The principle of staying within planetary environmental boundaries is now recognised internationally. This idea is also endorsed by a wide variety of professional groups and institutions and is today reflected in the internationally recognised Sustainable Development Goal (SDG) 13: ‘Take urgent action to combat climate change and its impacts’ (UN 2015). The built environment makes a significant contribution to greenhouse gas (GHG) emissions and thus to climate change. The science is clear (IPCC 2018) and there is broad acceptance of the need to drastically reduce GHG emissions from the built environment over a very short timeframe (fewer than 30 years). Past approaches to energy demand and GHGs have been based on incremental reductions from a baseline year (often 1990 is used). However, this approach fails to sufficiently account for the more meaningful small remaining amount of GHGs that can safely be emitted and how that is shared. It also fails to account for life-cycle emissions in the case of construction products and construction works.
The importance of the GHG emissions and the resulting environmental impacts associated with the built environment is often underestimated due to its cross-sectoral character. Around 30% of all energy-related GHG emissions worldwide come from the operation of residential and non-residential buildings, including direct and indirect emissions (GlobalABC 2018). Another 10% arises in connection with the manufacturing of construction products for building construction and renovation and is caused by energy- and process-related emissions (IEA 2019).
Given the many different actors (both up- and downstream in the supply chain) in creating, operating, maintaining, refurbishing and re-purposing the built environment, a set of key questions for climate protection (e.g. staying within a 1.5–2°C range of global warming) arises:
This special issue examines these questions. It provides a process for creating a coherent set of targets and indicators based on the safe planetary boundary for GHG emissions. The intention is for these indicators to inform decisions and actions for the wide array of built-environment actors.
To limit global warming, the remaining total carbon budget is defined as an overall global target (IPCC 2018). Due to progress in climate science, it may be necessary to further adjust climate-protection goals in terms of the level of GHG reductions, and the timing of those reductions.
One approach is to set carbon budgets, which can be global, national, regional, local, specific project or sectorial. In apportioning a budget of GHG emissions (typically expressed as CO2e), there are two essential aspects. First, the sum of the parts must not exceed the overall global budget. Second, the process must be accepted as fair, robust and transparent (Klinsky & Mavrogianni 2020). Such goals and approaches are indispensable elements of an overall strategy. They provide the wide range of built-environment actors with the necessary target values and assessment criteria. In order to support targeted climate protection activities with manageable principles, methods and tools, another important element is required: the establishment of assessment rules.
The development of carbon metrics for buildings and cities is understood here as a basis for the quantitative determination of GHG emissions, combined with the aim of assessing them. Assessment can be undertaken by comparing carbon metrics with absolute or relative benchmarks.
A carbon metric can be understood as a standard of measurement of GHG emissions. It is based on transparent, verifiable, traceable, and reliable GHG accounting and assessment at all scales. Such standards are usually developed by a larger group of actors in which scientists as well as representatives of other interested parties are involved, including from politics and industry, as well as consumers and non-governmental organisations. Examples of relevant international standards in the context are shown in Table 1. In addition to the examples mentioned, the principles and rules for a carbon performance assessment based on the determination of a 100-year global warming potential (GWP 100) of emitted GHGs are also part of European standards for the environmental performance assessment of construction works, developed in the context of CEN TC 350.1
|ISO 14064-1: 2018||Greenhouse gases—Part 1: Specification with guidance at the organization level for quantification and reporting of greenhouse gas emissions and removals||Describes how organisations (e.g. real estate development companies and property investment funds) can report and communicate greenhouse gas emissions|
|ISO 21929-1: 2011||Sustainability in building construction—Sustainability indicators—Part 1: Framework for the development of indicators and a core set of indicators for buildings||Lists global warming as a main indicator in assessing the contribution of buildings to sustainable development; points to the consequences of climate change in other dimensions of sustainability|
|ISO 14067: 2018||Greenhouse gases—Carbon footprint of products—Requirements and guidelines for quantification||Applicable to products of all kinds|
|ISO 21931: 2010 (in revision)||Sustainability in building construction—Framework for methods of assessment of the environmental performance of construction works—Part 1: Buildings||Formulates building-specific requirements for the life-cycle assessment of buildings, including in relation to GWP 100 indicator|
|ISO 16745-1: 2017||Sustainability in buildings and civil engineering works—Carbon metric of an existing building during use stage—Part 1: Calculation, reporting and communication||Specially developed for determining GHG emissions during the use phase of buildings|
|ISO 16745-2: 2017||Sustainability in buildings and civil engineering works—Carbon metric of an existing building during use stage—Part 2: Verification||Specifies the requirements for the verification of a carbon metric calculation for GHG emissions of an existing building during the use stage|
|ISO 21678: 2020||Sustainability in buildings and civil engineering works—Indicators and benchmarks—Principles, requirements and guidelines||Formulates the requirements for the development of performance levels/target values and the description of benchmarks—can be used for GWP 100 and carbon budgets|
|ISO 21930: 2017||Sustainability in buildings and civil engineering works—Core rules for environmental product declarations of construction products and services||Formulates the requirements for the provision of unassessed information on resource use and the effects on the environment in the life-cycle of construction products, including GWP 100|
|ISO 14040: 2006||Environmental management—Life cycle assessment—Principles and framework||Establishes the basic principles for life-cycle assessment (LCA)|
|ISO 14044: 2006||Environmental management—Life cycle assessment—Requirements and guidelines||Provides information on the implementation of LCAs|
|ISO 14026: 2017||Environmental labels and declarations—Principles, requirements and guidelines for communication of footprint information||‘[P]rovides [the] principles, requirements and guidelines for footprint communications for products addressing areas of concern relating to the environment’a|
|ISO 14080: 2018||Greenhouse gas management and related activities—Framework and principles for methodologies on climate actions||Supports the management of GHG emissions as well as the preparation and implementation of mitigation measures|
The emergence of related standards within the framework of international or national standardisation bodies is not a prerequisite for recognised measurement regulations. Groups, committees or organisations can agree on standardised calculation and assessment processes (e.g.RICS 2017; WRI & WBCSD 2013; BSI 2011, UNEP 2009, WRI 2014, EU 2017; CMCE 2016). Key questions are whether and to what extent compromises arise as a result of a consensus-oriented approach and if this leads to deviations away from purely scientific positions.
In addition to the standards mentioned in Table 1, the International Organization for Standardization (ISO) provides a complete overview of the results of international standardisation activities in the area of climate change (ISO 2018).
A consensus-based and standardised measurement specification must also meet all the scientific requirements relating to the overall limits of GHG emissions. The following information should therefore always be publicly available:
Carbon metrics depend on energy information on the type, source and quality of emission factors and/or the data source. For large-scale observation it is possible to create measurement by remote sensing.
Any kind of institution or organisation producing and using a specific metric must have clear responsibility and authority for the oversight, validity and accuracy of the measurement regulations; the selection of groups (organisations or individuals) involved in the development, and whether there are reasons for the validity to be limited in time. In addition, it is possible to specify which actors can have an influence on GHG emission reductions, with which measures, whether there are interactions or conflicting goals with other indicators, or what are the relevant up- or downstream effects and processes.
The elegance of a carbon metrics approach is that it is scalable and therefore can take an overall GHG emissions target and translate that to a specific circumstance. The objects examined in the built environment range from individual buildings to neighbourhoods, districts and cities or from the building stock of institutional and individual owners to the regional and national building stock of the federal states as well as from the areas of need of individual households (here for housing) to economic sectors and areas of action.
When determining GHG emissions in buildings, the carbon metrics approach considers the complete life-cycle. This increases the type and number of objects under assessment because buildings are comprised of a large variety of different types of products, and each individual product can be an object of assessment by itself. As a basis for determining the carbon performance of buildings, data on GHG emissions during the manufacturing and disposal of construction products, as well as for construction processes, energy provision, transport and other services, are required. Requirements for the completeness of models describing the building structure and its life-cycle are important, possibly in conjunction with cut-off rules.
Construction works (i.e. buildings and infrastructures) have special features compared with other goods. They have a long lifespan and useful life, the need for repair and refurbishment, and sometimes the need to adapt to changing use requirements (or behaviour) and/or environmental conditions. The importance of dealing with the factor of time factor is clear. Many decisions are made that only have an impact in the medium to long term. The necessary assumptions and scenarios create uncertainties that must be dealt with appropriately.
The GHG emissions that arise in cities (or that are assigned to them from ‘imported’ GHGs) require the specification of clear system boundaries. A decision is needed about whether to use a production- or consumption-based accounting approach, or a combination of both. Compared with buildings, the complexity of the assessment task is increased.
This short overview alone makes one thing clear: the complexity, interdisciplinarity and multi-scale/multi-actor/multi-sector perspective of the built environment entails a comprehensive approach to the definition and assessment of carbon metrics. It is important that methodological consistency is maintained across all details and levels of action, that results are presented in both aggregated and disaggregated forms, and that gaps in the determination of GHG emissions are avoided by overlapping perspectives. The need to resolve this complexity in the interests of manageability is also apparent. Individual measurement regulations for specific applications are one approach. In addition to the features mentioned above, they must be described in such a way that the following information enables context-specific selection and application:
It is essential to clearly define measurement regulations and system boundaries. The narrower are the system boundaries for determining GHG emissions, the easier allegedly it is to avoid or compensate for them. Defining climate- or culturally specific environmental target values is a key task, e.g. for GHG emissions in the life-cycle in kg CO2e/m2.yr. A prominent example is SIA 2040 in Switzerland (SIA 2017). Specific measurement regulations have been and are required for this. If future climate neutrality is sought for the built environment, then zero or net zero can be seen as a uniform goal. However, this can be achieved with different means and in compliance with specific measurement regulations adopting specific system boundaries. To reduce the risk of manipulation, a transparent and comprehensible measurement rule is an indispensable basis for statements on climate neutrality. A future carbon budget of zero for the built environment is also a budget in terms of a target value. Specific measurement regulations are still required to show that all GHG emissions could actually be avoided or compensated for.
The main purpose of carbon metrics is to aid decision-making and actions at many different levels. It provides an understandable currency of targets that different actors can understand and implement. As the research in this special issue shows, it provides a process for determining a coherent set of targets, and not all buildings will have the same target. In addition, it can provide clear data on whether targets are actually being met.
Politicians in their role as legislators are one potential user group of carbon metrics. In connection with the societal responsibilities to preserve the natural basis of life and safeguarding the future, it is inevitable that legal requirements will limit GHG emissions for the life-cycle of buildings. Several European countries are already working on this. One possible approach is to formulate binding requirements in the form of laws, but to refer to an international or national standard for the calculation and assessment rules—a carbon metric. Data on GHG emissions can also provide the basis for design and decision-making for regional policy-makers and their experts.
The requirements and options for reducing GHG emissions must correspond to the area of work and responsibility of the many different actors, be integrated into decision-making processes and be adaptable to the specific circumstances of the object under assessment.
Other relevant stakeholder groups can use the metrics in the design process (designer, client), provide compliant data (manufacturer, service provider), take on special tasks (auditors, providers of sustainability assessment systems and tools, creators and providers of databases), use the results for their own decision-making processes (valuation professionals, banks) or check compliance with requirements (legislators, funding institutions).
Carbon metrics can also support other decision-making processes. In particular, it can inform management tasks by specific actors in their specific area of responsibility. Composed across all levels of action, it provides a clear system of measurement that is both shared, consistent and comprehensive by embracing all factors influencing GHG emissions in the built environment. This will help planners, mayors, clients, investors, designers, contractors, facility managers, tenants/occupants and material/component suppliers to understand whether a project, building, neighbourhood or portfolio is within the specified range of GHGs. It will also focus actions on how to achieve the target.
The papers contained in the special issue cover a broad spectrum of questions. They will be of interest to the scientific community, policy-makers, educators and leading practitioners. Their range illustrates how wide and complex the scientific discussion is for the accounting, assessment and management of GHG emissions and their undesirable effects on the global climate. The respective objects of assessment in the papers range from individual building products and components to whole buildings, neighbourhoods and cities, and up to institutional, regional and national building stocks. The economic sectors relevant to the construction sector, the field of action ‘construction and operation of buildings’ and the area of need ‘housing’ are also covered. Table 2 provides an overview of all the papers. With the exception of a contribution each from Australia and North America, all publications were written by authors from Europe. This shows how intensely the discussion is conducted on the European continent, which aims to become climate neutral by 2050 (EC 2020).
|T. Lützkendorf||The role of carbon metrics in supporting built environment professionals (Editorial)||10.5334/bc.73|
|G. Habert, M. Röck, K. Steininger, A. Lupísek, H. Birgisdottir, H. Desing, C. Chandrakumar, F. Pittau, A. Passer, R. Rovers, K. Slavkovic, A. Hollberg, E. Hoxha, T. Jusselme, E. Nault, K. Allacker & T. Lützkendorf||Carbon budgets for buildings: harmonising temporal, spatial and sectoral dimensions||10.5334/bc.47|
|K. W. Steininger, L. Meyer, S. Nabernegg & G. Kirchengast||Sectoral carbon budgets as an evaluation framework for the built environment||10.5334/bc.32|
|R. Frischknecht, M. Alig, C. Nathani, P. Hellmüller & P. Stolz||Carbon footprints and reduction requirements: the Swiss real estate sector||10.5334/bc.38|
|M. Kuittinen & T. Häkkinen||Reduced carbon footprints of buildings: new Finnish standards and assessments||10.5334/bc.30|
|B. Bordass||Metrics for energy performance in operation: the fallacy of single indicators||10.5334/bc.35|
|T. Lützkendorf & R. Frischknecht||(Net-) zero emission buildings: a typology of terms and definitions||10.5334/bc.66|
|M. Balouktsi||Carbon metrics for cities: production and consumption implications for policies||10.5334/bc.33|
|T. Fawcett & M. Topouzi||Residential retrofit in the climate emergency: the role of metrics||10.5334/bc.37|
|A. Parkin, M. Herrera & D. A. Coley||Net-zero buildings: when carbon and energy metrics diverge||10.5334/bc.27|
|E. Hoxha, A. Passer, M. R. M. Saade, D. Trigaux, A. Shuttleworth, F. Pittau, K. Allacker & G. Habert||Biogenic carbon in buildings: a critical overview of LCA methods||10.5334/bc.46|
|C. E. Anderson, K. Kanafani, R. K. Zimmerman, F. N. Rasmussen & H. Birgisdóttir||Comparison of GHG emissions from circular and conventional building components||10.5334/bc.55|
|B. Waldman, M. Huang & K. Simonen||Embodied carbon in construction products: a framework for quantifying data quality in EPDs||10.5334/bc.31|
|M. Schmidt, R. H. Crawford & G. Warren-Myers||Integrating life-cycle GHG emissions into a building’s economic evaluation||10.5334/bc.36|
The spectrum of contributions ranges from a review of the development of indicators in the context of energy performance assessments of buildings (Bordass) to the clarification of current methodological questions on the use of wood in the assessment of GHG emissions in the life-cycle of buildings (Hoxha et al.) and the development of national assessment methods and standards (Kuittinen & Häkkinen) up to various applications, exemplified by a comparison of building components (Anderson et al.) and residential retrofit (Fawcett & Topouzi) or up to carbon metrics for cities (Balouktsi). But the topic goes far beyond the spectrum of the contributions presented here. Many other activities are occurring in the development and use of carbon metrics. Table 3 shows the key themes that individual papers address.
|Author||Object of assessment/scale||Methodological aspect||Application|
|Product/component||Building||Urban district/neighbourhood||City||Building stock||Construction/real estate sector||Indicator development||Assessment method||Creation of a carbon budget||Reduction requirements||Policy-making||Regulation/standardisation||Tool development||Provision of data||Design: new construction||Design: retrofit||Economic evaluation||Macroeconomics|
|Habert et al.||■||■||■||■||■||■||■||■|
|Steininger et al.||■||■||■|
|Frischknecht et al.||■||■||■||■|
|Kuittinen & Häkkinen||■||■||■||■|
|Lützkendorf & Frischknecht||■||■||■||■|
|Fawcett & Topouzi||■||■||■|
|Parkin et al.||■||■||■|
|Hoxha et al.||■||■|
|Anderson et al.||■||■||■||■|
|Waldman et al.||■||■||■|
|Schmidt et al.||■||■||■|
As an introduction to the collection of publications, Bordass is recommended. This paper presents the basic principles and elements surrounding a metric, and also discusses experiences from the past decades in the UK and the problems associated with benchmarking. The paper focuses initially on the operation of buildings and thus on the operational part of an energy and carbon performance assessment. Other publications in this special issue complement this approach by examining the embodied aspects over the complete life-cycle.
An important and newly emerging areas of research is macro-economic considerations relating carbon to the built environment. This involves both cross-sector considerations and the specification of carbon budgets (Steininger et al., Habert et al. and Frischknecht et al.). To support this topic, an illustration is offered here that shows the existing relationships between sectoral considerations, the dynamics of the building stock as a field of action, the life-cycle of individual buildings and the areas of need of households (Figure 1).
Several key points can be drawn from the papers individually and collectively:
Based on the present papers as well as on the additional literature, an attempt can now be made to answer questions a)–h) provided above:
Several recommendations arise from this special issue for public policy:
Carbon performance assessment (involving valuation assessors, funding agencies, investors, etc.) has wider implications for design and construction practice. These include the following:
1CEN Technical Committee 350—Sustainability of construction works. European Committee for Standardization (CEN). Retrieved from https://standards.cen.eu/dyn/www/f?p=204:7:0::::FSP_ORG_ID:481830&cs=181BD0E0E925FA84EC4B8BCCC284577F8/.
The guest editor thanks all colleagues who have enriched this special issue with their contributions. Despite the difficult circumstances of the COVID-19 pandemic, it is thanks to the excellent cooperation of the authors, reviewers, the editor Richard Lorch and the team at Buildings & Cities that this special issue is published in a timely manner before the World Sustainable Built Environment Conference (https://beyond2020.se/).
The author has no competing interests to declare.
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