Carbon Dioxide Removal: A Comparative Analysis of Biological and Mechanical Approaches

Photo by Tak-Kei Wong via Unsplash.

By Kaylee Wei

With the goal of identifying global solutions to the unprecedented environmental and economic challenges that the world faces due to climate change, the 2023 United Nations Climate Change Conference (COP28) is currently underway in Dubai, United Arab Emirates. As countries negotiate the outcomes of the conference, carbon capture and storage (CCS), a process intended to capture man-made carbon dioxide (CO2) at its source and store it permanently underground, has been suggested as a means for mitigating CO2 emissions while the international community debates phasing out the use of fossil fuels. 

A recent study by the University of Oxford estimates that net-zero pathways heavily dependent on CCS could cost at least $1 trillion more annually than scenarios involving renewables. Additionally, CCS is not as effective as phasing out fossil fuels in terms of limiting emissions.

In a recent briefing to the CO2 Plenary Group, senior Economics in Context Initiative researcher June Sekera, explores the effectiveness, resource efficiency and co-impacts of two different approaches to carbon dioxide removal—mechanical and biological—and argues that biological methods are superior to the mechanical methods on all three aspects. The briefing by Sekera was based on two years of research she conducted alongside a team of scientists and economists, the results of which were published in the journal PLOS Climate

The urgency of reducing the level of atmospheric carbon dioxide 

Sekera emphasized the pressing need for action to reduce the level of CO2 in the atmosphere, as global CO2 emissions have surged to a staggering 39 billion tons annually, with the United States contributing a substantial 5 billion tons per year. Figure 1 shows this rapid increase in carbon emissions, which is mainly caused by increased fossil fuel combustion, with ecosystem destruction a key contributor as well.

Figure 1: Annual total CO2 Emissions, By Region

Source: Carbon Dioxide Information Analysis Center (CDIAC); Global Carbon Project (GCP).
Methods of carbon dioxide removal 

Many policymakers have been advocating carbon dioxide “removal” as a mitigation method. 

In her briefing, Sekera compares two primary methods of carbon capture and removal: mechanical methods that include techniques such as Carbon Capture and Storage (CCS) and Direct Air Capture (DAC), and Biological Carbon Dioxide Removal (BCDR).

CCS aims to capture CO2 at its source; it removes no CO2 from the atmosphere while DAC involves mechanically capturing CO2 from the ambient air. Sekera argues that a key challenge with both these methods is that they often fall short in effectively capturing and removing CO2, and in some cases may even add to overall CO2 emissions. 

BCDR, on the other hand, employs natural processes such as forest preservation and maintenance, as well as ecosystem conservation and regenerative agriculture to capture CO2 from the atmosphere. Data gathered by Sekera and co-researchers show that these biological methods are capable of removing substantial amounts of CO2 from the atmosphere. Their projections show that BCDR methods could capture over two gigatons of CO2 in the US in 10-20 years, a significant contribution to reducing atmospheric CO2.

Effectiveness of carbon removal methods

Which method of carbon removal is more effective at removing CO2 from the atmosphere? Sekera points that the CCS method “does not remove anything from the air and it barely captures CO2 at its source.” Worse, almost all CCS projects in the US use the captured CO2 to extract new oil from existing oil wells via a process the industry calls “enhanced oil recovery” (EOR). When EOR is done using CO2 that has been captured at coal-fired power plants, the overall process can emit 3.4 to 4.7 tons/CO2 for each ton of CO2 removed, making this method entirely counter-productive. Significantly, all CCS projects at power plants in the US have failed

While DAC in theory can remove CO2 from the atmosphere, projections from the 2022 Intergovernmental Panel on Climate Change (IPCC) report shows that there is no effective carbon removal from the DAC method until 2030; by 2100 only 1.02 Gigatons of CO2 be removed using this method. 

Compared to the mechanical methods, the Biological Carbon Dioxide Removal (BCDR) can be highly effective. The same IPCC report estimates that with biological methods, 0.86 Gt of carbon dioxide will be removed by 2030, and 4.19 Gt will be removed by 2100. As shown in Figure 2, biological sequestration methods with improved practices and ecosystem restoration have a much higher potential for carbon sequestration compared to the mechanical methods.

Figure 2: Annual amount of potential CO2 removal by biological methods with improved practices

Source: Sekera et al., Carbon dioxide removal – What’s worth doing? A biophysical and public need perspective, PLOS Climate, Feb. 14, 2023.
Co-Impacts of carbon removal methods

The evidence on the collateral impacts of the different methods of carbon removal clearly show that the BCDR method has positive ecological impacts, enriching ecosystems and extending benefits to local communities, while the co-impacts of mechanical methods are overwhelmingly harmful.

Sekera lists several positive co-impacts of BCDR methods, including flood control, improved soil fertility, watershed protection, reduced air pollution, drought mitigation and water conservation. In contrast, mechanical CDR methods are associated with several adverse co-impacts, such as increased risks of pipeline ruptures, leakage of CO2 from underground storage and potentially reduced crop yields. With mechanical carbon storage, as the Institute for Energy Economics and Financial Analysis, points out, “it is impossible to guarantee that stored CO2 will stay underground,” making the method not only ineffective, but also engendering dangers from the escaping plumes of dense CO2, which can be lethal to humans and animals.

Cost evaluation

Sekera’s analysis indicates that mechanical CDR methods can be 25 to 50 times more expensive than biological CDR methods. Table 1 shows one estimate mechanical DAC method could cost anywhere from $500-$1,100 to capture one ton of carbon, compared to biological methods such as reforestation and improved forest management costing $20 or less, and improved agricultural practices costing $100 or less for the same amount for CO2 removal. 

The substantially higher costs per ton of CO2 removed by DAC emphasize the economic feasibility and advantage of investing in reforestation and other biological approaches, at the scale required for addressing climate change effectively.

Table 1: Costs for mechanical capture and biological sequestration of 1 ton/CO2/yr

Source: Sekera et al., Carbon dioxide removal – What’s worth doing? A biophysical and public need perspective, PLOS Climate, Feb. 14, 2023. Note: *Lower cost estimates exist in the literature (including $100/ton) but generally come from sources close to industry; upper range estimates are generally derived from thermodynamic considerations.
Embracing Nature-Based Solutions for Combating Climate Change

In the ongoing battle against climate change, the selection of effective, efficient and sustainable CDR methods is crucial. Sekera’s research shows that the biological methods of carbon removal are effective, resource efficient and have low costs and positive co-impacts, compared to the mechanical methods powered by fossil fuels that are ineffective, inefficient and have high costs and many negative co-impacts, as shown in Figure 3. 

Figure 3: Carbon Accountability Dashboard

Source: Sekera et al., Carbon dioxide removal – What’s worth doing? A biophysical and public need perspective, PLOS Climate, Feb. 14, 2023.

Current policies on carbon removal are focused on market-based solutions of subsidies, tax credits and other incentives for mechanical carbon removal methods. In the US, over $11.7 billion went towards mechanical carbon capture projects from 2010-2020 and an additional $12 billion was included in the 2021 Infrastructure and Jobs Act, but to date there has not been significant funding for biological sequestration methods. 

Sekera strongly emphasizes the need for increased public investments in biological carbon sequestration. This involves supporting communities, Indigenous groups, farmers and public agencies in adopting and expanding regenerative agricultural practices, reforestation and restoration of wetlands, grasslands and forests. Supporting and enhancing these biological systems offer a practical path towards a sustainable future, while avoiding the adverse effects associated with mechanical methods. It is crucial to redirect efforts towards solutions that genuinely work for the planet and for the benefit of future generations.


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