Time-differentiated life cycle assessment and dynamic carbon budgeting for decarbonising the Aotearoa New Zealand building sector : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Natural Resource Management at Massey University, Palmerston North, New Zealand

Abstract

The building sector is a major contributor to global greenhouse gas (GHG) emissions, and is characterised by the long service lives of buildings and associated whole-of-life building GHG emissions. Despite the increasing policy emphasis in countries around the world on achieving net-zero emissions within specified time horizons, and setting carbon budgets for specific years, existing conventional life cycle assessment (LCA) climate change assessment methods are based on static assumptions and aggregating emissions over time. This limits their usefulness in informing timely and effective mitigation strategies. The thesis therefore addresses the challenge of aligning building sector GHG emissions with time-differentiated climate change targets and associated carbon budgets - where the aim is to support more effective decision-making for building sector decarbonisation.

The thesis is focused on two major research areas. First, it applies two time-differentiated LCA approaches - dynamic and prospective LCA - to assess the timing and evolution of GHG emissions across building life cycles. Two industry case studies are used to test different time-differentiated LCA methods: (i) a steel eccentric braced frame representing a structural system in a multi-storey building subject to temporal dynamics (such as potential seismic-related reconstruction and technology changes at end-of-life treatment), and (ii) an office fit-out case study with recurring emissions associated with interior refurbishment over the service life of a multi-tenant building. The results demonstrate that dynamic - as opposed to static - LCA provides additional insights on emission profiles over time, and in particular highlighting the relevance of early-stage mitigation actions. The case studies further show that circular economy strategies, including reuse and design for disassembly, can substantially mitigate climate change impacts, alongside prospective technological and socio-economic developments that may deliver additional progressive reductions in emissions over time. Secondly, a dynamic carbon budget allocation approach is developed for the national and building sectoral level in Aotearoa New Zealand. The method enables the quantification of annual remaining carbon budgets (RCBs) and carbon debts up to year 2050 to stay below the Paris Agreement 1.5 ºC warming levels, incorporating temporal dynamics that influence emissions over the years. It provides a range of RCBs by accounting for the influence of different sharing approaches and non-CO₂ emission global warming on the RCBs. This approach improves the robustness of dynamic RCBs and debts by: (a) avoiding reliance on a single allocation principle that may result in either overly restrictive or insufficiently ambitious mitigation pathways, and (b) addressing uncertainties associated with assumptions that future emissions will align with stringent non-CO₂ emission global warming levels.

In addition to these two main research areas, the thesis also explores alternative decarbonisation strategies for the office building sector in the context of the Construction 4.0 concept. Although the technology improvements and data management techniques promoted in Construction 4.0 offer significant potential to reduce emissions over the lifetime of buildings, research on current applications is largely limited to studying the construction phase. This research demonstrates how integrating (i) Construction 4.0 themes - such as the circular economy and digital technologies - with (ii) a whole-of-life perspective informed by time-differentiated assessment approaches provides additional insights. In particular, it highlights the need to prioritise reuse of building materials and elements in the near-term future.

The thesis contributes methodological, conceptual, and practical advancements in the practice of LCA by integrating the timing of GHG emissions into prospective climate change impact assessment informed by the Intergovernmental Panel on Climate Change (IPCC) shared socio-economic pathways (SSPs) and downscaled RCBs. This approach provides new insights into how time-differentiated LCA approaches can support the development of effective decarbonisation pathways for the building sector using Construction 4.0 strategies.