**4. Discussion**

Cement and steel, together with diesel use in construction processes and material transports, account for the majority of the CO2 emissions associated with building and infrastructure construction (cf. Figure 6). While the analysis in the study has served to improve the current estimate of the climate impact of building and transport infrastructure construction, it is still associated with a degree of uncertainty. To provide well-grounded decision support for the climate transition ahead, it is important that su fficient resources and competence are allocated so that development of emissions can be properly evaluated and so that the e ffects of planned measures and policies can be assessed before implementation.

Based on the updated estimate, this roadmap served to illustrate how the basic materials industry and supply chains for buildings and transport infrastructure construction are a ffected, in terms of energy and material use and associated greenhouse gas emissions, by di fferent technical choices. The study also aimed to illustrate the timing of measures needed to reach intermediary and long-term emission reduction targets. The results show that it is possible to reduce CO2 emissions associated with construction of buildings and transport infrastructure by at least 50% to 2030, applying already available measures, and reach around 90% emissions reductions by 2045, while the energy use may be reduced by varying degrees (6–19% to 2030 and 16–37% to 2045), indicating that strategic choices with respect to process technologies and energy carriers may have di fferent implications particularly on the energy use over time. It is worth noting that no pathway reaches zero carbon emissions, which is why it is important to further investigate the potential for and accounting of negative emissions (e.g., carbon capture of biogenic emission) and carbon sinks (e.g., use of long-lived wood products in construction) as to enable an approach towards net-zero emissions by 2045. The measures proposed in this roadmap could (and perhaps should) also be backed by strategies to avoid building by exploring alternatives and by repurposing assets, as well as reduce the floor area per capita by smarter floor plans and increased shared spaces [38,43,158].

This study, in alignment with previous analysis, as reported in, e.g., Gerres et al. [34], demonstrates the importance of ensuring su fficient availability of sustainable biomass/bioenergy, electricity and hydrogen. The urgency in upscaling these energy sources becomes particularly evident as experience shows that planning, permitting, and construction of both support infrastructure (renewable energy supply, electricity grid expansion, hydrogen storage, CCS infrastructure) and piloting and upscaling to commercial scale of the actual production involve long lead times. Strategic planning for key support infrastructure therefore needs to be initiated as early as possible, even if not all uncertainties will be fully resolved.

As there are already today known measures and technologies which can reduce emissions to zero, from circularity and material e fficiency measures, biofuel or biomaterial substitution, electrification (direct or indirect) with renewable electricity, and/or carbon capture and storage, the challenge to meet climate targets is not only a technological challenge but relative to economics and financial risk, particularly since the current climate policy is too weak [159]. Indeed, large scale demonstration of key processes is required to obtain confidence in technologies, gain experience, and reduce financial risk, but technologies are available at high maturity levels. This would also serve to reduce the uncertainties inherent in the span of emission reduction potential from di fferent abatement measures found in literature (ref. Figure 5).

A key message from this work (as illustrated in Figure 10) is the importance of simultaneously focusing on short- and long-term abatement measure. With this statement, we that that the pursuit of 'low-hanging fruits' (e.g., material substitution and e fficiency measures) cannot be an excuse for not acting to lay the foundation for the high-cost long lead-time measures (zero-CO2 basic materials) required to reach deep decarbonization. Vice versa, we cannot let the promise, e.g., of low-CO2 steel or cement, be an excuse not to act to unlock the potential for measures that already exists today. Successful decarbonization of the supply chains for buildings and infrastructure, including the production of basic materials, will involve the pursuit—in parallel—of emission abatement measures with very different characteristics. Consequently, to facilitate the transition, the support tools box will need to encompass a variety of policies and strategies.

**Figure 10.** Successful decarbonization of the supply chains for buildings and infrastructure in less than three decades will require the parallel pursuit of emission abatement measures with very different characteristics. Figure adapted from Vogt-Schilb and Hallegatte (2014) [160].

The results thus illustrate the importance of intensifying efforts to identify and manage both soft (organization, knowledge sharing, competence) and hard (technology and costs) barriers and the importance of both acting now by implementing available measures (e.g., material efficiency and material/fuel substitution measures) and actively planning for long-term measures (low-CO2 steel or cement). Unlocking the full abatement potential of the range of emission abatement measures that are described in this study will require not only technological innovation but also innovations in the policy arena and efforts to develop new ways of co-operating, coordinating, and sharing information between actors in the supply chain. Key priorities include, e.g.,

	- ◦ avoid building (where possible),
	- ◦ re-using old assets,
	- ◦ recycle building materials and components,
	- ◦ optimize material use, and
	- ◦ shift to low-CO2 materials and services.

early on, as well as planning for agility and endurance in the face of the unforeseen (e.g., delays, changing market conditions). Similar planning processes, including identification of designated strategic areas/zones, have previously been carried out for wind and hydro power [165,166].

	- ◦ Establishment of an (public or private) umbrella organization with the responsibility to oversee and support the low-CO2 transition.
	- ◦ Securing new competence by including low-CO2 building and construction as a central part of the in upper secondary school and higher education.
	- ◦ Training of active practitioners (engineers, architects ... ).

It is of course also important to continue to find ways to sharpen existing climate policies, such as the EU-ETS and renewable policies, most important being to make them as long term as possible [35]. There is no guarantee that investments in the development and implementation of hydrogen direction reduction in the steel industry, CCS in the cement industry, nor other low-carbon technologies for industrial applications will pay off [172,173]. However, choosing not to, or failing to act within the next few years, to create the economic, organizational, and infrastructural conditions that could facilitate a shift towards low-CO2 production and practices will severely compromise the chances of a successful decarbonization of the steel and cement-industries, as well as the supply chains for buildings and transport infrastructure, up to the year 2045.

Although the findings reported in this paper draw primarily on Swedish experiences, with some of the conclusions valid only under certain conditions and circumstances, it is clear that many of the challenges that have been raised here, which must be overcome to achieve a transition to zero-CO2 production and practices in the supply chains for buildings and infrastructure, are universal [43,174,175]. Whereas rapid improvements of the climate performance of the use phase (i.e., related to heating and cooling) of the existing and new building stocks is a key priority in many parts of the world, it is equally important to take measures to reduce the climate impact of the construction process and the production and supply of building materials.

From a global perspective, this is important, not the least, since there are still many regions of the world where much the of the buildings and the infrastructure to provide shelter from the elements, mobility for people and goods, and infrastructures for the supply of water, electricity, and heat remains to be built. Estimates sugges<sup>t</sup> that more than half of the urban infrastructure that will exist in 2050 has ye<sup>t</sup> to be built [175,176] and that total global floor area of buildings will double within the next three or four decades [43,174].
