The built environment emits 12 GtCO2-eq. annually, accounting for 21% of global greenhouse gas emissions, with steel and cement production alone causing 2,16 GtCO2-eq. While significant progress has been made in providing renewable energy for the use phase, the focus now shifts to embodied emissions and the resource consumption of building components and materials. About 28% of the world´s available resources are consumed by the built environment. In view of increasing resource scarcity and shrinking carbon budgets, measures to promote the circular economy are crucial in order to reduce absolute material consumption, take appropriate account of environmental impacts and retain building components with high environmental invest.
Life Cycle Assessment (LCA) is the established method for assessing the environmental impact of products and processes, but lacks suitable measures to evaluate emissions over time and in order to account for the building´s age. A time horizon of 50 years is usually set in LCA of buildings. Since a fixed time horizon equals to a cut-off, it can be considered as an infinitely high discount in year 50. This frequently leads to the environmental impact in year 51 being neglected, which is inappropriate regarding environmental valuation.
This paper introduces and tests a time-dependent accounting method combining LCA and the Bern Carbon Cycle Model (BCCM). The concept bridges the gap between LCA and economic valuation by introducing a method that incorporates time in LCA and internalizes long-term environmental impacts. It enables a genuine environmental comparison between the adaptive reuse of existing buildings and demolition and new construction, taking into account the building´s age and the existing embodied emissions. The BCCM computes a regressive function of an initial impulse – the environmental investment in a building – with a remaining fraction in the atmosphere of about 40% after 100 years and 24% after several centuries.