QUANTIFYING THE SUNK CARBON COSTS OF CITIES: A case study of 50 years of construction in Odense, Denmark

Book chapter, 2023

Constructing buildings and infrastructure in cities generates immense emissions of carbon dioxide. Understanding these emissions can aid in low-carbon urban planning and development. Current efforts to estimate these emissions use the Carbon Replacement Value method, which calculates the carbon cost of replacing the urban stock using current technologies. The CRV is useful for forecasting the carbon costs of near-term urban development. However, the perspective of historical emissions from technologies from the time of construction is missing. Quantifying historical emissions would improve our understanding of the embodied carbon spent to build the cities we live in today. This study proposes a methodology to estimate the Sunk Carbon Cost of urban material stocks. This method combines urban material cadastral maps with temporally dynamic life cycle assessment (LCA) of previous construction technologies to quantify and spatialize the historical carbon investment in the urban built form. We demonstrate the method by estimating the historical carbon emissions from concrete in all construction since 1961 (~105,000 individual buildings) in the city of Odense, Denmark. We estimate that 20.2 Mt of concrete was used in these buildings which released 3.0 Mt of carbon dioxide. We find that the sunk carbon cost for all construction since 1961 may be up to ~36% greater than estimates using carbon replacement value, but that these differences decrease for newer buildings. The spatial analysis identifies low-density settlement patterns as spatial hotspots of historical emissions, highlighting the material and carbon efficiencies of high-density neighborhoods. Applying this method to a city’s entire material stock can identify hotspots of embodied emissions across the urban fabric and suggest how to best utilize spent carbon through the adaptive reuse of existing buildings and low-carbon urban design.