output of that technology increases; this empirical observation is referred to as technological learning. While the cost reduction happens via a variety of mechanisms, the core observation is that cumulative experience with a technology results in “learning-by-doing”, which over time, increases efficiency and lowers cost. Learning is greater in competitive industries and modular technologies, so modular forms of DAC may have higher potential for today’s funds to support quick learning. We explore this dynamic in a model, published here.
As examples of the expected price trajectory of DAC: Climeworks has a roadmap to achieve $200/ton; Carbon Engineering projects costs of $94-$232/ton; McQueen et al 2020 identify scenarios achieving <$300/tCO2; Fasihi et al 2019 review and recalculate estimate from prior studies to find a range of $99-$388/tCO2; the US National Academies of Sciences assesses the potential costs of various DAC systems in depth and finds as low as $89/ton to be feasible; Lackner and Azarabadi describe a pathway to $100/ton for modular DAC systems.
Giving Green’s Assessment of DAC
While expensive relative to other carbon offsets, we see DAC projects as one of the most certain ways to remove CO2 from the atmosphere. We recommend one DAC project: Climeworks.
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Lackner, K. S., & Azarabadi, H. (2021). Buying down the Cost of Direct Air Capture. Industrial & Engineering Chemistry Research, 60(22), 8196–8208. https://doi.org/10.1021/acs.iecr.0c04839.
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