Multi-sectoral efforts are required for decarbonising the building sector: a case in Hong Kong

The building sector is a major contributor to global carbon emissions, accounting for 39% in 2021. Meeting the Paris Agreement's 1.5°C target and global carbon neutrality goals necessitates deep decarbonization of this sector. Decarbonization requires collaborative efforts across multiple sectors because the building sector is an end-use sector dependent on various fuels and materials. For instance, electricity significantly impacts operational emissions, while the manufacturing sector's cleaner production practices affect embodied carbon in building materials. Existing studies often focus on individual mitigation measures within the building sector or make simplified assumptions about other sectors, lacking a comprehensive multi-sectoral perspective. This research addresses this gap by proposing a new framework for analyzing the interconnectedness of various stages, contributing sectors, and measures in decarbonizing the building sector.

Previous studies on building sector decarbonization primarily used top-down regression models to predict mitigation effects based on socioeconomic factors. These studies often focused on measures within the building sector itself, treating other sectors' contributions as exogenous factors, leading to potential biases. While some research identified key driving factors (electricity consumption efficiency, clean power, material consumption), they lacked comprehensive decarbonisation strategies incorporating relevant contributing sectors and their interconnected effects. Process-based life cycle assessment (LCA) offers a potential solution, but even LCA studies often isolate measures within individual sectors, failing to capture the full life cycle impact and the intricate interactions between different stages, sectors, and mitigation measures.

This study introduces a novel "stage-sector-measure" analysis framework based on process-based LCA and bottom-up modeling. The framework analyzes the building sector across six life cycle stages: material production, material transportation, construction, operation, demolition, and waste transportation and disposal. Four contributing sectors are identified: manufacturing, transportation, building, and electricity. Twenty-three mitigation measures across six categories are identified and linked to these stages and sectors. A two-phase calculation method quantifies the mitigation effects of each measure. The first step quantifies carbon mitigation effects from changes in key factors across contributing sectors. The second step quantifies how the measures affect these factors (emission factors of materials, vehicle shares, fuel mix for construction and operation, electricity emission factors). Hong Kong is selected as a case study due to its high-rise, high-density urban environment. Building stock and new/demolished building floor area predictions from 2020-2050 are used, and sensitivity analysis assesses the impact of key parameter fluctuations on mitigation results. Specific equations are detailed in the paper for each stage's calculation, considering various factors such as material consumption, transportation distances, energy intensities, emission factors, and carbon removal rates.

The study projects an 84.4% reduction in Hong Kong's building sector emissions by 2050 under the implemented decarbonization scenario. The electricity sector is identified as the most significant contributor to mitigation, accounting for 71.8% of the total effect, largely due to alternative fuel mixes and carbon capture and storage (CCS). Cleaner production of concrete and steel accounts for 62.9% of mitigation in the material production stage. Alternative fuel mixes and CCS contribute significantly to mitigation in other stages (42.2-87.7%). Analysis by life cycle stage shows the largest mitigation potential lies in material production, construction, operation, and demolition stages during 2020-2030. The study further breaks down mitigation effects by contributing sector. The electricity sector shows the largest contribution, followed by the building sector. The transportation sector initially has a negative contribution before 2040 due to the higher emission factors of electric vehicles compared to diesel vehicles before electricity generation decarbonization. The analysis also reveals that the clean electricity measures have the largest contribution, accounting for 71.8% of the carbon mitigation effects. Building energy efficiency and cleaner production of building materials also contribute significantly. Sensitivity analysis identifies the carbon removal rate by CCS as the most sensitive parameter, followed by building stock floor area and electricity generation emission factors.

The findings emphasize the crucial role of multi-sectoral collaboration in achieving deep decarbonization of the building sector. The significant contribution of the electricity sector highlights the importance of transitioning to cleaner energy sources. The large impact of cleaner production of building materials underscores the necessity of addressing embodied carbon emissions. The initial negative contribution of the transportation sector before 2040 illustrates the complex interactions between different sectors and measures, highlighting the need for a holistic approach. The framework's ability to quantify these interactions improves the accuracy of decarbonization pathway predictions compared to previous studies that relied on exogenous predictions or only considered individual sectors. The results support the feasibility of achieving carbon neutrality in Hong Kong's building sector by 2050 through integrated multi-sectoral strategies.

This study provides a novel "stage-sector-measure" framework for comprehensively evaluating building sector decarbonization from a life cycle perspective, highlighting the necessity of multi-sectoral efforts. The application in Hong Kong demonstrates the potential for substantial emission reductions (84.4% by 2050). The findings emphasize the dominant role of the electricity sector and the importance of addressing both operational and embodied emissions. Future research should refine predictions by incorporating detailed mechanisms for measure application, conduct a cost-benefit analysis for measures, and expand the system boundary to include additional sectors and emerging technologies.

The study's predictions on mitigation measure application rely on policy requirements and reports, lacking detailed mechanistic explanations. For instance, building energy efficiency improvements are assumed to increase annually at certain percentages without considering factors like human behavior, building design, or appliance efficiency ratings. Economic analysis is also lacking; a cost-benefit analysis would strengthen the policy recommendations. Finally, the system boundary could be expanded to include additional sectors and technologies that may become more significant in the future.