How can carbon be stored in the built environment? A review of potential options

Abstract

In order to reach carbon neutrality, GHG emissions from all sectors of society need to be strongly reduced. This especially applies to the construction sector. For those emissions that remain hard to reduce, removals or compensations are required. Such approaches can also be found within the built environment, but have not yet been systematically utilized. This paper presents a review of possible carbon storage technologies based on literature and professional experience. The existing technologies for storing carbon can be divided into 13 approaches. Some are already in use, many possess the potential to be scaled up, while some presently seem to only be theoretical. We propose typologies for different approaches, estimate their net carbon storage impact and maturity, and suggest a ranking based on their applicability, impact, and maturity. Our findings suggest that there is an underutilized potential for systematically accumulating atmospheric carbon in the built environment.

KEYWORDS:

• Climate change mitigation
• built environment
• greenhouse gas emissions
• carbon neutrality
• carbon storages
• negative emission technologies

1. Introduction: the needs and approaches for removing carbon from the atmosphere

1.1. Carbon neutrality and the built environment

 We are quickly consuming the allowable carbon budget that is left before the globe is estimated to warm beyond 1.5 degrees Celsius. According to the Intergovernmental Panel on Climate Change (IPCC 2018), this remaining quota of greenhouse gas (GHG) emissions for this 1.5 warming pathway is 570 Gt CO₂e and 800 Gt CO₂e for a 2-degree warming scenario. Moreover, the construction sector needs urgent strategies to balance its GHG emissions as one-third of GHG emissions, approximately 40% of primary energy demand and half of raw material consumption, can be attributed to the constructions sector (Herczeg et al. 2014).

To stay within these budgetary frames, GHG emissions need to be reduced and GHG removals increased. When emissions and removals are balanced, a state of ‘carbon neutrality’ is reached. Cumulative carbon budgets have been proposed for countries (Gignac and Damon 2015), and some have set targets for reaching carbon neutrality: Norway and Uruguay by 2030, Finland by 2035, Sweden by 2045, and the European Union by 2050 (United Nations Environment Programme 2019; Perissi et al. 2018).

Should the construction sector pursue carbon neutrality, the emissions and removals of carbon within the sector should be balanced. According to recent policy developments in the EU, as reported by Frischknecht et al. (2019), there seems to be a tendency towards setting top-down carbon quotas for buildings. In this approach, the carbon budget for the construction sector is derived, from acts, such as the Nationally Determined Contributions of the Paris Agreement (United Nations 2015), or from compatible climate legislation. Exemplary methodology for allocating national emissions to the real-estate sector has been published in the EU (Hirsch et al. 2019) with Germany working on maximum allowable carbon emissions and a minimum amount of stored carbon for buildings (Hafner 2017).

1.2. Technologies for removing carbon from the atmosphere

There are both natural and technological means for removing GHGs from the atmosphere. Natural removals include carbon sequestration through photosynthesis, accumulation of organic matter into soils, uptake of carbon into aquatic ecosystems, and slow geological processes, such as the weathering of rocks.

There are also numerous existing and emerging carbon dioxide removal (CDR) technologies or negative emission technologies (NET) for removing CO₂ from the atmosphere. Certain NETs fix ambient CO₂ via the transition to stable compounds. Others produce a stream of high concentrated gaseous or liquid CO₂, which can be stored or utilized for products. Some of these concepts use photosynthesis to achieve CO₂ removal, either by storage in biomass or by conversion to a different form for long-term storage. Others use sorbents to directly capture CO₂. There is a broad range of possible NETs that include afforestation (Caldecott, Lomax, and Workman 2015), biochar (Caldecott, Lomax, and Workman 2015; Napp et al. 2017), energy from biomass with carbon capture and storage (Caldecott, Lomax, and Workman 2015; Napp et al. 2017; McGlashan et al. 2012), direct air capture (DAC) (Caldecott, Lomax, and Workman 2015; Napp et al. 2017; McGlashan et al. 2012), ocean liming (Caldecott, Lomax, and Workman 2015; Napp et al. 2017; McGlashan et al. 2012), accelerated chemical weathering of rocks (Caldecott, Lomax, and Workman 2015), ‘artificial trees’ (McGlashan et al. 2012), and soda-lime processes (Napp et al. 2017; McGlashan et al. 2012).

Carbon Capture and Storage (CCS) is a group of CDR technologies that is perhaps most actively referred to in the current climate policy discussion as it is seen as a key strategy for achieving CO₂ emission reduction targets (Leung, Caramanna, and Maroto-Valer 2014). CCS includes various processes and technologies to remove CO₂ from emission sources in industrial and energy sectors, such as the production of cement and steel (Napp et al. 2017). After capturing and separating, CO₂ is compressed for transport and long-term storage or utilization (IPCC 2005). Captured carbon can be stored in former oil fields, geological formations (mineral carbonation through the reaction of carbon with magnesium and calcium [Caldecott, Lomax, and Workman 2015]), or even at sea floor. However, environmental concerns may limit its use.

Both CDRs and NETs may have cost implications. Their application into construction industry, for example, has been suggested to cause price increases of 20–30% for steel and 20–80% for cement (Material Economics 2019).

1.3. Objectives and structure of the study

Despite the alarming environmental and economic predictions as well as international calls for action (World Green Building Council 2019), it appears that the scale and costs of the required efforts to decarbonize the construction sector are poorly understood (Giesekam, Tingley, and Cotton 2018). In addition to drastic emission reductions, the material and energy intensive constructions sector should not overlook the potential of creating carbon storages and pools within its value chains.

This paper compiles an overview of the existing scientific and professional understanding of approaches to storing carbon into the built environment. We present various alternatives, estimating their maturity and their potential for storing or sequestering carbon. Through this evaluation, we intend to raise awareness of the construction sectoŕs own potential to alleviate its harmful impacts on the climate and possibly benefit from the evolving emission trading mechanisms or compensation business. In particular, this review aims at providing building designers with an overview of the available options and their applicability to current design work.

The study is organized as follows: In the introduction, the needs and logics are presented for storing carbon into the built environment. The Methods section explains our research approach as well as presents the concepts utilized: technology readiness levels and typologies for storing carbon in the built environment. All identified approaches are then described in the Results section with their maturity and impact being explained. In the Discussion chapter, we summarize the results as well as propose a ranking of different technologies for storing carbon.

2. Methods

2.1. Identification and description of approaches

To identify various approaches, our searches were conducted within the literature using search strings composed of the words carbon, storage, sequestration, construction, built, and building. In addition, specific search words and strings were employed to deepen the coverage of the search within the literature of the identified approach. Relevant expert reports and professional literature were also reviewed during this procedure.

To analyse and discuss the options for storing, removing, and sequestering carbon in the construction sector, these options are divided into three main typologies (Table 1): Carbon captured off and stored on the site, carbon captured and stored on the site, and carbon captured on and stored off the site. We have used the building site as a system boundary for carbon storage and sequestration, because this study is targeted at building designers. City planning differs from building design and would require a different approach to the topic, for instance, the consideration of transport and services.

Table 1. Typologies for carbon storage in the built environment.

Typology		Examples
	1. Carbon captured off and stored on the site	Bio-based and CO2-based building materials: Wood, bamboo, bioplastics, CO2-cured concrete
	2. Carbon captured and stored on the site	Plants and soils on the site, green roof and facades, carbonization of concrete
	3. Carbon captured on and stored off the site	Bioenergy with carbon capture and storage, direct air capture, artificial photosynthesis

In addition to these three typologies, the identified approaches were categorized in respect to their temporal impacts. Some of them uptake carbon before the building or structure is built, some during its use phase, and some after its use. Here our point of reference was the commonly used life-cycle modules of buildings and infrastructure works, as defined in the European standard EN 15643–2 (European Committee for Standardization 2011). This paper uses the terms ‘before use phase’ (life-cycle Module A of EN 15643-2), ‘use phase’ (Module B), and ‘end-of-life phase’ (Module C).

This paper only focuses on the captured and stored carbon in relation to the site. We do not compare the GHG emissions of the presented approaches to the quantity of captured or stored carbon. Thus, the life-cycle GHG balances of the various approaches are beyond the scope of this paper and would require detailed studies. Similarly, this study does not include estimations on the temporal aspects of the stored carbon. Most building products are long-lasting and possess the potential to hold carbon in them for decades or longer as material inherent property. However, if the product is burnt or decomposes at its end-of-life, the carbon is released back into the atmosphere (unless technologically captured). The longer the carbon remains stored in a product, the greater become its climate benefits. This is especially relevant for bio-based products, which should preferably hold the biogenic carbon longer than the time required for their source environment to restore its carbon balance (Brandão et al. 2013; Levasseur et al. 2012; Seppälä et al. 2019). Additionally, there is a need to differentiate between the carbon storage related to a site (as relevant for this paper) and the calculation of carbon pools on a national level. Here, the carbon stock level cannot be solely estimated from the quantity of carbon uptake. To assess the GHG impact of deviating development pathways, the accounting has to be contrasted against a reference case as shown in earlier studies (Rüter 2017; Hafner and Rüter 2018). In this paper, we compare various possibilities to capture and store carbon to increase the carbon stock. The potential climate reduction and mitigation potential which is to be depicted on a national level and calculated as carbon sink is not included.

According to the glossary of GHG inventories (IPCC 2006), captured and stored carbon is referred to as a carbon pool. Examples of carbon pools are living biomass (above and below-ground), dead organic matter (including wood and litter), and soils. ‘The quantity of carbon contained in a “pool”, meaning a reservoir or system which has the capacity to accumulate or release carbon’ (FAO 2004) is called carbon stock. The impact of changes in the carbon stock on GHG mitigation for climate protection is often referred to as carbon sink, although it could also act as a net source of emissions. To estimate the carbon stock magnitude of selected pools, inventory can be applied as well as flux data methods. The latter are based on information on the magnitude of carbon inflow to a pool as well as its carbon outflows (Rüter 2017).

2.2. Estimation and ranking of the maturity and carbon storage potential

To create a certain comparability of the implementation process of different technologies, a uniform framework is necessary. For this purpose, the Technological Readiness Levels (TRLs) (ISO 16290, European Committee for Standardization 2013) are used, which are explained in Table 2. Decisive for the use of TRLs, also for comparability with the reviewed scientific literature, is the subdivision into three main groups: Basic research on a laboratory scale (TRL 1–3), development and testing on a small scale (TRL 4–6), and implementation and testing on a large scale (TRL 7–9).

Table 2. Technological readiness levels (TRLs) for technologies as well as effects of carbon storage and utilization.

TRL	Definition	Description
1	Basic principles observed and reported	First scientific research observes and reports basic principles of the technology/ effect and begins to convert to applied science.
2	Formulation of the application	The formulation of the technology concept leads to the invention of a practical application. The verification of the established theses requires further scientific assessment, which may not yet be available at this level.
3	Proof-of-concept	Proof of the functionality of the technology and its components. TRL 3 marks the starting point of applied research and the start of early-stage development.
4	Technology validated in laboratory environment	The technology and the interaction of its basic components is tested for functionality in a laboratory environment.
5	Technology validated in relevant environment	The basic system components are assembled and completed by specific supporting elements in order to be tested in a relevant environment.
6	Technology demonstrated in relevant environment	Technology model is demonstrated in the relevant environment. All critical functions of the technology need to be verified in the relevant environment to reach TRL 6.
7	Prototype demonstration in operational environment	TRL 7 requires an actual technology proto-type demonstration in the operational environment.
8	Technology complete and qualified	At TRL 8, the technology has proven to work in its final form and under expected conditions. At this point, the development of the system is completed in most cases.
9	Technology/ Effect is used on a large scale	The technology is established in its final form and is operated under full-scale conditions. Additionally, a (‘passive’) effect of technologies, which has CO2 storage potential and is used on a large scale, is classified at TRL 9.

While estimating the maturity of the identified approaches, it was found that their TRLs are estimated differently in different sources (Napp et al. 2017; McGlashan et al. 2012; Bui et al. 2018; Element Energy Ltd et al. 2014). On the other hand, several applications have been utilized for thousands of years (e.g. bio-based materials). As the scope of this study was not to perform an estimation of the TRLs of the identified approaches, we have carefully compared the TRLs and climate impact potentials presented in different sources favouring the most recent and peer-reviewed sources. However, this leaves our study with a range of uncertainty that was beyond our control.

To be able to identify the carbon storage potential of the approaches, their carbon storage or capture capacity were listed based on scientific literature. This provided a range of numeric values for the impact, either in units of carbon or carbon dioxide. In this article, all units have been converted into both C and CO₂ based on the atomic weight ratio of CO₂ molecule to C atom (44/12). Furthermore, we have simplified the comparison of building-level solutions by reporting the values either per kg of product or per m² of floor area (depending on which units were used in sources); however, for landscape-level solutions, all values have been converted into emissions per hectare (ha). Unfortunately, not all background data allowed us to convert these values. Therefore, in the final conclusions, only the potential range could be estimated by using a simplified range (low – medium – high). Moreover, it should be noted that we have not only considered and evaluated technologies, but also included accompanying effects (e.g. carbon storage in wooden buildings or carbonation of concrete).

Finally, all the results are summarized into a ranking matrix consisting of two axes: carbon storage potential and applicability for the built environment. We emphasize that the summary matrix is a general overview and does not describe the conditions of a site or project. Case-specific variables may considerably alter the applicability, maturity, and storage potential.

3. Results

3.1. Identified approaches for storing carbon into the built environment

We identified over twenty approaches for storing carbon in the built environment and grouped them into 13 groups (Table 3). Four of these were approaches in which carbon was captured off but stored on the site, which is our system boundary. Seven approaches were identified in which carbon was both captured and stored on the site. In addition, we found two approaches in which carbon was captured on the site but stored off it. Table 3 presents all identified approaches and Figure 1 illustrates them. Sections 3.2–3.4 describe each of these approaches and briefly summarize the main sources of further scientific or professional information.

Figure 1. Approaches for storing carbon in the built environment.

Table 3. List of identified approaches for storing carbon in the built environment.

Typology	Approach
1. Carbon captured off and stored on the site	1.1 Bio-based construction materials
	1.2 Biochar
	1.3 Mineralization of CO2 into concrete
	1.4 CO2-based plastics
2. Carbon captured and stored on the site	2.1 Carbonation of cement-based materials
	2.2 Enhanced weathering
	2.3 Natural photosynthesis
	2.4 Enhanced photosynthesis
	2.5 Artificial photosynthesis
	2.6 Uptake of carbon into soil
	2.7 Living building materials
3. Carbon captured on and stored off the site	3.1 Bioenergy with carbon capture and storage (BECCS)
	3.2 Direct air capture (DAC)

3.2. Carbon captured off and stored on the site

3.2.1. Bio-based construction materials: wood

The ClimWood2030 study (Rüter et al. 2016) quantifies the five ways in which the EU forest sector contributes to climate change mitigation: carbon sequestration and storage in EU forests, carbon storage in harvested wood products in the EU, substitution of wood products for functionally equivalent materials and substitution of wood for other sources of energy, as well as displacement of emissions from forests outside the EU. Through a scenario analysis, it presents the consequences for GHG balances in the EU of possible policy choices at present. A precondition for all this timber use is sustainable forestry and, accompanying that, parallel active reforestation.

The carbon content in wood can be calculated according to EN 16449 as 50% of the mass from wood at zero percent moisture content (European Committee for Standardization 2014). However, a range of carbon content in different parts of different trees can vary between 0.42–0.61 kgC/kg (1.54–2.24 kgCO₂/kg) (Thomas and Martin 2012).

For wooden material, there is a variety of studies on carbon storage and wooden buildings (Takano, Highes, and Winter 2014; Heeren et al. 2015; König 2016). They demonstrate that on the level of construction material in product stage, wooden material displays advantages in terms of carbon storage capacity (named biogenic GWP in new EN 15804:2020); therefore resulting in lower GHG emissions in the product stage. Carbon storage is balanced out over the life-cycle, as carbon storage is calculated as emission at the end-of-life stage. However, it exists for as long as the buildings exist, and can be extended into the next life-cycle if the products are reused or their materials recycled without losing their carbon content (Hafner and Schäfer 2018).

The global climate benefits of wood construction have been calculated by various studies, such as Churkina et al. (2020) and Amiri et al. (2020). Both studies show the potential of storing carbon in buildings on global level and the advantages thereby. The national implications of carbon storage are modelled in several studies. Hafner and Rüter (2018) report the potential GHG impact of wood consumption in the building sector in Germany based on an insinuated future increase of the market share of timber buildings. They also quantified the reduction potential through substitution of energy intensive materials with material choice with lower emissions, showing that there is a substantial potential to lower GHG budget by timber buildings (Hafner and Schäfer 2017). Kalt (2018) presented a similar approach for Austria, showing the influence of increased carbon storage in timber buildings, and Vares, Häkkinen, and Vainio (2017) discuss the potential for Finland.

The amount of carbon storage in buildings depends on the size of the building and the bio-based construction materials used and it can vary from 109–300 kgCO₂/m² gross floor area, depending on the chosen structural solution (Vares, Häkkinen, and Vainio 2017). In general, the highest amount can be found in buildings made with massive cross-laminated timber (CLT) (Hafner and Schäfer 2018). As the use of timber is current mainstream industrial construction, we rank its TRL as 9.

3.2.2. Other bio-based construction materials

In addition to wood, many organic fibres are used in construction. The variety is wide, but bamboo, straw, and hemp are typical examples.

The carbon sequestration of different bamboo species in their growing habitats has been examined in several articles over the past years (Leksungnoen 2017; Han et al. 2013; Teng et al. 2016; Yuen, Fung, and Ziegler 2017; van der Lugt, Vogtländet, and Brezet 2009), and the factors affecting carbon sequestration are scientifically well described. In regard to the carbon storage of bamboo-based construction, the number of studies is clearly smaller than the number of similar studies for wood-based construction products. The climate impact studies of bamboo construction include reports on the carbon footprint of bamboo wall panels (Ramirez et al. 2014), bamboo flooring (Gu et al. 2019), bamboo scaffoldings (Laleicke et al. 2015), or humanitarian emergency shelters made from bamboo (Kuittinen and Winter 2015). We rank the TRL of bamboo as 9, due to its wide commercial and vernacular use in construction.

Straw is a typical residue from agriculture that has been used for construction purposes since time immemorial. The most typical modern construction uses include straw bales that are attached between loadbearing timber studs or straw-clay mix that can be cast into different structures. The carbon storage potential of straw bale construction has been studied and compared to that of biochar (Mattila et al. 2012) to typically used bricks or blocks (Gonzalez 2014), and to other typical building materials (Sodagar et al. 2010). Although straw in construction is to a certain extent vernacular, it is widely used, thus allowing it be ranked at TRL 9.

Hemp is used regionally in vernacular or ecological construction. Its fibres can also be used to make concrete or composites (Schwarzova, Stevulova, and Melichar 2017). The annual carbon sequestration potential of industrial hemp plantation is approximately 0.67 t/ha (Pervaiz and Sain 2003) and the carbon content of hemp around 2 kgCO₂/kg (Butkutė et al. 2015). While Jami, Rawtani, and Agraval (2016) suggest hemp concrete to be ‘carbon negative technology’, Pretot, Collet, and Garnier (2014) showed that in hemp concrete wall structures, the lime-based binder is accountable for the largest environmental impacts. Therefore, cement substitutes for hemp concrete have been considered in a bid to improve its environmental performance (Kidalova, Terpakova, and Stevulova 2011). We rank the TRL of hemp in various forms of construction at 7–9, as some of the composite applications continue to evolve.

In addition, there is a wide range of bio-based materials that can be applied in construction. Several agricultural residues and waste, such as tomato stalks or potato peels; or vernacularly applied raw materials, such as seaweed, willow, algae or grasses, have been used in traditional construction and piloted for new buildings (3XN_GXN 2016; Vandkunsten Architects 2013). In experiments, composites have been produced by binding together agricultural residues and other waste materials with the filaments of growing mycelium fungi (Jones et al. 2018). The TRLs of these experimental or vernacular curiosities of construction can be placed in the range of 2–8, depending on the case.

3.2.3. Biochar

Biochar is made from biomass (such as straw, wood, bamboo, cotton, sediments, sludge, and manure) through pyrolysis and has typically been used for soil fertilization and the removal of pollutants. Pyrolysis can simultaneously create energy and biochar.

There are a number of research projects in which the potential of biochar for sequestering carbon into soils has been studied, especially regarding its use in agricultural land (Purakayastha et al. 2015; Galinato, Yoder, and Granatstein 2011; Roobroeck et al. 2019). Its potential to store carbon in soils is reported to range between 0.9 and 3.7 tCO₂/ha/a (Smith 2016), although the beneficial consequences of enhancing plant growth and carbon sequestration are difficult to predict. However, the global carbon storage potential is reported to range between 0.9 and 3 Gt (McLaren 2012), and up to a ‘high’ 4 Gt CO₂/a (Napp et al. 2017).

Biochar has potential as an additive in certain construction materials. Several reports are available for the use of biochar for asphalt, clay plaster, cement panels, and mortar (Zhao et al. 2014; Schmidt 2013; Gupta and Kua 2017; Wang et al. 2019). The reported contents of biochar vary strongly from 0.025% to 50%, depending on the product experiment. Within this amount of biochar, the carbon content varies from 46 up to 98% of the original amount of carbon of the used biomass (Gupta and Kua 2017). Thus, although clear numbers for estimating the carbon content in the end-products are not reported, we estimate that the overall carbon content and storage potential as an additive to construction products remains low.

Differences were found regarding the TRL for biochar as a NET. Napp et al. reported a range 1–4 (Napp et al. 2017), whereas McLaren suggests a higher TRL range 4–6 (McLaren 2012). However, in comparison to a range of other NETs, biochar may bring emission reductions with less disadvantages, as concluded by Smith (2016).

3.2.4. Carbon captured and utilized for CO₂-cured concrete

Mineral carbonation of concrete is a process similar to geological weathering of rocks. In the process, CO₂ is captured using CCS technologies; thereafter, it is used to cure concrete during which it reacts with alkaline metal minerals and forms carbonates. As a result, CO₂ is permanently stored in the concrete product (Sanna et al. 2014). The quantity of CO₂ in different concrete mixes has been reported to vary from 41 kg/m³ (cement with limestone powder) (Tu et al. 2016) to 69 kg/m³ (Wollastonite-Portland cement) (Huang et al. 2019a). In addition to storing carbon, the CO₂-cured building elements may offer reductions of global warming potential (GWP). Huang et al. (2019b) conducted a life-cycle assessment of several different concrete blocks that had been cured with CO₂. Their findings suggest that the GWP of a CO₂-cured Wollastonite-Portland cement (WPC) block can be approximately 30% lower than in typical steam-cured concrete block made with ordinary Portland cement (OPC). However, the cost of CO₂-curing may become an issue when compared to storing CO₂ in geological deposits (sedimentary rocks), which is cheaper as transportation costs of CO₂ can be reduced (Sanna et al. 2014).

Based on the process descriptions provided by manufacturers and recent reports, we estimate that the maturity of CO₂-cured concrete varies between TRL 4–6, and that its mitigation potential is medium (Napp et al. 2017; Element Energy Ltd et al. 2014; CarbonCure 2019; Rosen 2020). However, the utilization of CO₂ for concrete also has direct linkage to CCS technology and its maturity. If the CO₂ used in the process does not originate from the atmosphere or CCS process, the environmental gains are questionable.

3.2.5. Carbon captured and utilized for plastics

Although the total mass of plastics has been documented to remain below 1% of the weight of a building (Kuittinen, Häkkinen, and Vares 2019), the construction sector is the second largest consumer of plastics, using around 20% of the global production (Geyer, Jambeck, and Law 2017). A variety of plastics can be produced from biomass or captured CO₂.

The use of CO₂ as a feedstock for plastics has been progressing. For example, CO₂ together with biomasses (agricultural waste, lignocelluloses, food waste, and industrial by-products) can be utilized to produce polycarbonates (Cui et al. 2019). Similarly, CO₂ can be processed into polyesters (Murcia Valderrama, van Putten, and Gruter 2019) or polyurethane (von der Assen and Bardow 2014). Polypropylene can be produced from both biomass and CO₂ (Bazzanella and Ausfelder 2017).

Bioplastics are already commercially available for several purposes. In theory, they could evolve into a NET, if the production process would utilize low carbon energy and be equipped with CCS. Currently, the production of CO₂ or biomass-based plastics requires energy and causes GHG emissions. von der Assen and Bardow (2014) conducted LCA on CO₂-based polyurethane (a typical plastic for construction insulation products) concluding that although CO₂-based process had lower emissions than a conventional option, the end results are not producing negative emissions. Thonemann (2020) showed that the CO₂ mitigation benefits of CO₂-based chemical production greatly depend on the chosen process and product.

Based on the literature cited above, the maturity of bioplastics is reported to range between TRL 1–7, depending on the feedstock and chemical processes involved. However, the scientific reports clearly contradict the self-supplied information of some companies on the industrial-scale production of materials, such as bio-based plastics. Some estimations on the potential of storing captured or bio-based carbon into plastics are provided (up to 210 MtCO₂/a in 2050, Bazzanella and Ausfelder 2017). Due to lack of evidence, we chose not to speculate if this mitigation potential could also be directly allocated into construction plastics following their 20% share of all plastics produced. Nevertheless, because of the marginal weight of plastics in a building, we estimate their carbon storage potential as low.

3.3. Carbon captured and stored on site

3.3.1. Carbonation of concrete and cement-based materials

During the production of cement and concrete, CO₂ is emitted. A certain amount of it can be re-absorbed by carbonation over the life-cycle of the concrete elements. Carbonation is a natural chemical reaction occurring in concrete in which CO₂ from the atmosphere enters the pores of concrete and reacts with calcium hydroxide Ca(OH)₂ transforming into calcium carbonate CaCO₃. Thus, it is a counter-reaction to calcination, in which limestone (CaCO₃) breaks into CaO and CO₂ during the production of cement clinker. The carbonation of concrete depends in equal parts on material attributes and the surrounding parameters (Monteiro et al. 2012). The temporal absorption of carbon by carbonation in concrete is determined by a root function, while considering the carbonation depth. Consequently, the longer the observation period, the higher the absolute carbon uptake, whereas the annual carbon uptake gradually decreases. The production of calcium carbonate slightly increases the strength of the concrete compared to concrete in which no carbonation occurs (Chi, Huang, and Yang 2002).

In the literature, percentage values are mostly provided, which refer to the carbon emitted during production and indicate a reabsorption rate. On the global level, Cao et al. (2020) estimated that up to 30% of the total emissions (resulting from calcination and energy use) of cement production may be reabsorbed through carbonation. However, according to Lagerblad (2005) and EN 16757 (German Institute for Standardization 2017), no more than 75% of the maximum CO₂ absorption potential over a sufficient period of time can be assumed. This is because not all concrete is in contact with the air. For example, concrete rubble is often recycled for road construction (buried in underground structural layers), whereas carbonization requires direct contact with the CO₂ in the air. Moreover, the maximum theoretical CO₂ absorption depends on the amount of reactive CaO in the binder of the concrete. Hence, for a common assessment of the maximum absorption capacity, the value of 75% is widely used in the literature as a general practical maximum. Cho et al. (2015) have determined a CO₂ absorption rate of fly-ash-blended concrete structures for a lifetime of 20 years at 3.79%, and for 100 years at 8.47%, respectively, of the CO₂ emitted during manufacture. Nilsson (2011) examined the CO₂ absorption of different concrete structures over a period of 100 years. For indoor structures, he determined that 0.9–6.1 kgCO₂ was absorbed per m², depending on the coating. In comparison, for outdoor structures, he calculated values of 0.9 for weather-exposed, and 2.8 kgCO₂/m² for sheltered structures. In addition, a detailed guideline for calculating the CO₂ absorption of concrete by carbonation is supplied as part of the Environmental Product Declarations (EPD) by EN 16757 (German Institute for Standardization 2017) and must be considered in the CO₂ balance of concrete structures, as concluded by Possan, Felix, and Thomaz (2016).

In conclusion, the maturity of carbonation of concrete and cement-based materials is TRL 9, since concrete structures are built on a large scale resulting in carbonation having a corresponding influence. In the built environment, carbonation occurs wherever concrete is used for construction, hence indicating its high applicability. However, the degree of climate potential is influenced by coatings; there is often an effort to avoid carbonation (as it leads to the corrosion of reinforcement steel bars). Furthermore, the carbon-intensive production of cement-based materials contrasts with the climate benefits of carbonation. Therefore, we estimate the overall climate potential as medium, although some applications for utilizing concrete rubble in gabions for infrastructure works can possess a higher potential.

3.3.2. Enhanced weathering

In addition to natural weathering, during which rocks are dissolved by various surface chemical reactions (Carroll 1970) and the resulting bicarbonates carried into ocean deposits (Berner, Lasaga, and Garrels 1983), the process can be enhanced by grinding suitable rocks to maximize the surface area, then dispersing them over farmland and forest areas. Enhanced weathering can be used to sequester CO₂ on a large scale, especially in warm and humid climates as reported by various studies (Schuiling and Tickell 2010; Hartmann et al. 2013; Taylor et al. 2016). Depending on the type of rock used, approximately 0.3–1.1 tonnes of CO₂ per tonne of rock could be removed (Moosdorf, Renforth, and Hartmann 2014; Renforth 2012). Lefebvre et al. (2019) recently raised the importance of considering the life-cycle assessment of all process steps of improved weathering to obtain realistic estimates of the amount of CO₂ sequestered.

Enhanced weathering does not appear to be an effective approach for carbon storage at the level of the built environment. The required time scale for significant storage potentials is lengthy when considering the timely relevance of climate action. In addition, the large land use of free areas, such as forests or cropland, is limited within a densely planned built environment. Moreover, enhanced weathering may require considerable amounts of either external process energy, for example, as in the case of grinding olivine rock into soils (Renforth, Pogge von Strandmann, and Henderson 2015), or ‘supercritical’ conditions in which both temperature and pressure are extremely high (Garcia et al. 2010). Therefore, we rate the applicability as low.

Enhanced weathering is classified at a maturity level of TRL 2–3 (Haszeldine et al. 2019); therefore, further basic research is required for particularly environmentally relevant influences. Considering the possible carbon absorption rates of enhanced weathering, we rate the climate potential within the built environment as medium.

3.3.3. Natural, enhanced, and artificial photosynthesis on the site

Natural photosynthesis for sequestering carbon on site is an ecosystem service that is essential for climate change mitigation. Naturally, its TRL is on the highest level 9. The potential of natural photosynthesis to uptake and store carbon varies significantly depending on the plant (trees, bushes or herbaceous), growth conditions (including maintenance and fertilization), climate (both micro and macro climatic conditions), and maintenance methods. The applicability of natural photosynthesis is also high in most built environment.

Urban trees can hold good potential for sequestering carbon. Their annual sequestration potentials are reported as 5.9 tCO₂/ha/a in Mexico (Velasco et al. 2014), 8.1 tCO₂/ha/a in China (Chen 2015), and 10.3 tCO₂/ha/a in the U.S.A. (Nowak et al. 2013). Furthermore, the alternative forestry method of Japanese botanist Akira Miyawaki (1998) offers a concept for planting micro-sized, very dense, highly biodiverse and fast-growing forests that have been widely tested in urban and rural areas across the world (Schirone, Salis, and Vesella 2011; Ottburg et al. 2018). In one Belgian report (Manuel 2020), the carbon storage potential of the Miyawaki micro forests and their soils is estimated to exceed 598 tCO₂/ha with the sequestration potential reaching 5.1 tCO₂/ha/a; however, there do not seem to be adequately peer-reviewed studies on the climate aspects of the Miyawaki method.

In green roofs, the potential for carbon sequestration is reported to vary from 0.3 kgCO₂/m²/a (Heusinger and Weber 2017) to 7.1 kgCO₂/m²/a (Luo et al. 2015), depending on conditions and variables. Nevertheless, the majority of this sequestered carbon is stored for only a short time, as herbaceous plants decompose naturally over growing seasons. Therefore, the carbon dynamics for lawns and herbaceous plants are included in the estimations of carbon uptake of soils.

In enhanced photosynthesis, the natural process is cultivated for improving the yield of solar fuels. For example, microalgae and cyanobacteria, which normally grow as epiphytes on various surfaces and utilize photosynthesis in their metabolisms, can be cultivated for fuels, such as butanol and methanol in photobioreactors. Such reactors can be shaped into building components and integrated, for example, into façades or roofs. Building-integrated photobioreactors can produce heat and biomass while preventing the building from overheating and sequestering CO₂ from the flue gases of the building (Wurm and Pauli 2016; Wolff et al. 2015). However, only seven reports were found on experiments of building-integrated photobioreactors. The technology appears to suffer from several vulnerabilities related to both the growth factors of microalgae or the construction of the reactor panels (Talaei and Mahdavinejad 2019) resulting in us interpreting that the current applicability in the built environment is low. The TRL for enhanced photosynthesis is reported to range from 4–6 (McLaren 2012).

In fully artificial photosynthesis, solar energy (photons) and CO₂ are converted into chemical energy, thus mimicking natural photosynthesis. It can be used to develop carbon-negative solar fuels as well as processes for turning CO₂ into ethanol (Gurudayal et al. 2017), or the photocatalytic splitting of water into hydrogen (Kim et al. 2016). Although buildings can be equipped with artificial photosynthesis systems, actual building-integrated applications were not found. The TRL for artificial photosynthesis in the production of solar fuels is reported in the range of 1–3 (Napp et al. 2017). If solar fuels can be used for the energy demands of the same building, the requirements for additional infrastructure or transports are avoided, although the CO₂ emissions from the burning of fuels would remain. Therefore, we estimate that both carbon storage potential and applicability in the built environment are currently low.

3.3.4. Soil organic carbon in the site

Carbon accumulates in soils because of organic processes; indeed, soils around the planet are the largest terrestrial carbon stock. The potential of soil to store carbon varies considerably based on the climate, soil type, vegetation, erosion, microbial activity, pollution, and other factors.

The global amounts of carbon accumulated in croplands are reported to vary from 0.4–8.6 GtCO₂e per year (IPCC 2019). In urban areas, the reported amount of carbon stored into soils can vary from 213 tCO₂/ha (Raciti, Hutyra, and Finci 2012) to 741 tCO₂/ha (Edmondson et al. 2012) and the annual accumulation of soil organic carbon from 3 tCO₂/ha/a (Raciti et al. 2011) to 11 tCO₂/ha/a (Vasenev and Kuzyakov 2018) (depending on e.g. sampling depth). This potential can be enhanced e.g. with the use of biochar in the soil (see 4.1.2). Interestingly, research findings show that the annual accumulation of carbon into urban soils can be much higher than in natural areas (Edmondson et al. 2012; Raciti et al. 2011; Vasenev and Kuzyakov 2018), although the initial conversion of the natural landscape into the built area may have caused the carbon content of the soil to decrease (Trammell et al. 2017). Furthermore, this content seems to be high, especially in residential and public urban zones, according to a wide meta-analysis by Vasenev and Kuzyakov (Vasenev and Kuzyakov 2018). As uptake of carbon into soils is a natural phenomenon, its TRL is 9 and applicability high.

3.3.5. Living building materials

In vernacular architecture, living plants have been used for load-bearing structures. For instance, in rural India, ‘living bridges’ have been grown from the aerial roots of ficus trees (Ludwig et al. 2019). In addition, large-scale living ‘arboscupltures’ have been created for recreational parks (Gale 2011). So-called biohybrid constructions have been classified into four groups by Heinrich et al. (2019): (1) biological organisms supported by scaffolds, (2) biological energy sources in buildings, (3) plants grown into loadbearing building components, and (4) forming building components from amorphous living material.

The carbon storage potential of living tree structures is similar to that of timber (see Table 4). However, from the viewpoint of carbon storage, the uptake of carbon happens during the use of the structure. In addition to trees, growing and gradually expanding fungi have also been experimented with for building skins and components (Pownall 2019). Although living structures have probably been used since the evolution of humankind (and by other species as well) and even patented in the 1920s in Germany (Wiechula 1926). The applications to modern construction are experimental with them being limited to structural test and outdoor canopies or pergolas. Therefore, based on the available literature, the current maturity of the approach can be estimated in the range of TRL 3–5 (Ludwig, Schwetfeger, and Storz 2012; Gale 2011; Urbanist 2017). Table 5 provides a summary of approaches for capturing carbon off site and storing it on site.

Table 4. Summary: carbon captured off and stored on site.

Approach	Application to construction	Carbon content^a (kgC/kg, unless otherwise specified)	Carbon dioxide content (kgCO2/kg, unless otherwise specified)	Maturity (TRL)	References
1.1 Bio-based construction materials^b
Wood	Load-bearing structures, surfaces, claddings, insulation, furniture, engineered products	0.5^c (range 0.42–0.61)	1.84 (range 1.54–2.24)	9	Thomas and Martin (2012), Rüter et al. (2019)
Cellulose fibre	Insulation	0.39^c–0.41	1.42–1.50	9	Rüter et al. (2019), Ekovilla (2018)
Cardboard and construction paper	Vapour barriers, insulation, engineered products	0.41^c	1.50	9	Rüter et al. (2019), Ekovilla (2018)
Bamboo	Load-bearing structures, surfaces, claddings, furniture, engineered products	0.55–0.67	2.02–2.46	9	Vogtländer, van der Velden, and van der Lugt (2014)
Hemp	Insulation, composites	0.55–0.57	2.02–2.09	7–9	Butkutė et al. (2015)
Cork	Flooring, insulation, composites, engineered products	0.56	2.06	9	Zribi et al. (2016)
Straw	Insulation, composites (vernacular)	0.48	1.76	9	Gao et al. (2016)
Sheep wool	Insulation (vernacular)	0.5	1.84	8	Dénes, Florea, and Manea (2019)
Seaweed	Insulation, composites, facades (vernacular, regional)	4.1 t/ha/a	15.0 t/ha/a	7	Duarte et al. (2017)
Herbaceous plants (global average)	Insulations (vernacular, regional), composites	0.45–0.48	1.65–1.76	2–7	Ma et al. (2018)
Composites from agricultural residues (e.g. tomato, potato)	Insulation, composites, facades (experimental)	Depends on the biomass and its share in the composite		4	3XN_GXN (2016)
Composites from mycelium (fungi)	Insulation, composites, facades (experimental)	(No references found; also depends on the components of the composite)		2–4	Jones et al. (2018)
1.2 Biochar
Biochar for landscaping	An additive to growing medium or soils	0.25–1.01 t/ha/a (1.1 Gt/a globally)	0.9–3.7 t/ha/a (4 Gt/a globally)	1–6	Napp et al. (2017), Smith (2016)
Biochar for asphalt	Filler	No data found		1–4	Gupta and Kua (2017)
Biochar for cement	Filler	No data found		1–4	Gupta and Kua (2017)
1.3 Mineralization of CO2 into construction products
CO2-cured concrete	Loadbearing structures, surfaces, infrastructure works	1.0 Mt/a in EU	3.8 Mt/a in EU	4–6	Napp et al. (2017)
1.4 CO2-based construction plastics
CO2-based plastics	Insulation, pipes, ducts, profiles, films	No data found		3–5	Element Energy Ltd et al. (2014)
Bio-feedstock-based plastics	Insulation, pipes, ducts, profiles, films	No data found		1–3	Napp et al. (2017)
Biopropylene	Films, sheets, textiles	No data found		6–7	Bazzanella and Ausfelder (2017)

Note: The figures in this table are examples from scientific literature. For LCA, representative databases or EPDs should be referred to.

^a Carbon at 0% moisture content.

^b The numbers shown serve to show the range of carbon calculated in the literature. For any LCA calculations in the built environment, appropriate environmental product declarations or LCA databases should be referred to.

^c For calculations of carbon storage capacity of the built environment on the national level, the IPCC (Rüter et al. 2019) provides default conversion factors.

Table 5. Summary: carbon captured and stored on the site.

Application	Application in construction	Potential for C storage	Potential for CO2 storage	TRL	References
Carbonation of cement-based materials
Carbonation of concrete	Surfaces and components that are not vulnerable to corrosion of rebars	0.25–1.66 kg/m^2	0.9–6.1 kg/m^2 ^a	9	Cho et al. (2015), Jacobsen and Jahren (2001), Haselbach and Thomas (2014), Pade and Guimaraes (2007), Nilsson (2011), Jacobsen and Jahren (2001), Haselbach and Thomas (2014), Pade and Guimaraes (2007), Nilsson (2011)
		4–8% of emissions during manufacturing^b
Carbonation of recycled concrete	Landscaping, fills, gabions	Up to 41% of emissions during manufacturing^b		9	Collins (2010)
Enhanced weathering
Enhanced weathering of ultramafic rocks (e.g. olivine, dunite)	Surfaces or ground layers in landscaping	0.22–0.30 kg/kg	0.8–1.1 kg/kg	2–3	Moosdorf, Renforth, and Hartmann (2014)
Enhanced weathering of mafic rocks (e.g. basalt, dolerite)		0.08 kg/kg	0.3 kg/kg	2–3	Renforth (2012)
Photosynthesis in urban context
Natural photosynthesis	Trees (urban context)	1.6–2.8 t/ha/a	5.7–10.3 t/ha/a	9	Chen (2015), Velasco et al. (2014), Nowak et al. (2013)
	‘Miyawaki’ micro forests (including soils)	Uptake: 1.4 t/ha/a Storage: 160 t/ha	Uptake: 5.1 t/ha/a Storage: 598 t/ha	8	Manuel (2020)
	Bamboos (including above and below ground biomass)	16–24 t/ha/a	59–88 t/ha/a	9	Shanmughavel and Francis (1996), Castañeda-Mendoza et al. (2005)
	Lawns	0.08–0.30 t/ha/a	3.01–1.09 t/ha/a	9	Raciti et al. (2011), Smith et al. (2018)
Enhanced photosynthesis	Photobioreactors integrated into buildings	No data	No data	4–6	McLaren (2012)
Artificial photosynthesis	Technical installations integrated into buildings	No data	No data	1–3	McLaren (2012)
Carbon uptake into soils
Soil carbon uptake (general)	Landscape architecture, green infrastructure	0.03–1.3 t/ha/a	0.11–4.77 t/ha/a	9	Smith (2016)
Soil carbon uptake (general) enhanced with biochar	Landscape architecture, green infrastructure	0.73–2.6 t/ha/a	2.70–9.54 t/ha/a	9	Smith (2016)
Soil carbon uptake in urban areas	Landscape architecture, green infrastructure	0.82–3 t/ha/a	3.01–11.01 t/ha/a	9	Raciti et al. (2011), Vasenev and Kuzyakov (2018)
Green roofs	Herbaceous plants on top of water-proof layers	0.08–1.93 kg/m^2	0.3–7.1 kg/m^2	9	Heusinger and Weber (2017), Luo et al. (2015)
Living building materials
Living trees	Loadbearing structures, surfaces	Comparable to timber (appr. 50% of dry mass)		3–5	Ludwig et al. (2019)
Living fungi	Surfaces, bio-bricks	No data found		3–4	Pownall (2019)

^a Indoor structures, depending on the coating, and outdoor structures, based on the exposure to rain, 100 years assessment time.

^b In the literature reviewed, assessment time frames of between 20 and 100 years were used.

3.4. Carbon captured on the site and stored off the site

3.4.1. Bioenergy with carbon capture and storage

Bioenergy with CCS (BECCS) includes the combustion of organic materials in a power plant and capturing the emitted carbon from combustion. A similar process without CCS would not be a NET, but remain a low-carbon technology (Napp et al. 2017; McGlashan et al. 2012). Nevertheless, on the level of a city, municipality, or a region, BECCS could well be a feasible solution, despite the land requirement being in direct competition with other sectors, such as food cultivation or forestry (Napp et al. 2017; McGlashan et al. 2012; Arasto et al. 2014). In addition to the required land area, there may be difficulties in supply chains related to the transport and storage of biomass (Bennett et al. 2019).

BECCS itself does not appear to be relevant on a building level, but can be linked to the built environment through the treatment of organic municipal waste (Pour, Webley, and Cook 2017). The limitations of BECCS are in the storage solution, due to the finite nature of usable geological formations. As an alternative to geological storage, the captured CO₂ from the biomass power plant can be converted into carbon-based materials and products as described in Sections 3.2.4 and 3.2.5.

Based on the literature, the maturity of BECCS can be estimated in the range of TRL 3–6 (Caldecott, Lomax, and Workman 2015; McLaren 2012). Further, it can be stated that both the applicability to the built environment and the climate potential in the built environment are low because we identified no BECCS systems on a building level that would utilize any organic material produced on the site. Nonetheless, agricultural production, in which manure is used as feedstock for energy sources, such as biogas, could theoretically fulfil this requirement; however, no examples of BECCS in such a context were identified. In BECCS applications, the ultimate fate of captured carbon remains a question that defines the feasibility of the approach.

3.4.2. Direct air capture

Direct air capture (DAC) is a technology for capturing CO₂ directly from the atmosphere (Williamson 2016). For the built environment, Dittmeyer et al. (2019) portray the concept of integrating a DAC system into an air conditioning unit, powered by renewable electricity. This solution can collect CO₂ from buildings and convert it into hydrocarbon fuels. The CO₂ captured by DAC needs to be processed and compressed for transport before it is used or stored. For building-integrated DAC to qualify as NET, the emissions from processing, transport, and use or storage must not exceed the amount of captured carbon (Lackner 2009). As the options for permanent CO₂ storage are still limited, the continued use of CO₂ represents an alternative solution. DAC can be used to achieve a closed carbon cycle by using the CO₂ to produce short-life synthetic fuels (Fasihi, Efimova, and Breyer 2019), but it could also be used for chemical construction products, as described in Section 3.2.5.

Table 6 shows the reported outcomes for three practical cases, the Frankfurt Fair Tower office building, a typical grocery store, and an apartment building. Based on the available literature, the maturity of DAC can be estimated in the range of TRL 1–6 (Caldecott, Lomax, and Workman 2015; McLaren 2012). We conclude that the applicability of building-integrated DAC in construction is higher than its climate impact because the latter is highly dependent on the storage solution. Therefore, we rate the current applicability in the built environment as medium, and the climate impact in the built environment as low despite the figures for sequestration potential appearing to be promising.

Table 6. Summary: carbon captured on the site and stored off the site.

Approach	Application in construction	Potential for carbon sequestration			TRL	References
Bioenergy with carbon capture and storage (BECCS)	Waste incineration plants	0.7 tCO2/t of wet municipal solid waste			3–6	Pour, Webley, and Cook (2017), Caldecott, Lomax, and Workman (2015), McLaren (2012)
Direct air capture (DAC)
Office building	Air-conditioning (10,00,000 m³/h)	28–57 kgC/m^2/a	104–209 kgCO2/m^2/a	250 kg/h^a	1–6	Dittmeyer et al. (2019), Caldecott, Lomax, and Workman (2015), McLaren (2012)
Grocery store	Air-conditioning (50,000 m³/h)	73–87 kgC/m^2/a	270–319 kgCO2/m^2/a	14 kg/h^a	1–6	Dittmeyer et al. (2019), Caldecott, Lomax, and Workman (2015), McLaren (2012)
Residential building	Air-conditioning (140 m³/h)	2.8–3.4 kgC/m^2/a	10–12 kgCO2/m^2/a	0.2 kg/h ^a	1–6	Dittmeyer et al. (2019), Caldecott, Lomax, and Workman (2015), McLaren (2012)

^a Production rate of liquid hydrocarbon fuels.

Table 7. Comparison of identified approaches for storing or sequestering carbon in the built environment based on the literature reviewed. Darker shading in the cells indicates better ranking.

Table 8. Ranking of the carbon storage or sequestration potential of different construction applications. Darker shading in the cells indicates better ranking.

4. Comparison of the available options

4.1. Maturity, impact, applicability, and timing

We compared the maturity, impact, and applicability of the identified approaches. In addition, their timing of carbon uptake and storage were examined. The results are summarized in Table 7.

The estimation of maturity of various approaches is based on their respective TRLs (Column a. of Table 7). From that viewpoint, bio-based construction materials, the accumulation of carbon into soils, carbonization of concrete, and natural photosynthesis exhibit the highest maturity. On the other hand, artificial photosynthesis, DAC, and enhanced weathering, as well as living structures appear to exhibit the lowest maturity.

The timing of carbon uptake and storage may be important aspects, as there is an urgency to remove large quantities of carbon from the atmosphere within the next few decades. Considering this aspect (Columns b. and c. of Table 7), most of the approaches uptake carbon and store it during the use phase of the built environment. Fewer approaches uptake carbon at the beginning of the life-cycle: bio-based materials, biochar, CO₂-cured concrete, and bio-based plastics (i.e. typologies of Category 1).

As the carbon uptake or storage potential of different approaches is documented in the literature found in a non-comparable manner, it is difficult to precisely compare impacts. Such a comparison would also require a clear definition of a use case, against which the comparisons would be measured. Moreover, speculation of the impact has a temporal aspect: the current situation differs from that of the future, and predicting the policy development or market fluctuations was not within the scope of this study.

The applicability of the approaches is largely dependent on the case. Most conventional approaches, such as bio-based construction materials, are theoretically applicable everywhere, if not as a loadbearing frame, then in supplementary structures and surfaces. If the building site allows, utilization of ecosystem services (vegetation and soils) is applicable to most cases. Emerging technological solutions (DAC, artificial or enhanced photosynthesis, BECCS) are currently less applicable to most construction projects due to their costs and maintenance requirements. The same applies to the emerging or marginal, albeit very inspiring, solutions of living structures and building skins.

5. Discussion

5.1. Summary of the findings

Although several negative emission technologies are being developed, their maturing seems to require considerable amounts of time and funds. Therefore, as McGlashan et al. (2012) conclude, it would be unrealistic to assume that they would notably impact GHG levels in less than 20 years. Thus, mitigation efforts should be prioritized. Similarly, the main approach in the built environment should also be the reduction of GHGs. Simultaneously, carbon pools should be built to support global efforts aimed at removing carbon from the atmosphere.

Based on our review, we have compiled a ranking of the practical approaches for currently available carbon removal and storage technologies in the built environment (Table 8). The ranking is based on the conclusions and comparisons arrived at in this article, and our experience as practising architects. It should be read with the understanding that regional differences should always be considered when there are plans to adopt these approaches.

Approaches that are mature and have sufficient impacts on climate mitigation include the use of organic construction materials, especially timber and bamboo, even when compared with all possible identified approaches for storing carbon in the built environment. These construction products are widely applicable, but their use holds potential for upscaling. Focusing on the building sector results shows a positive influence on overall carbon balance for wood and bamboo material use in buildings and cascade use by increasing the recovery of solid wood or bamboo products. However, it is of the utmost importance that the service life of bio-based products is extended beyond the time required for their source environment to restore their natural carbon equilibrium. Moreover, it is equally important to prevent bio-based products from releasing their carbon content back into the atmosphere at their end-of-life. In addition, the use levels of forests must be maintained at optimal levels to ensure that their carbon stock capacity is ensured and enhanced. Therefore, more urban trees and ecosystem services are needed.

Our review indicates that there are mature technologies that are not yet used to their optimal potential. These include the ‘Miyawaki’ micro forests as well as urban soils, that both seem to exhibit a strong potential for very cost-efficiently sequestering carbon. This potential could be further accelerated by using biochar.

The currently carbon-intensive production of cement-based materials may be slightly compensated for by carbonation, but the avoidance of CO₂ emissions in the production phase should be prioritized. The mineralization of CO₂ into concrete holds significant potential for the future. Although the technology route from CCS to concrete is still immature, it could offer possible ways to safely store CO₂ into deposits within the built environment. As emission trading schemes and sustainable investing evolve, this may also encourage the development of new business models.

Negative emission technologies, such as DAC and BECCS, do not yet directly link to the built environment. First and foremost, more attention needs to be paid to development in order to achieve a higher level of maturity before the concept can be transferred to the construction sector. Moreover, BECCS is linked to the global biomass potential, especially concerning large-scale biomass supply chains. The biomass needs of competing sectors may reduce the actual biomass potential for BECCS (McGlashan et al. 2012). DAC does not require biomass for the process; therefore, unlike BECCS, it does not compete with other bio-based sectors (Gutknecht et al. 2018). One of the main advantages of DAC is the use of atmospheric CO₂. Hence, this technology is not linked to any emission source and does not compete with large-scale carbon capture (Napp et al. 2017). Rather, DAC is considered as an alternative and the main potential carbon source from which to produce synthetic fuels (Gutknecht et al. 2018). The limiting factor for these technologies could ultimately be geological storage. In the end, DAC and BECCS require the effective development of CO₂ storage technologies to be considered as carbon removal technology (Caldecott, Lomax, and Workman 2015).

Carbon- or bio-based plastics are an alternative form of carbon storage, which is already being used in certain applications in buildings. However, since the quantity of the material in buildings is marginal, the storage potential remains low.

5.2. Recommendations

Our study has also revealed the need to further process the results, eventually allowing them to be directly applicable in practice. Therefore, we propose a set of recommendations for researchers, designers, clients, product manufacturers, and policymakers.

1. Researchers

   1. Verify and localize the values of the approaches for different countries, regions, or cities

   2. Provide the values in units that are directly applicable to city-planning (e.g. tCO₂/ha) and building design (e.g. kgCO₂/m²)

   3. Provide these values for developers of major BIM and LCA software

2. Designers

   1. Start evaluating the carbon storages and pools in projects

   2. Provide estimations of site- and building level carbon storages for clients

   3. Request information from product manufacturers

3. Clients

   1. Create demand for carbon pools and storages in the built environment

   2. Set minimum requirements for carbon uptake and storage in green public procurement of buildings and infrastructure works

   3. Introduce carbon uptake and storage as a parameter in architectural competitions

4. Product manufacturers

   1. Include carbon storage data and scenarios in the Environmental Product Declarations of construction products where possible

   2. Study options for selling emission compensations through long-term or permanent carbon storages in products

5. Policymakers

   1. Support the long-term storage of carbon into the built environment through policy development and normative requirements

   2. Channel funding for accelerating the development of promising approaches (based on Tables 7 and 8)

   3. Empower activism by offering citizens, NGO’s, property developers and funders with practical instructions on ways to store carbon in the built environment.

Based on our review, the potential of the built environment as a carbon storage appears yet to be discovered and taken into systematic use. Therefore, it would be essential to rapidly utilize mature technologies for storing carbon in our buildings and infrastructure. These long-living assets should be taken into use in the global efforts to mitigate the anthropogenic climate change. At the same time, it should be ensured that increasing carbon stocks in the built environment would not cause collateral emissions or decreases of other carbon pools.

Finally, we underline the importance of adopting an active role in climate-conscious building design. Approaching carbon neutrality requires that an iterative design process is applied, in which the emissions are reduced and the stocks simultaneously increased.

Disclosure statement

No potential conflict of interest was reported by the author(s).