Carbon dioxide (CO2) is the most abundant Greenhouse gas (GHG) in our atmosphere. To combat 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 imminently exist to reach sustainable levels of GHG. An important avenue for removing CO2 is Direct Air Carbon Capture and Sequestration (DACS). This is a process wherein a machine pulls CO2 from the surrounding air and then permanently stores that CO2, often underground, to prevent it from contributing to our warming planet.
We currently recommend one carbon capture and sequestration project, Climeworks, which sells offsets directly from its website.
DACS as a carbon offset
Carbon Capture and Sequestration (CCS) can involve pulling CO2 from industrial facilities or from the ambient air, either passively through natural processes like photosynthesis and Enhanced Weathering or actively through machines in a process called Direct Air Capture (DAC). Because of how much CO2 is already in the atmosphere or projected to be released regardless of effort to prevent it, capturing and storing CO2 that already exists in the atmosphere is an important lever in fighting climate change. This paper focuses on Direct Air Capture and Sequestration projects as opposed to other types of CCS such as enhanced weathering or carbon capture from industrial facilities, which we will deal with in a separate paper.
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. While harvesting CO2 for commercial sources may be carbon-negative even if the CO2 is eventually re-emitted, we focus our attention on DAC projects that permanently sequester CO2 (i.e. DACS projects). 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 (and here).
There are a few elements to establishing causality of DACS projects:
Successful carbon capture
Leakage of CO2
Energy cost of running machines
Byproducts of sequestration (like production of oil)
We tackle each in more detail below.
Successful carbon capture
DACS projects are technically difficult. The process requires capturing CO2 from the air, which requires advanced technology that is only just beginning to be developed in a scalable fashion. For a DACS project to be successful, it needs to actually capture CO2. To determine the causality of a DACS project, there needs to be evidence that the company’s claims to successfully removing carbon are valid.
DACS projects also need to show that they have sequestered the CO2 they captured. This is one of the most challenging parts of these projects, often requiring that the CO2 be injected deep underground. There are many stages in which this process can go wrong, with CO2 leaking back into the atmosphere. To establish the causality of DACS projects, we must look at whether the companies are successfully sequestering carbon.
Leakage of CO2
Even after CO2 has been sequestered, there is a chance that it can leak back into the atmosphere. To get around this, many projects find geological formations where the CO2 will have a difficult time leaking, such as underneath impermeable rocks. However, this is not a foolproof method of sequestering carbon and so we need to investigate whether projects had a method for preventing the leakage of CO2 before we can comment on the causality of their work in reducing GHGs.
Energy cost of running machines
The process of removing CO2 from the atmosphere and then sequestering it underground is energy-intensive and, currently, fairly inefficient. While there is some amount of investment in this process that should likely happen to get the technology to a place where it is more efficient, it is important that we evaluate the energy cost of running the DACS machines as part of understanding whether they are reducing total GHGs.
Byproducts of sequestration
Carbon dioxide can be injected into oil wells to produce oil that can’t 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 using DAC to fuel EOR results in more or less emission 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, if the captured carbon is subsidized, this becomes a subsidy to oil companies. This article provides a good discussion of the pros and cons of using carbon capture for EOR. Giving Green does not recommend offsets that use DAC for EOR.
The carbon capture market in general does not currently rely heavily on carbon offsets. Instead, it mainly focuses on recycling CO2 for other commercial uses (e.g. like EOR or beverages). But DACS projects that sequester the carbon without commercial gain, are likely to rely almost entirely on carbon offsets or philanthropy. We see the DACS carbon offset market as important for encouraging the growth of this industry and in running specific projects.
The cost of DACS currently far exceeds the amount of money that comes in through carbon offsets, meaning that most funding still comes from private capital looking for commercial uses or for eventual profits from carbon offsets. We think it is likely that most DACS projects would not continue in the absence of carbon offsets as a whole but that it is hard to directly tie your specific offset to CO2 removed. We thus view the additionality of most of these projects as mixed.
DACS project have both large up-front capital costs as well as large operational costs (such as electricity). DACS projects must keep a steady flow of revenue coming in to pay these operational costs, and therefore additional offset money theoretically allows them to run the machines for more time. Also, most DACS projects are modular, so offset money 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 DACS 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. There are several different levels of permanence that we have found in the sector. Some projects attempt to use geological formations to keep the CO2 from leaking. We have a skeptical view on these projects, with worry that the CO2 will eventually leak out. However, there are some projects that use natural geological processes to convert the CO2 into a rock form. We see these projects as highly permanent.
DACS projects do not currently offer co-benefits.
Assessment of DACS projects
While expensive relative to other carbon offsets, we see DACS projects as one of the most certain ways to remove CO2 from the atmosphere (as long as the carbon is not used for EOR). We recommend one DACS project: Climeworks.
This work is preliminary and subject to change. Questions and comments are welcome at firstname.lastname@example.org.
“Background Information about Geologic Sequestration.” EPA, Environmental Protection Agency, 6 Sept. 2016, www.epa.gov/uic/background-information-about-geologic-sequestration
“The Concept of Geologic Carbon Sequestration.” USGS , Mar. 2011, pubs.usgs.gov/fs/2010/3122/pdf/FS2010-3122.pdf
Brennan, S.T., Burruss, R.C., Merrill, M.D., Freeman, P.A., and Ruppert, L.F., 2010, A probabilistic assessment methodology for the evaluation of geologic carbon dioxide storage: U.S. Geological Survey Open-File Report 2010–1127.
Sundquist, Eric, Burruss, Robert, Faulkner, Stephen, Gleason, Robert, Harden, Jennifer, Kharaka, Yousif, Tieszen, Larry, and Waldrop, Mark, 2008, Carbon sequestration to mitigate climate change: U.S. Geological Survey Fact Sheet 2008–3097.
“What’s the Difference between Geologic and Biologic Carbon Sequestration?” What's the Difference between Geologic and Biologic Carbon Sequestration?, 2020, www.usgs.gov/faqs/what-s-difference-between-geologic-and-biologic-carbon-sequestration?qt-news_science_products=0#qt-news_science_products
Douglas W. Duncan and Eric A. Morrissey. “The Concept of Geologic Carbon Sequestration, Fact Sheet 2010-3122.” USGS Publications Warehouse, 2010, pubs.usgs.gov/fs/2010/3122/.
“CCS Explained.” UKCCSRC, 13 Dec. 2019, ukccsrc.ac.uk/ccs-explained/
Roberts, David. “Pulling CO2 out of the Air and Using It Could Be a Trillion-Dollar Business.” Vox, Vox, 4 Sept. 2019, www.vox.com/energy-and-environment/2019/9/4/20829431/climate-change-carbon-capture-utilization-sequestration-ccu-ccs
“Geologic Sequestration in Deep Saline Aquifers.” Geologic Sequestration in Deep Saline Aquifers | EARTH 104: Earth and the Environment (Development), 2020, www.e-education.psu.edu/earth104/node/1094
Elliott, Rebecca. “Carbon Capture Wins Fans Among Oil Giants.” The Wall Street Journal, Dow Jones & Company, 12 Feb. 2020, www.wsj.com/articles/carbon-capture-is-winning-fans-among-oil-giants-11581516481#:~:text=Chevron%20has%20invested%20in%20companies,natural%20gas%20or%20making%20cement
Kintisch, Eli. “Can Sucking CO2 Out of the Atmosphere Really Work?” MIT Technology Review, MIT Technology Review, 2 Apr. 2020, www.technologyreview.com/2014/10/07/171023/can-sucking-co2-out-of-the-atmosphere-really-work/
Peters, Adele. “We Have the Tech to Suck CO2 from the Air–but Can It Suck Enough to Make a Difference?” Fast Company, Fast Company, 17 June 2019, www.fastcompany.com/90356326/we-have-the-tech-to-suck-co2-from-the-air-but-can-it-suck-enough-to-make-a-difference