Hope For CO2 Removal

2.1 Selected CDR Options

Following the scoping of CDR options from Borchers et al. (2022), we here give only a short overview of the general features of 14 selected CDR options for Germany, with detailed information and description of the options to be found in the aforementioned publication. First, we include two DACC and one ERW CDR options, which use chemical processes to capture CO₂ out of the atmosphere. Furthermore, we include six bioenergy combined with carbon capture (BECC) options, which combine biological and chemical carbon capture and are therefore called hybrid options. To complete the BECC and DACC options, we added one concept for geological storage solutions for Germany, again based on Borchers et al. (2022). Finally, CDR options that capture CO₂ through photosynthetic processes and accumulate carbon in above or below-ground biomass are described in the biological carbon capture section, which incorporates three concepts that involve changes in agricultural practices, and two concepts of ecosystem restoration (peatlands and seagrass meadows).

2.2 Assessment Framework

The assessment of the CDR options for Germany follows the suggested framework by Förster et al. (2022) along six dimensions. In the following, we will give a short overview of the indicators considered in the environmental, technological, institutional, economic, societal and system utility dimensions (for an overview of the assessment framework and the respective evaluation scale, see Förster et al., 2022).

The environmental dimension assesses how the deployment of a CDR option could potentially affect the atmosphere and terrestrial, aquatic and marine ecosystems. The impact variables are in line with commonly used impact assessment metrics (UBA, 2020). Effects on the atmosphere include emissions from changes in terrestrial and marine ecosystems, local climatic effects and noise. Effects of CRD deployment on terrestrial, aquatic and marine ecosystems are assessed in terms of spatial demands and related trade-offs, effects on biodiversity and soils as well as effects on water quality and quantity.

The technological dimension assesses the potential for deployment and upscaling of CDR options based on technological performance. This includes the efficiency of a CDR option in particular in terms of energy use (net energy balance) and capacity for CO₂ removal (CO₂ reduction and removal efficiency per energy unit). Market maturity is determined by the TRL as well as the compatibility with existing infrastructure. Lastly, the compatibility with the future energy system is evaluated with respect to the CO₂ collecting effort and the ability to access low carbon energy carriers.

The economic dimension relates to costs of deploying CDR options, the effects this has on the domestic economy and possible barriers for CDR investments. Accordingly, the marginal cost for removing CO₂ from the atmosphere is included in the assessment of the market costs, that is, the business cost of a given CDR option at this point in time. As costs of a CDR option can change over time, this is likely to alter also their relative cost vis-à-vis other CDR options, which is considered by also assessing the dynamic cost efficiency. This is done by including future cost reductions due to technological enhancements, cost reductions per unit of CDR when upscaling the production (economies of scale), and the marketability of co-produced goods (indicating economies of scope). External effects of CDR options, that is, impacts on third-party actors that are not taken into account by the actor causing them (e.g., negative or positive impact on water quality) are also considered in the economic dimension but are assessed in the environmental dimension to avoid double consideration in the assessment. Another cost category analyzed is transaction costs related to CDR deployment (e.g., for market screening, access and transaction, insurance and meeting regulatory requirements). The assessment includes transaction costs occurring for regulators and for actors involved in deploying CDR measures. The effects on the domestic/regional economy are assessed in terms of additional domestic value and employment. Investment barriers to CDR options are assessed by the share of capital cost in total cost (capital intensity), the specificity of the investments, and the revenue risk.

The institutional dimension addresses the policy landscape in which CDR options have to operate, taking a political and legal perspective on the maturity of CDR options and the feasibility of deploying CDR within existing laws and regulations, administrative capacities and accounting frameworks. Political (and institutional) maturity assesses the CDR options' position in the policy cycle (e.g., agenda setting, adoption of legislation, policy evaluation). The political acceptability is assessed by public and policy support for CDR options within the political debate, governmental support for research of a specific CDR option, as well as by the level of recognition of the role of CDR climate strategies at national and regional scale. Legal and regulatory feasibility addresses possible legal conflicts related to CDR options. It may be assessed by potential conflicts with existing legal requirements, the CDR options' conformity with human rights, and various environmental and conservation laws, particularly with climate laws. The assessment also addresses the demand for additional regulatory effort. Finally, transparency and institutional capacity include the assessment of existing monitoring, reporting, and verification (MRV) systems, the integration of CDR in national reporting of carbon emissions, and the integration of CDR in carbon markets. Beyond that, the institutional capacity is also assessed by the presence of capabilities for using adaptive and responsive approaches for governing the deployment of CDR technologies and whether the deployment of a CDR option requires additional administrative effort.

The social dimension assesses how CDR options are perceived by the public, the social context, associated costs or benefits in societal terms, the extent to which stakeholders are included and can participate in CDR deployment, as well as ethical implications. The public perception of CDR options evaluates the perceived risk of a CDR option, and the trust in institutions, as this has been shown to be a cause for resistance to technology deployment (Markusson et al., 2020; Waller et al., 2020; Winickoff & Mondou, 2017). The assessment of social co-benefits or costs includes potential impacts on health and employment. Inclusiveness and participation are found to increase public trust in technological projects and are assessed by the participation of the public during the planning and execution steps, the dialog on national and regional levels, and the transparency throughout the process. Ethical considerations are assessed by evaluation of the discursive legitimation, the CDR options' effect on intergenerational equity/justice, as well as regarding ethical reservations of resource use. The social context of CDR implementation is assessed by previous experiences with large-scale development projects and the corresponding local narrative.

The system utility dimension describes the potential of CDR options to remove emissions necessary to close the gap for achieving a net-zero CO₂ system in 2050. Taking factors like the availability of biomass and the number of bioenergy plants attainable for retrofitting (relevant for BECC), costs and access to renewable energy supply (relevant for DACC), and available area (relevant for biological options) into account, we attempted to estimate the CDR potential within the German context. CO₂ emissions avoidance potential is assessed by the amount of avoided current emissions to the system in the short and long term, respectively. Emissions potentially avoided in the future are not considered. For assessing the permanence of CO₂ storage of a CDR option the natural persistence of the respective storage reservoir is considered in terms of decades, centuries to millennia (including risks due to natural and human-caused disturbances). CDR options are also assessed for the possibility to measure and verify their contribution to removing and storing CO₂ as well as possible uncertainties involved in such estimates.

2.3 Evaluation Scales

To present the results in an easy-to-read way, we introduce a traffic light system (see Förster et al., 2022) to indicate the effort required to overcome hurdles for the deployment of the assessed CDR options. Green indicates that the implementation of a CDR option is likely to be possible under current conditions (high feasibility) involving no or few hurdles for implementation. Yellow means that there are hurdles of medium magnitude to the implementation that require additional effort to be overcome. Red indicates that the implementation of a CDR option is currently not feasible (low feasibility) with considerable hurdles for implementation. In addition, we indicate if an indicator was “not applicable” for certain CDR options (gray), or if insufficient or ambiguous data was found for the assessment (white).

3.1 System Utility Assessment

We find that relative to the removal need based on estimates of remaining emissions between 32 and 70 Mt CO₂/year for Germany by mid-century (Kopernikus-Projekt Ariadne, 2021; Mengis et al., 2022; UBA, 2021), seven out of 14 options are estimated to provide significant annual removal in the order of magnitude of 10% or more of remaining emissions (F1 is yellow or green, Figure 1). More specifically, our estimates for BECC-based CDR potentials range from 0.5 to 29.9 Mt CO₂/year, where paludiculture and macroalgae for biogas CHP (0.5 and 0.8 Mt CO₂/year, respectively) show the lowest removal potential, and mixed biomass for biogas CHP, wood biomass for pyrolysis for biochar production and woody biomass for combustion CHP (12.6, 14, 29.9 Mt CO₂/year, respectively) show the highest removal potential (Borchers et al., 2022; see Supporting Information S1 for details). If we assume that DACC in heat, ventilation and air-conditioning systems are installed in 15% of the largest buildings in Germany, the CO₂ capturing potential would amount to 15 Mt CO₂/year. If constrained by renewable energy supply by mid-century DACC-farms carbon removal potential would be limited to about 16 Mt CO₂/year (Kopernikus-Projekt Ariadne, 2021). All BECC and DACC options would have to be combined with geological storage for which the storage capacity in discontinued oil and gas fields amounts to an order of magnitude of 2.200 Mt CO₂ (Michael et al., 2011). In addition, saline aquifers on and off-shore could hold another 20,000 Mt CO₂ (Knopf & May 2017). Finally, the scaled potential of natural sink enhancement (NSE) CDR options in Germany was estimated to range from 0.1 to 6.3 Mt CO₂/year, where seagrass restoration and cover crops on agricultural soils show the lowest removal potential (0.1 and 1.7 Mt CO₂/year, respectively), and terrestrial enhanced weathering, and improved crop rotation on arable soils show the highest removal potential (4 and 6.3 Mt CO₂/year, respectively; Borchers et al., 2022; see Supporting Information S1 for details).

Some of these CDR options bring about the additional systemic effect of emissions avoidance (F2, Figure 1). This is true for almost all biomass- and biogas-based bioenergy CHP options, where fossil coal or gas can be replaced by biogenic fuels thereby reducing emissions for electricity and heat production (Borchers et al., 2022). For the rewetting of peatlands the systemic effect of emissions avoidance could be up to 43 Mt CO₂/year by 2050 (Tanneberger et al., 2021), which is found to be more relevant than the removal potential. Noteworthy is the opposite effect of emissions avoidance for the chemical carbon capture options, for which their high energy demand especially in the near term would likely cause an increase in fossil emissions (F2 is red, Figure 1).

Concerning the durability of carbon storage and risks by anthropogenic or natural perturbations (F3, Figure 1), the DACC and BECC options rely on geological storage, for which several thousands of years of storage with close to zero leakage and low natural risk of perturbations are found (Banks et al., 2021; Kempka et al., 2014). Noteworthy is the higher risk of anthropogenic recovery of the stored CO₂ for later usage, if depleted oil and gas fields were to be used for CO₂ storage. Both pyrolysis and gasification of biomass produce products, for which we assume storage, but which bear a risk of anthropogenic usage. For the CDR options that do not depend on geological storage, durability ranges from thousands of years for enhanced weathering and rewetted organic soils (Löschke & Schröder, 2019; Borchers et al., 2022, respectively), over centuries to millennia for the seagrass meadows (Borchers et al., 2022), to decades to centuries for different agricultural practices to increase top soil carbon (Dynarski et al., 2020; Mutegi et al., 2013; Poeplau & Don, 2015). CDR removal based on natural ecosystems is more prone to carbon storage disturbances (e.g., Fuss et al., 2018; Poeplau et al., 2011). Climate change impacts and anthropogenic disturbances (e.g., changes in the occurrence of pest infestations, forest fires and land use change) may alter carbon permanence. For seagrass meadows, carbon storage is sensitive to storm events, ocean warming, and seawater depth and quality. Hence the degradation of seagrass could lead to large losses in its function of storing carbon.

All CDR options seem to be monitorable in principle (see F4, Figure 1). For CO₂ storage in geological reservoirs, geophysical methods are widely employed to monitor possible leakages. For marine and terrestrial options increasing carbon stock, well-established measuring options for soil/sediment carbon stock changes exist. However, the uncertainty due to temporal and spatial variability within the carbon stocks reduced the overall accuracy with which CO₂ sequestration and therefore gross negative emissions can be reported.

3.2 Environmental Assessment

We find that for all biomass-based CDR options the indicator for area demand (A2.1) is key to determine environmental impacts: the higher the area demand for biomass production the more land use competition and environmental impacts are to be expected. This is in particular the case for the BECC option involving biomass combustion in power plants (WCom), which is expected to increase biomass demand and thereby area demand (A2.1 is red, Figure 1) to meet the combustion capacity. As a consequence, it is to be expected that WCom has negative environmental impacts in particular for biodiversity (A2.2; Birdsey et al., 2018; Schlesinger, 2018). In contrast, the BECC options of gasification of woody biomass to liquid fuel (WGas) and the pyrolysis of woody biomass for biochar production (WPyr) assume to be integrated in the current use of fuelwood without the need of increasing biomass production, likely causing no additional environmental impacts (A2.1 is yellow, Figure 1). The CDR concept of retrofitting available biogas plants with carbon capture technology (MxBG) includes the assumption that biomass use was to stay within current levels. However, competition for land and water (e.g., for irrigation) would persist and together with the use of fertilizers and pesticides, MxBG is expected to involve a range of negative environmental impacts (A2 and A3 are red, Figure 1). This concerns in particular negative impacts on water quality and biodiversity (e.g., Babin et al., 2021; Haakh, 2017; Kirschke et al., 2019; UBA, 2014).

CDR options involving changes in agricultural practices by introducing changing the land-use to forest (agricAFF), cover crops (agricCC) and adjusted crop rotation for enhancing soil carbon storage (agricCR) are expected to have a range of positive environmental effects by potentially enhancing biodiversity and water and soil quality (A2 and A3 mostly green, Figure 1; e.g., Thapa et al., 2018). In particular CDR options focusing on enhancing the carbon sink potential of ecosystems such as paludiculture for biogas and bioenergy production combined with carbon capture (BECC-PalBG), and the restoration of peatlands (PReW) or seagrass meadows (SeaG) are expected to have positive environmental impacts in particular for biodiversity, soil and water quality (A2.2, A3.1–A3.4 are green, Figure 1; e.g., Gaudig et al., 2014; Joosten et al., 2013; Reusch et al., 2021). This indicates that ecosystem-based CDR options are likely to create multiple benefits to the environment.

Synergies between CDR options could possibly be harnessed when combining CDR options involving ecosystem restoration with BECCS. Peatland restoration (PReW) combined with paludiculture for biogas and bioenergy production with carbon capture (BECC-PalBG) is an example, where ecosystems are restored and managed for enhancing soil carbon and biodiversity conservation, while at the same time also providing options for biomass production that can be used for BECCS. However, shortly after rewetting peatlands a peak in emissions of non-CO₂ greenhouse gases like methane and nitrous oxide occurs (Tanneberger et al., 2021).

There are knowledge gaps and research needs in particular related to indirect environmental impacts related to indirect land use effects in the case of BECCS and indirect impacts from energy use in the case of DACCS.

In particular for biomass-based CDR options environmental impacts are site-specific and dependent on local conditions and the type of management practices applied. For this assessment, we assume that the applied CDR options would follow sustainable management practices that are in line with environmental regulations (e.g., not exceeding thresholds for the use of pesticides and fertilizers or avoiding leakage of chemical substances of technical appliances). However, already current land management practices come with significant environmental impacts and related negative impacts are therefore likely to continue to persist, as it is the case, for example, for the leakage of nitrogen to water bodies (Kirschke et al., 2019; UBA, 2014). As environmental conditions differ locally, the environmental impacts of CDR measures will have to be reassessed at site-level when moving from national feasibility studies to local scale implementation. The presented assessment using the traffic-light system indicates trends in environmental impacts that can be expected from CDR implementation. These will have to be complemented with site-based assessments in order to understand the location specific implications.

3.3 Technological Assessment

The energy requirement differs significantly between the CDR approaches (B1, Figure 2). Chemical CDR options are most energy consuming, as they must cover their energy demand by external supplies (e.g., Fasihi et al., 2019; Heß et al., 2020; Moosdorf et al., 2014). Although the carbon capture processes for both BECC and DACC are energy intensive, part of the heat and/or power production in bioenergy plants may be used on site to cover the demands of energy generation and CO₂ capture processes, so that no additional energy input is needed. Furthermore, DACC comes with higher effort for CO₂ capture than BECC, as almost its whole energy demand is related to the capture process, whereas in case of BECC only a part of produced energy is used for CO₂ capture—from 15% to 33%, depending on the option: 15% for gasification (WGas), 20% for biogas options (**BG), 24% for biomass combustion (WCom), and 33% for pyrolysis (WPry) (e.g., Thrän et al., 2020). If combined with CO₂ storage, the technology efficiency of BECCS and DACCS will further decrease, as there is energy demand associated with geological storage as well (e.g., Wiese & Nimtz, 2019). In comparison, biological CDR options have a very low energy demand, mainly needed for the initial implementation of the CDR option (e.g., Smith, 2016). Additionally, they do not have energy needs for capture and storage of carbon as those take place via natural processes (e.g., photosynthesis).

Biological CDR options also present the highest degree of maturity (B2 is green, Figure 2), as they are already deployed on different scales. Also, most of the BECC options are technically mature (B2 mostly green, Figure 2) and may build on already established bioenergy and infrastructure (Thrän et al., 2020). However, in case of macroalgae and paludiculture based BECC, the infrastructure for biomass supply would still need to be substantially developed (e.g., rewetting peatlands, launching offshore rings for macroalgae farming) (B3 is yellow/light red, Figure 2; e.g., Buck & Buchholz, 2004). Further development effort is also needed for DACC options to enhance their cumulative CO₂ capture capacity (B2 is light green and light red, Figure 2). There are 19 DACC pilot plants in operation in other countries (e.g., in Iceland and the US; IEA, 2021), but only few small low-temperature-DACC modules (as necessary for DACC-HVAC) tested in laboratories, which makes this option ready for deployment within a decade or later (Dittmeyer et al., 2019; Heß et al., 2020). ERW have been tested in a few field studies, however, achieved mixed results indicate a need for further investigations (Andrews & Taylor, 2019; Löschke & Schröder, 2019).

Additionally, BECC and DACC need the integration of the carbon storage elements (see GEOSTOR, Figure 2), whether it be domestically or abroad. In Germany, many elements of storage infrastructure would still need to be developed, including determining the storage sites and construction of injection wells, preparation of the monitoring system around the storage location, and establishing CO₂ collection networks to deliver CO₂ to storage sites (B3, B4.1 are red, Figure 2).

3.4 Economic Assessment

The business or market cost of CDR options can be a first indication of their value and is usually expressed as cost per unit of carbon removed (Fridahl et al., 2020). Marginal CO₂ removal costs tend to be lower for biological options (C1.1 are mostly green in Figure 2), sometimes even negative costs are indicated, as in the case for cover crops (Fuss et al., 2018). Peatland rewetting is assumed to involve relatively low costs (Couwenberg & Michaelis, 2015), while afforestation of croplands shows a very wide range in cost estimates (Fuss et al., 2018). However, the marginal removal costs of biological options are highly side specific and thus cannot simply be transferred to the German context. Furthermore, ecosystem-based CDR options often require scarce land resources, with the exception of agricCC, which means that they tend to have high opportunity costs (see C1.2 mostly red, Figure 2). Similar considerations also translate to biomass-based hybrid options. In general, chemical and hybrid options are characterized by comparably higher marginal removal costs (Beerling et al., 2020; Heß et al., 2020; IEAGHG, 2013; Kearns et al., 2021; Strefler et al., 2018) as they rely on technological equipment and recurring costs for inputs (energy, feedstock etc.). Due to the hypothetical nature of some of the analyzed CDR options and/or incomplete, ambiguous or lacking information on their market costs in general, for the specific (technological) setting of the CDR options, or for the German context, it reveals to be difficult to give definite estimates on the marginal removal costs for a number of CDR options (C1.1 are mostly white for tech CDR options, Figure 2). However, the notion “no data” should not automatically be interpreted as there being no data at all on the cost of the respective CDR option (see details in Supporting Information S1).

In the evaluated CDR options, cost reduction potential by technological progress seems to be limited (C2.1 is red and yellow, Figure 2). In case of BECC higher potential is seen for CO₂ capture, rather than the bioenergy generation, as the latter is delivered by mature technologies (e.g., combustion, pyrolysis). Moreover, part of the cost may also be covered by revenues coming from sales of jointly produced goods, for example, heat and electricity produced by BECC (C2.3 yellow for BECC, Figure 2). For DACC options, cost reductions of scaling up operations (economies of scale) are expected to be quite significant, since mass production of installations is likely to reduce its cost (Heß et al., 2020). In comparison, such aspects of technological progress and economies of scale are expected to have less potential for reducing costs in biological options.

Private transactions costs, for example, for using relevant markets, setting up necessary contracts and complying with regulations, tend to be moderate to high for most of the CDR options (see C3.2, Figure 2). For chemical and hybrid options transaction costs for the erection of plants as well as for establishing supply chains/markets for inputs and outputs play a major role. For biological options often the high number of actors involved drives the transaction costs if new regulations have to be complied with and new markets need to be used, which is partially caused by the scattered ownership of private forest and agricultural land in Germany. The same applies for example, to decentralized DACC in HVAC systems which includes a high number of actors when applied on a larger scale as well as a larger number of relevant regulations.

The potential for increases in domestic value added provided by the deployment of the CDR options seems rather limited. This is due to little value added potential in general (as e.g., in the case of cover crops or the management of (existing) seagrass meadows) or the fact that the manufacturing and/or installation of equipment is (partially) done by companies from abroad (which might apply e.g., for DACC and BECC options).

An important barrier to investments in the CDR options can be caused by the expectation of a high amount of sunk costs in case the investment fails. This risk increases with the capital intensity of the CDR option (i.e., the overall costs of the CDR measure involves a high share of capital cost), the specificity of the investment (i.e., the financial loss when assets would be applied for other purposes than the envisaged CDR option) as well as with the risks of the expected revenues. Due to low investment needs, biological options tend to possess a rather low capital intensity while hybrid and chemical options that require the erection of technical facilities come along with rather high capital intensity. However, as DACC appliances show high operating cost (due to their high energy consumption) their capital intensity tends to be lower compared to BECC options. Meanwhile, they show a very high specificity of investment, since the technical facilities can barely be used for other purposes and hence would be a stranded investment if DACC turns out to have no economic viability. The same applies to the equipment of existing bioenergy plants with carbon capturing facilities. Biomass-to-liquid plants could switch to the production of other gases for industrial use which makes their investment less specific than those of other BECC options. Since for biological options the carbon is often fixed in (marketable) biomass, selling off the biomass if the CDR case fails remains an option and reduces the specificity of the investment.

The assessment of the revenue risk is challenged by the fact that many of the CDR options do not generate CDR related revenues (as e.g., seagrass meadows) or are not established yet. Thus, the institutional setting of a potential revenue scheme is unclear by now (e.g., DACC or ERW). This puts a high revenue risk on such options from today's perspective. The revenue risk is lower for options that are remunerated for climate protection contributions by a fixed payment scheme such as the EU's common agricultural policy (which applies to afforestation of croplands (agricAFF) and cover crops (agricCC)). BECC options are assessed to have a moderate revenue risk, as technology-related risks are rather low due to the high maturity of these technologies. However, BECC revenues partially are dependent on the development of the EU emissions trading system, which has shown a high volatility in the past and is subject to political discretion, thereby putting a certain risk on the revenues of these facilities. In the case of macroalgae as a feedstock the revenue risk can be assumed to be higher since failing algae yields in Germany (e.g., due to pests or technical challenges) can barely be substituted as established markets are missing.

3.5 Institutional Assessment

In general, institutional arrangements, policies, and laws are more developed for established measures considered as CDR options. For example, land use practices involving paludiculture for biogas and bioenergy production combined with carbon capture (BECC-PalBG), afforestation (agricAFF), enhancing soil carbon sequestration through peatland rewetting (PReW) and cover crops (agricCC) are already practiced and implemented today. These options are also characterized by greater acceptance in the policy debate (E2.1), conformity with existing regulations concerning human rights (E3.2), environmental laws (E3.3) and climate laws (E3.4). Hence, the regulatory effort related to these CDR options is comparatively low (E3.5) (see Figure 3).

However, this is not the case for CDR options involving carbon capture and storage (CCS). BECCS and DACS options consist of multiple components with BECCS including land use for biomass production, bioenergy generation and DACCS requiring technologies for air capture and ultimately technologies for CCS. Different institutional arrangements apply for each of these components. Accordingly, these more complex CDR options require a diversity of institutional arrangements that can pose hurdles to CDR implementation.

In the case of BECCS, the components of bioenergy generation are already well established. Hence the current policy landscape and institutional arrangements facilitate the implementation of the bioenergy component of BECCS. However, this is not the case for the carbon storage (S) component. For example, the federal states of Mecklenburg-Vorpommern, Lower Saxony and Schleswig-Holstein have completely excluded carbon dioxide storage for their territories (Deutscher Bundestag, 2018). The reason is that carbon storage is highly contested in the public and policy debate in Germany (E2.1), with policies and institutional arrangements currently not supporting the implementation of carbon storage. Hence, the geological storage of carbon (GEOSTOR, Figure 3) is rather in an early stage of the policy cycle (E1.1). This is also true for DACCS: while the technologies for DAC are being tested, the CCS component is restricted by the lack of implementation options for carbon storage. Accordingly, the CCS component of BECCS and DACCS is currently limiting the application of these CDR options in Germany. This is reflected in the German National Climate Strategy, which indicates that the potential for CCS options should be examined but it does, however, not explicitly call for the implementation of BECCS and DACCS options (BMUB, 2016) (E2.3). Nevertheless, all CDR options are currently assessed through government-supported research (E2.2).

The same applies to the Monitoring Reporting and Verification (MRV) systems for CDR options (E4.1). While components of MRV systems exist for land-use related CDR options (paludiculture-based biogas CHP—PalBG, afforestation of croplands—agricAFF, peatland rewetting—PReW), there is no MRV system for BECCS and DACCS options. Hence these options are also not integrated into the carbon market (E4.3).

Knowledge gaps exist in particular with a view to those CDR approaches which are in an early stage of development such as ERW or seagrass restoration (SeaG) (Figure 3). Empirical research on other technologies whose results can be used for extrapolation is largely missing. In addition, the institutional aspects are difficult to quantify and the assessment remains tentative.

3.6 Social Assessment

Assessment of the social criteria is challenging, as societal dimensions affected by the different CDR options are subject to diverging definitions and inherent heterogeneity. The public perception of CDR approaches for instance results from different perspectives of stakeholders as that can be classified as individuals, households, industries and economic sectors, or the government. Individual perspectives are shaped by different preferences and circumstances and are furthermore dynamic and can change out of intrinsic or external motivators. In most cases, policy shapes the framework in which the different CDR concepts are presented, but diverging preferences about or exposure to concepts, knowledge or availability (from a technological or economic side) influences perception, acceptance, participation, and contexts the options can be assessed in.

As a result, the assessment is often lacking data or providing ambiguous information about CDR options. This applies especially to the social context (D5), where, due to the different TRLs, assessment of previous experience or local narratives is not available, although it is stated that for example, acceptance of technology options increases if there is exposure and past experience (Wüstenhagen et al., 2007). Acceptance, which can be understood as a consequence of successfully considering the social dimension (Figure 3), is crucial for successful implementation of options. For inclusiveness/participation, data is sparse and ambiguous for for example, paludiculture-based biogas CHP (PalBG), where national dialogues exist. Still, transparency is high only for the biomass part, but low for carbon capture, which leads to the category classified as medium (D3.3 yellow). Also, participation is, as it is a key measure to foster acceptance (Stadelmann-Steffen & Dermont, 2021), difficult to assess due to data availability and implementation status.

As for the hybrid and chemical solutions co-benefits can be found for gasification and paludiculture-based options regarding health and economic co-benefits for employment through increased business opportunities. This is also the case for macroalgae-based biogas CHP (MABG), ERW, and geological carbon storage (GEOSTOR). Employment co-benefits can also help in lowering societal barriers to acceptance, but ambiguous or economically detrimental effects from losing jobs, often indicating a structural change, can societally affect options negatively. Perceived risk for hybrid options and for storage options is also rather high, which is partly mirrored in issues with ethical considerations. This applies especially for geological storage, where social reservations are high, possibly due to no exposure and lacking knowledge and transparency. Looking at BECC options, there exist considerable barriers, as uncertainty regarding the effects, which are often paired with significant negative actions (e.g., competition for land use among options and natural resources in general), harm acceptance. Ethical resource use is the major issue here, as treating hybrid CDR options as a mitigation deterrence shifts the mitigation burden away from other sectors (Carton et al., 2020). For DACC, the resource use can compromise energy security, which is also an ethical concern that as a last consequence, affects acceptance negatively.

Regarding tendencies of the assessment of the options, the social dimension of biological options involving NSE is overall more positive than for hybrid or chemical options, where no clear-cut picture can be made. Health as a co-benefit of the options, meaning additional recreational use or better air or water quality often goes hand in hand with options also posing lower perceived risk. This applies for example, to afforestation (agricAFF) or restoration of seagrass meadows (SeaG). CDR options like these are also rated better considering ethical matters of intergenerational equity (D4.2) or through discursive legitimation (D4.1). This is something that applies to most nature-based solutions, as they are societally less invasive, so acceptance is granted easier. Among the hybrid options, paludiculture- and macroalgae-based biogas CHP (PalBG and MABG) are the ones with the overall most positive outlook, as co-benefits and inclusiveness increase the feasibility of the social dimension. However, such options for more ecosystem-based solutions also require land, which can lead to land use conflicts and lower acceptance by certain land user groups. Tampering with nature is socially frowned upon, which can be an additional reason for barriers in acceptance (Wolske et al., 2019).

4.1 Limitations of the Study

This analysis provides a first comprehensive assessment of selected CDR options for Germany across multiple thematic areas and disciplines. However, the focus of the study comes with inherent limitations, which we would like to point to in this section.

First, given the rather coarse assessment scale of the traffic light system, this analysis often provides qualitative information on general trends related to the feasibility of CDR options within the German context. As the analysis is in part based on expert judgments, subjective views and biases cannot be excluded, and might deviate from other relevant stakeholder perspectives. Furthermore, as environmental conditions differ between sites, locally specific assessments could identify regional differences in the feasibility of CDR options. Therefore, site-specific assessments (e.g., as part of environmental impact assessments) are needed for better understanding the location specific implications. Locally more specific assessments of CDR options within a particular local context (e.g., pilot sites) might lead to different conclusions.

The comparability of the selected CDR options' assessment is limited due to the differences in the implementation scales with respect to their annual removal rate. While the maximum removal scale for each option was chosen, the fact that the annual rates vary substantially impacts among others the options environmental assessment for example, with respect to area demand and its associated impacts. Beyond that, a thorough assessment of the socio-political and legislative dimension would benefit from the development of context-specific implementation scenarios, including information on relevant actors, stakeholders and impacted communities.

Finally, the selected options are not a comprehensive list of possible CDR options for Germany, but was chosen based on the available CDR option portfolio from Borchers et al. (2022). In particular marine-based CDR options are under-represented in this exercise.