Carbon dioxide removal (CDR) sometimes referred to as negative emissions technology (NETs) or simply carbon removal can be divided into two major technological approaches, carbon capture (CC) and direct air capture (DAC). While CC reduces carbon emissions at the point of production (eg the flue gases in smoke stacks), DAC siphons CO2 out of the ambient air. According to the IPCC, technologies that capture and sequester carbon (eg geosequestration which stores captured carbon underground) are a critical part of the required efforts to minimize the worst impacts of climate change. CC, DAC, and geosequestration are not competing technologies, they are best understood as different interlocking parts of the climate action puzzle
Interest in CC and DAC began to intensify in the wake of the Paris Climate Agreement in 2016 (for a brief summary of these technologies click here for a more detailed assessment click here) That interest was ratcheted up after Joe Biden and the Democrats passed the Inflation Reduction Act (IRA) in the summer of 2022 which contains important provisions that support both CCS and DAC.
Why do we need CDR?
There have been thousands of studies documenting the veracity of anthropogenic climate change and its wide-ranging catastrophic impacts have been extensively investigated by researchers. There is a clear and unequivocal consensus that human-generated greenhouse gas (GHG) emissions are driving the climate crisis. Atmospheric carbon emissions have been steadily rising since the industrial revolution and along with these increases, we have seen consistent increases in global average temperatures. The research conclusively supports the finding that to minimize further warming we must radically reduce atmospheric GHGs in the near term, particularly carbon dioxide (CO2) as it is known to be the GHG that is driving the largest increases in average sea and air temperatures.
The newly released report called State of Climate Action 2022, finds that we are not on track to meet our climate goals (50 percent emissions reduction by 2030 and zeroing them out by 2050). This is in addition to IPCC reports which indicate that we are not reducing global emissions and we are ebbing ever closer to the upper-temperature threshold limits. We are currently at 421 ppm of CO2 in the atmosphere and to keep temperatures from breaching the upper limit we need to stay below 450 ppm, this means we need to remove and store between 120–160 GtCO2 (Mac Dowell, Fennell, Shah & Maitland, 2017).
There has been a non-stop barrage of scientific warnings about the urgent need for climate action. These warnings are supported by a strong body of evidence that convincingly demonstrates the need for CDR as one crucial part of a wide array of climate solutions. Together these efforts give us a high probability of being able to keep temperatures from exceeding the upper threshold limit of 2 degrees Celsius above pre-industrial norms.
CC refers to a suite of technologies that pull CO2 from the flue gases in a smokestack before it escapes into the air. CC is a mature and deployable technology (Mac Dowell, Fennell, Shah, and Maitland, 2017) that is an important part of efforts to decarbonize the global economy (Matter, 2016).
In 2009 the IEA estimated that globally, over 200 power plants need CC technology by 2030 in order to prevent temperature rises of over 3°C (IEA, 2009) and a more recent report stated that all fossil fuel-based power plants must be fitted with CC (IPCC, 2018). CC can be widely deployed (Global CCS Institute, 2018) and research from MIT suggests the technology can reduce human-generated CO2 to 80 percent of 1990 levels by 2050 (Paltsev, et al, 2007). After more than a decade of “spectacular technological achievements” and “unrelenting progress” (Welch, 2018), cc may be on the cusp of enormous growth. Moving forward CC needs to increase its efficiency and reduce the costs of the energy needed to power it,
As of 2019, there were 19 large-scale CC facilities in operation around the world. (Jones, 2020) by 2021 there were 22 such plants operating or under construction (Quest, n.d.) and a total of 40 carbon capture projects had been announced in 13 different European countries. In 2021 the U.S. was already the global leader in carbon removal with 13 carbon capture facilities and 30 under construction in a variety of industries. The number of CC plants is expected to grow in the wake of the passage of the IRA. There are also carbon capture projects in the Middle East, Asia, the Pacific, Africa, and Latin America.
Direct air capture
DAC is a relatively new technology that draws carbon dioxide out of the ambient air using fans that push large volumes of air through scrubbers that remove carbon molecules. DAC and Natural Climate Solutions (NCS) are the only approaches we have to safely remove carbon from the ambient air. A side-by-side comparison of the two approaches reveals that DAC is far more efficient than NCS (Lant, 2017). DAC does not require soil or water so it can be positioned almost anywhere so unlike NCS it will not interfere with food production (Peters, 2017). A wide range of DAC technologies are being field-tested (Andrews, 2018) but the most common employs solid amine sorbents (Realmonte et al, 2019).
Some researchers have expressed confidence that DAC can fill both fill the gaps as we decarbonize our economy (Welch, 2018) and be part of our long-term carbon removal plan. DAC could suck up 0.5 to 5 GtCO2/year by 2050 and up to 40 GtCO2/year by 2100 (Cho, 2018). Other researchers conclude that DAC could remove and sequester up to 30 GtCO2/year between 2070–2100 (Realmonte et al, 2019). Some researchers have concluded that DAC will provide half of our CO2 capture needs (Kramer, 2020) and enable us to achieve the goals laid out in the Paris Climate Agreement (Realmonte et al, 2019).
While DAC is more expensive than CC it is also highly scalable (Nemet et al, 2018). To achieve the goals of the Paris Agreement 30,000 plants would need to capture 30 GtCO2/year and some researchers peg the maximum scale-up rate at 1.5 GtCO2/year (Realmonte et al, 2019). Like CC, the high cost of DAC may be mitigated by scaling and finding inexpensive, zero-emissions energy and heating solutions for these plants. Many researchers have logically concluded that DAC may be the most promising CDR technology in the long term (Roberts, 2019). Models clearly demonstrate that without DAC there is virtually no hope of keeping temperatures within the upper threshold limits (Realmonte et al, 2019).
According to IEA, there are currently 18 DAC plants operating around the world but there are many more plants that will come online in the next couple of years, particularly in the US. The oil giant Occidental is going to take advantage of government support afforded by the IRA to build 30 new DAC facilities on 106,000 acres of land in Texas. These facilities will be positioned over a geologic reservoir that can reportedly store up to 3 billion metric tons of CO2.
Geosequestration of carbon
Once the carbon is captured it must then be sequestered to keep it from returning to the atmosphere, most commonly this is accomplished by burying it deep underground in a process known as geosequestration. To remove significant amounts of CO2 from the atmosphere both CC and DAC require sequestration infrastructure, indeed the utility of CDR as a climate solution depends on its ability to safely and permanently store CO2 (Matter, 2016). When CC is partnered with sequestration it is referred to as carbon capture and sequestration or CCS, when DAC is partnered with sequestration it is called direct air capture and sequestration or DACS.
Sequestered carbon can be permanently locked in basaltic rock formations and other underground locations including depleted oil wells and deep saline aquifers (Andrews, 2018). Finely ground olivine-rich rock can absorb up to its weight in CO2 and basalt and volcanic rock can absorb CO2 equivalent to approximately 20 percent of its weight.
CO2 that is injected into underground basalt formations turns into rock in less than two years (Matter, 2016) and as stated by the IPCC, more than 99 percent of the sequestered CO2 will remain sequestered for at least 1,000 years (IPCC, 2005) This is a far more durable solution than NCS. Locking CO2 in trees and vegetation is inherently leaky because it has a limited lifespan and can be released by fires or floods (Kramer, 2020).
We can geosequester around 90 percent of the CO2 produced by big emitters like coal or gas-fired power plants (Hardcastle, 2016). At a pilot plant in Iceland CarbFix has demonstrated that it can sequester over 95 percent of captured CO2 (Matter, 2016). To meet our climate goals we will need to store 120–160 GtCO2 (Mac Dowell, Fennell, Shah & Maitland, 2017). The theoretical global CO2 storage capacity is estimated to be significantly more than enough (ibid). Another study found that the pore space in sedimentary rocks around the globe far exceeds all the CO2 that must be removed from the atmosphere (Folger, 2018) some suggest we can sequester hundreds of trillions of tons of CO2 (Kramer, 2020).
As of 2019, there are only five dedicated geological CO2 storage locations operational worldwide (Kramer, 2020). The longest-running sequestration operation is the North Sea’s Sleipner gas field, where 1 million tons of CO2 have been sequestered since 1996 with no leakage (Global CCS Institute, 2018). The world’s largest dedicated geological storage site is Chevron’s liquefied natural gas project in Western Australia which the company says can sequester up to 4 million tons of CO2 each year. The only dedicated geological storage site in the US is the Illinois ethanol demonstration plant (Kramer, 2020).
If we combine all the CDR technologies in the world today they remove less than 0.1% of global emissions (Jones, 2020). So while CDR is a viable solution, the scale of the challenge is daunting and our window to act is rapidly closing. We are in danger of surpassing tipping points from which we may not be able to recover. It is not hyperbole to say that we are on the cusp of the collapse of civilization. We still have time to avert the worst impacts of climate change but we must act soon and we must act at every level. As Occidental’s plans suggest DAC technology can be fully deployed in a bit more than a year.
To be a viable solution the technology will need to be massively scaled and we must ensure that it is used to decarbonize the planet and not produce more emissions causing fuels. We have every reason to be suspicious of the fossil fuel industry and Occidental is a case in point. Occidental plans to use some of the carbon it captures to extract more hydrocarbons through a process called enhanced oil recovery (EOR). If we are serious about reducing emissions there are some things we should not do with captured carbon and expanding fossil fuel production is one of them. The fossil fuel industry has been the single largest private financial backer of carbon removal but if they are allowed to use these technologies to produce more fossil fuels we will not be able to meet our climate goals. Occidental is already using their DAC project in Texas to sell “net-zero oil” which is little more than manipulative greenwash. Given the fossil fuel industry’s track record, we have reason to be concerned about their involvement with CDR. This is an industry that cannot be trusted, they kill millions and there should be no doubt about their decades-long record of malfeasance. It can be argued that the fossil fuel industry’s disinformation efforts are one of the salient reasons why we have thus far failed to reduce emissions. Their actions are tantamount to crimes against humanity.
We need to evaluate the technologies and ensure that CDR is deployed to maximum effect while doing everything we can to minimize environmental impacts. CDR is indispensable but it should not be viewed as a replacement for emissions reduction nor does it decrease the need for other climate mitigation efforts. If we are to succeed in keeping temperatures below the upper threshold limits we need to deploy a wide range of mitigation efforts. Putting together the pieces of this complex jigsaw puzzle requires a carbon removal master plan that integrates CC, DAC, and geosequestration.
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