Recap and Commentary: National Academy of Sciences Report on Carbon Removal

Earlier today, the National Academy of Sciences (“NAS”) released a comprehensive study dedicated to carbon dioxide removal (“CDR”). To date, CDR has largely been relegated to the fringes of the conversation on climate change, despite the fact that major reports from the IPCC and the UN Environment Program have noted that CDR will likely be critically important for preventing climate change. Two factors have likely contributed to CDR’s position on the sideline for the climate conversation:

  1. CDR solutions have historically been conflated with the too-risky/speculative-to-even-research Albedo Modification (formerly Solar Radiation Management) “geoengineering” techniques
  2. Most CDR solutions cost more than other greenhouse gas (“GHG”) abatement approaches (e.g. solar, wind, energy efficiency, avoided deforestation, etc.), leaving little economic incentive for CDR approaches to develop organically.

The release of the NAS report takes important steps towards reducing both of these barriers for CDR to enter the mainstream climate change conversation. First, the NAS released two distinct reports – one on CDR and the other on Albedo Modification – with language explicitly stating that these two categories of “climate interventions” should not be analyzed together. Second, the report unequivocally endorses expanded R&D funding into CDR approaches, in hopes that such funding will enable the eventual commercialization of these CDR approaches.

The NAS analysis stops before identifying the necessary R&D required for developing and commercializing CDR solutions. But this report has hopefully cleared the way for this conversation to happen – along with the many other mainstream policy and industry discussions necessary for the development of CDR solutions.

The NAS study is worth the full read, but I have pulled out a handful of key sentences and figures from the report, below, along with some commentary on their context to the overall conversation on CDR and preventing climate change:

The definition of CDR according to the NAS:

The NAS study defines CDR as:

Carbon Dioxide Removal (CDR)—intentional efforts to remove carbon dioxide from the atmosphere, including land management strategies, accelerated weathering, ocean iron fertilization, biomass energy with carbon capture and sequestration (BECCS), and direct air capture and sequestration (DACS). CDR techniques complement carbon capture and sequestration (CCS) methods that primarily focus on reducing CO2 emissions from point sources such as fossil fuel power plants.”

Under the umbrella of CDR, the NAS identifies two broad classes of CDR approaches:

1. “Some carbon dioxide removal (CDR) strategies seek to sequester carbon in the terrestrial biosphere or the ocean by accelerating processes that are already occurring as part of the natural carbon cycle and which already remove significant quantities of CO2 from the atmosphere.”

2. “Other CDR approaches involve capturing CO2 from the atmosphere and disposing of it by pumping it underground at high pressure”

A graphic that I’ve created to understand how these CDR pathways are related to each other and to non-CDR pathways is below:

CDR pathways 3
CDR pathways 3

Note 1: the NAS study includes ocean iron fertilization, which I haven’t included in the above graphic because “previous studies nearly all agree that deploying ocean iron fertilization at climatically relevant levels poses risks that outweigh potential benefits.” In contrast, no other CDR approaches in the NAS study are given that assessment, and many others are even given endorsements on the grounds that they "can often generate substantial co-benefits.”

Note 2: Including “Carbon Sequestration on Agricultural Lands “ as part of a “Climate Intervention: Carbon Dioxide Removal and Reliable Sequestration” (emphasis added) report is an important win for advocates of soil carbon sequestration: many geologic sequestration proponents have called into question the permanence of soil carbon sequestration as a major issue with these approaches, which the NAS report largely doesn't raise as an issue.

Note 3: I have included biochar in the above graphic, but the NAS does not, citing "literature [on biochar] is still limited, and the impacts of utilization on net greenhouse gas emissions are not well defined."

The NAS report unequivocally separates CDR from “Albedo Modification” techniques…

The NAS study notes,

Carbon Dioxide Removal and Albedo Modification (i.e., modification of the fraction of short-wavelength solar radiation reflected from Earth back into space) have traditionally been lumped together under the term “geoengineering” but are sufficiently different that they deserved to be discussed in separate volumes.”

Separating CDR from Albedo Modification has been in the works for a long time – and I am hopeful that this report will definitively end the discussion of whether CDR falls under the geoengineering umbrella.

The below table from the NAS report highlights a number of key differences between CDR and Albedo Modification proposals. I have added commentary in italics in each cell:

figure 1.2
figure 1.2

...and links CDR as an extension of other GHG mitigation approaches.

“As a society, we need to better understand the potential cost and performance of CDR strategies for the same reason that we need to better understand the cost and performance of emission mitigation strategies—they may be important parts of a portfolio of options to stabilize and reduce atmospheric concentrations of carbon dioxide”

The report clearly states that the first-best option for preventing climate change is stopping GHG emissions, and that neither the development of CDR approaches nor the development of Albedo Modification approaches will change this finding.

The report’s first recommendation is that

“efforts to address climate change should continue to focus most heavily on mitigating greenhouse gas emissions in combination with adapting to the impacts of climate change because these approaches do not present poorly defined and poorly quantified risks and are at a greater state of technological readiness.”

The report goes on to reiterate that

“there is no substitute for dramatic reductions in the emissions of CO2 and other greenhouse gases to mitigate the negative consequences of climate change, and concurrently to reduce ocean acidification.” And by “helping to bring light to this topic area, carbon dioxide removal technologies could become one more viable strategy for addressing climate change.”

Figure 1.4 visualizes this hierarchy of desirable climate responses nicely:

figure 1.4
figure 1.4

The deployment of CDR techniques is limited by their cost, not by their riskiness or likely effectiveness (as is the case for Albedo Modification approaches).

“Carbon dioxide removal strategies… are generally of lower risk and of almost certain benefit given what is currently known of likely global emissions trajectories and the climate change future. Currently, cost and lack of technical maturity are factors limiting the deployment of carbon dioxide removal strategies for helping to reduce atmospheric CO2 levels.”


“Absent some new technological innovation, large-scale CDR techniques have costs comparable to or exceeding those of avoiding carbon dioxide emissions by replacing fossil fuels with low-carbon energy sources.”

The solar energy field in the 1970s provides a decent analog to the CDR field today. That is, the potential benefits from CDR are as clear as the potential benefits from solar were back in the 70s. But like solar in the 70s, CDR will not be able to scale up overnight.

Only CDR approaches can generate net-negative emissions... so if we need to deploy net-negative emissions to prevent climate change, we will need to have viable/scalable CDR solutions.

“industrialized and industrializing societies have not collectively reduced the rate of growth of GHG emissions, let alone the absolute amount of emissions, and thus the world will experience significant and growing impacts from climate change even if rapid decarbonization of energy systems begins.”


“Although such estimates of future deployment of carbon-free energy sources indicate that it may be possible to achieve a decarbonized energy system, great uncertainties remain regarding the implementation of such scenarios due to factors such as costs, technology evolution, public policies, and barriers to deployment of new technologies (NRC, 2010b)”

NAS to science funding government agencies – Part 1: more CDR R&D, please.

“Recommendation 2: The Committee recommends research and development investment to improve methods of carbon dioxide removal and disposal at scales that would have a global impact on reducing greenhouse warming, in particular to minimize energy and materials consumption, identify and quantify risks, lower costs, and develop reliable sequestration and monitoring.”

And from later in the report:

“Developing the ability to capture climatically important amounts of CO2 from the atmosphere and sequester it reliably and safely on scales of significance to climate change requires research into how to make the more promising options more effective, more environmentally friendly, and less costly. At this early stage successful development also requires soliciting and encouraging new synergies and approaches to CDR. Such research investments would accelerate this development and could help avoid some of the greatest climate risks that the lack of timely emissions reduction may make inevitable. The Committee recognizes that a research program in CDR faces difficult challenges to create viable, scalable, and affordable techniques, but the Committee argues that the situation with human-induced climate change is critical enough (see Chapter 1) that these CDR techniques need to be explored to assess their potential viability and potential breakthrough technologies need to nurtured as they arise.”

NAS to science funding government agencies – Part 2: more CDR R&D from everyone, please.

 “Several federal agencies should have a role in defining and supporting CDR research and development. The Committee recommends a coordinated approach that draws upon the historical strength of the various agencies involved and uses existing coordination mechanisms, such as the U.S. Global Change Research Program, to the extent possible.”

Other notes:

  • Page 25: We have to remove a lot of carbon to get back to preindustrial atmospheric CO2 concentration levels – “Reducing CO2 concentration by 1 ppm/yr would require removing and sequestering CO2 at a rate of about 18 GtCO2/yr; reducing CO2 concentrations by 100 ppm would require removing and sequestering a total of about 1800 GtCO2, or roughly the same amount of CO2 as was added to the atmosphere from 1750 to 2000.”
  • Page 26: an interesting history of CDR
  • Page 31: figure 2.2 comparing fossil CCS, DACS, and BECCS.
  • Page 39: Not good grades given to biochar for carbon sequestration purposes: “The residence time of biochar in situ is not well established (Gurwick et al., 2013). Although there has been research associated with the role biochar could play on carbon and nitrogen dynamics, the literature is still limited, and the impacts of utilization on net greenhouse gas emissions are not well defined (Gurwick et al., 2013).” and “Despite not being among the CDR approaches…”
  • Page 57: Interesting graph showing thermodynamics of DACS vs. other CCS approaches
  • Page 62: “In general seawater capture is much less technologically mature than air capture, so research in this area could yield potential benefits.”
  • Pages 72-81: Interesting summary of what characteristics of CDR approaches the NAS committee thought were most important.

In summary and conclusion, in two quotes:

“Even if CDR technologies never scale up to the point where they could remove a substantial fraction of current carbon emissions at an economically acceptable price, and even if it took many decades to develop even a modest capability, CDR technologies still have an important role to play.”


“If carbon removal technologies are to be viable, it is critical now to embark on a research program to lower the technical barriers to efficacy and affordability while remaining open to new ideas, approaches and synergies.”