Mitigating Hidden Climate Change Impacts of Timber Cities Critically Depends on Proactive Forest and Waste Management

Decarbonization options for buildings include using low-carbon cement and engineered timber. However, the long-term cumulative effects of urbanization, destinations of building materials at end-of-life, CO₂ uptake from cement carbonation, and biogenic sequestration from biomass regrowth on their climate change impacts remain unclear. Here, we assess the climate change impacts of these dynamic factors on future urban buildings for urban growth between 2025 and 2100, using dynamic life cycle assessment across 14 pathways under various short- and long-term scenarios. Construction of urban buildings using timber ('timber cities') can lead to a global temperature increase that is up to 0.023 K lower by 2100 than that caused by their construction using reinforced concrete ('reinforced concrete cities'). After 2100, timber cities can lead to a temperature increase similar to or higher than reinforced concrete cities if there is poor forest regrowth, high landfill gas release, and incineration. If timber recycling leads to forest aging or deforestation due to reduced motivation for forest regrowth, global temperature can significantly rise compared to a scenario in which timber is recycled while simultaneously maintaining the forest carbon sink, which is the most climate-friendly option. Important global actions to minimize the climate impacts of future cities are (1) to support rapid and large-scale implementation of timber buildings in response to current high urbanization; (2) to proactively develop land, forest, and waste policies that limit future temperature increases caused by poor forest regrowth, landfill gas release, and wood incineration; and (3) to adopt dynamic life cycle assessment and related indicators such as absolute global warming potential in the built environment for climate-related policymaking, rather than using only global warming potential.