The 80th LCA (life cycle assessment) Forum held on 9 June 2022 considered key issues in research and legislation for how carbon storage in buildings should be accounted for.

Key conclusions from this scientific meeting can be summarised as follows. CO₂ is a waste product and urgently needs a proper treatment, like phosphorous emissions in the 1960s and NO_X emissions in the 1980s, which were effectively being reduced with wastewater treatment plants and car catalysts, respectively. Biogenic carbon needs to be stored long-term (3000 to 8000 years which is equivalent to 100 to 300 generations) to be effective in reducing the rise of the global surface temperature. Temporary storage of biogenic carbon may be used to cap peak of temperature rise but only if fossil CO₂ emissions are drastically cut to net zero (i.e. including permanent carbon dioxide removals, CDR) at the same time. Temporary storage of biogenic CO₂ gives us a few decades time to develop effective CDR methods. CO₂ offsets shall be based on CO₂ removal and long-term storage. Technologies are currently not available and urgent efforts are needed to make them ready in large scales within the next few decades.

1 Introduction and overview

The 80^th LCA Forum at the Swiss Federal Institute of Technology Zurich (ETHZ) was opened with a welcome address given by ROLF FRISCHKNECHT (treeze, CH). Increasingly countries, national and international organisations are publishing white papers and guidelines on how to realise buildings that have net zero GHG emissions. While it may seem straightforward to quantify the GHG emissions caused by the construction, use and end of life of a building, the accounting and accountability of Carbon Dioxide Removal (CDR) is more disputed. How and under what conditions do biomass-based construction materials qualify as CDR? And how should biogenic carbon in buildings be modelled and its impact on the climate and the temperature increase assessed?

In a first session the role of biogenic carbon for the climate, and for buildings and community targets was discussed (Section 2). The next session was dedicated to the topic of biogenic carbon assessment in buildings LCA in different countries (Section 3). After a session of short presentations on biogenic carbon assessment in the building sector (Section 4) life cycle assessments of carbon mitigation measures and the role of carbon offsets in LCA were the topics of the last session (Section 5). Breakout groups discussed the topics of a) time dependent global warming potential (GWP) factors in carbon footprinting of buildings; b) temporal storage of biogenic carbon in buildings; and c) CDR and LCA (Section 6).

2 The role of biogenic carbon for the climate, buildings and community targets

CYRIL BRUNNER’s (ETH Zürich, CH) keynote considered the effect of temporarily stored biogenic carbon. Every tonne of CO₂ not in the atmosphere does also not contribute to warming (IPCC 2021). Removing CO₂ from the atmosphere (e.g. with the aid of additional vegetation) lowers the CO₂-induced warming. If the carbon is stored only temporarily (e.g. in building materials) then the climate benefits will be lost when the stored CO₂ is re-released (Matthews 2010). If CDR with temporary storage is used in a scenario in addition to consequent fossil fuel emissions reductions and permanent CDR, it can lower the peak temperature (Matthews et al. 2022), and subsequently reduce the number of endagered ecosystems and species. LCA needs to account for the uptake and re-release of stored CO₂ entirely and no discount rate on the re-release due to the storage duration should be applied.

GUILLAUME HABERT (ETH Zürich, CH) showed that buildings should no longer be considered as a problem, but as an opportunity to temporarily store carbon and counteract climate change. By incorporating fast growing biogenic material into buildings, it is possible to remove CO₂ from the atmosphere and lower the emission of construction. When these materials are combined with an optimised use of low carbon materials at building scale, it is possible to build climate neutral buildings today (Carcassi et al. 2022). Similar to forests where carbon stock is not considered per tree, but instead by hectare, the built environment's carbon stock should be considered at city scale and not at building scale (e.g. one biogenic meaterials-based building replacing another building of the same type) and maintaining the overall stock. Finally using biogenic by-products from agriculture should be preferred in order not to impinge on food production or create competing land usage.

NIKO HEEREN (City of Zürich, CH) represented the city of Zurich, which decided in 2021 to commit to ambitious GHG reduction goals. For the city administration direct (scope 1) GHG emissions must reach net zero by 2035 and indirect emissions must be reduced by 30% compared to 1990. As the city has been continuously phasing out fossil fuels for its building stock, the former reduction appears within reach. However, emissions related to material use are a mostly unresolved issue, because only few low-carbon construction materials are available on the market. The city is also expecting considerable growth for expanding infrastructure (schools, social housing, etc.) in the coming years. To meet the required reduction goals, the city's building construction office is working on a reduction pathway. The net zero strategy implies that remaining emissions in 2035 will need to be balanced by CDR. In a mandated study the potential of wood construction for reducing emissions and increasing stored carbon was assessed (www.stadt-zuerich.ch/holzbau-als-kohlenstoffspeicher). The results show that GHG emissions can be reduced by up to 12% by 2050 and up to 50% if carbon stored in buildings is also considered.

This considerable reduction potential illustrates the importance of biogenic carbon storage in building stocks. However, there exists no agreement in science and legislation how carbon storage in buildings should be correctly accounted for. This represents an important obstacle for the construction industry and must be swiftly addressed by policy and science.

3 How to deal with biogenic carbon in buildings LCA

BRUNO PEUPORTIER (Mines Paris - PSL, FR) presented the new French regulation for dwellings in France that came into force January 2022 and which includes requirements on GHG emissions related to products and operational energy (Ministère de la transition écologique 2020). To complement this regulation, design tools were developed for early design phases, aiming at a more science based approach with voluntary performance targets beyond regulatory requirements (Peuportier et al. 2013). The corresponding calculation methods were presented, focusing on biogenic carbon accounting. Example applications illustrated the environmental benefit and limits of biobased materials considering various impact indicators and resilience of buildings to heat waves.

MATTI KUITTINEN (Ministry of the Environment, FI) reported on the concept of carbon footprints and handprints for Finnish buildings. The government of Finland is pursuing carbon neutrality by 2035, and carbon negativity in 2040s. To support these ambitions, the fully revised Building Act includes a requirement for reporting building-related GHG emissions and sequestrations in a “climate declaration”. The methodology follows EU's Level(s) framework and EN 15978. This is complemented by a “carbon handprint”: climate benefits that would not be achieved without the building project (Kuittinen & Häkkinen 2020). In addition to reporting the module D benefits, the carbon handprint may include long-term biogenic or technical carbon storage, carbonation of concrete, and surplus renewable energy. Furthermore, finding a robust methodology for accounting for the GHG removals of urban trees, vegetation and soils is currently being investigated.

ALEXANDER PASSER (Graz University of Technology, AT) highlighted the IEA EBC Annex 72 consensus on biogenic carbon modelling and assessment in buildings LCA. Typically, in building LCAs, biogenic CO₂ is accounted for using two different approaches: the 0/0 (or carbon neutral) approach and the -1/+1 approach. The first considers by default that the uptake of CO₂ during the growth of the bio-based material is compensated by its release at the end of its service life. Consequently, the 0/0 approach considers only the contribution of gases from fossil sources to the GWP calculation. The -1/+1 approach, on the other hand, considers both the uptake during growth and the release at the end of life. Standards (EN 15804 2019) highlight that if the uptake is accounted for, the release must also be considered in end-of-life recycling, landfilling and incineration. The life cycle-based GHG emissions arising from the two approaches must be equal, the only difference being that with the -1/+1 approach one can track the biogenic carbon flows throughout the full life cycle. Considering the lack of consensus on the appropriateness of the different currently available methods to account for biogenic carbon in buildings, IEA EBC Annex 72 consensus aims to discuss the opposing views and derive recommendations based on the calculation guidelines published by the Intergovernmental Panel on Climate Change (IPCC 2021) and the increasing knowledge on carbon sources, sinks and deriving budgets.

4 Biogenic carbon modelling in the construction sector

CORNELIA STETTLER (Carbotech, CH) presented the required decline and path for net-zero GHG buildings. She talked about the necessary ambitious combination of different measures to follow this path and about the possible but limited role of biogenic carbon. A concept was presented for accounting biogenic carbon in the building assessment (Näf et al. 2021). The concept sets priority on the required emissions reduction and allows a partial accounting of biogenic carbon within given limits and under defined conditions.

ANDREW NORTON (Renuables, UK and CEI-Bois, BE) and HANSUELI SCHMID (Lignum, CH) talked about the urgent need to display the benefit of stored carbon in product footprints and building assessments to support policy, carbon credits and a more circular economy in a commonly defined way. There are many similar methods that can equate this benefit into a global warming potential calculation (Brandão et al. 2013; European Commission et al. 2021). As it is impossible to predict length of service for a product, they proposed the option to display calculations for a range of different lifespans as separate information, alongside an inclusion of the so-called dynamic LCA in GWP results, hereby incentivising longer lifespans and design-for-reuse in construction.

ILKKA LEINONEN (LUKE, FI) highlighted that there are currently no commonly accepted methods in LCA for handling the changes of biogenic carbon storage in biomass, dead organic matter and soil. A solution could be the application of general mass balance principles in all calculations of carbon flows. This would take into account all reductions and all increases in carbon storage and ensure that all changes are counted only once (no double counting). Efficient communication and decision making would be possible only if such counting principles are applied in a transparent way, making it possible to demonstrate the special features of biobased materials.

5 Carbon mitigation and carbon certificates

NIELS JUNGBLUTH (ESU-services, CH) presented carbon mitigation assessment options for tree planting over their life cycle. Some organisations offer to plant trees and promote this as a means of tackling climate change. But the calculation behind the offered carbon storage do not follow clear standards and lack transparency. The following issues need to be considered in a life cycle approach: the type of trees, area, seeds, planting, control trips, forest maintenance, irrigation and a binding guarantee over several decades. Risks of loss (e.g., by forest fires or deforestation) need to be considered. To guarantee carbon storage into future, it seems more relevant to protect existing forests/soils and functioning ecosystems than planting trees.

CHRISTIAN BAUER (PSI, CH) talked about CDR which is considered indispensable to limit global warming at 1.5-2 degrees. Several CDR options exist and their environmental performance, especially the net-effectiveness of CO₂ removal and potential trade-offs coming along with it, must be evaluated using LCA methodology. The status of the LCA literature regarding CDR options was reviewed (Terlouw et al. 2021). Despite a growing number of publications, several aspects remain to be addressed and the reliability of LCA results is often questionable. Main reasons are a lack of transparency, ambiguities in system boundaries, and mingling CO₂ removed from the atmosphere with avoided CO₂ emissions.

6 LCA in view of net zero buildings, break out group discussions

The discussions in the three breakout groups resulted in the following main messages, conclusions and recommendations:

a) The group discussing time dependent GWP factors in carbon footprinting of buildings acknowledged that the GWP and global temperature change potential (GTP) value of CO₂ are independent of the time of emission (physical reality). They nevertheless consider the timing of emissions and thus buying time essential to reach political climate targets. They agreed that methods that include temporary storage and delayed emissions make things complex but temporal carbon storage needs incentives. They propose to introduce an additional indicator and they questioned whether any kind of discounting should be applied too for fossil carbon stored in plastics.

b) The group discussing temporal storage of biogenic carbon in buildings prepared the following main messages: emission reduction is the primary measure before before balancing remaining (low) emissions with CDR. No clear preference was identified in the group with regard to time dependent GWP factors. Communities, cities and nations are recommended to install an inventory of biogenic carbon stored in infrastructures, monitor this stock and ensure that it does not decrease but rather increases. The introduction of a minimum threshold of biogenic carbon per m² building floor area is considered one possible measure. The carbon offset taxonomy (University of Oxford 2020) recommendation to give preference to CO₂ removals and long-term storage options (see Table 1) was endorsed by the majority of this group.

c) The group discussing CDR and LCA prepared the following four main statements: Avoid and reduce GHG emissions as much as possible before engaging in CDR. Ensure transparency in the carbon footprint accounting of buildings by keeping emissions separate from removals. Specify the scope of the assessment to determine the net amount of CO₂ removed and specify the permanence of the storage, giving preference to long-term (several thousand years) storage options. Address the risk of carbon leakages from “permanent” storage facilities.

Table 1: Taxonomy of Carbon Offsets.
Adapted from the Oxford Principles 2020 (University of Oxford 2020)

How is the offset generated?	Is carbon stored?	How is carbon stored?	Examples
Potentially avoided emissions	No	-	Forward-looking, counterfactual baseline: Renewable energy potentially replacing fossil fuels
Emissions reduction	No	-	Clear retrospective emissions data: N2O abatement; methane abatement
Emissions reduction	Yes	short	Changes to agricultural practices that retain already stored carbon
Emissions reduction	Yes	long	CCS on industrial facilities, fossil fueled power plants
Carbon removal	Yes	short	Afforestation, reforestation Soil carbon enhancement
Carbon removal	Yes	long	Direct air carbon capture and storage Biogenic energy carbon capture and storage Mineralisation, enhanced weathering

Notes

1. This conference report was jointly authored by: Rolf Frischknecht, Cyril Brunner, Guillaume Habert, Niko Heeren, Bruno Peuportier, Matti Kuittinen, Alexander Passer, Cornelia Stettler, Andrew Norton, Hansueli Schmid, Ilkka Leinonen, Niels Jungbluth, Christian Bauer. Special thanks to Rolf Frischknecht for coordinating and editing the individual contributions.

2. Conference presentations and videos are available for download from https://bit.ly/3QUQCLs

References

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