Urban Metabolic Flow in China’s Megacities Doubled by Material Stock Accumulation Since the 21st Century

Buildings, infrastructure, and durable goods play a critical role in urbanization, akin to bones and muscles that structure the human body. These stocks contribute to the exploitation of over half of the world’s resources and offer potential “urban mining” sources. However, the process of resource transformation regarding urban material stock growth and material flow alteration remains unclear. The metaphor of urban metabolism provides a new perspective to dissect this process, but current studies often spotlight only specific fragments, such as certain end-use types or materials. This study bridges this gap by establishing a comprehensive level-to-level analysis of urban “bone-muscle” metabolism in China’s megacities. This study presents a comprehensive analysis of urban metabolism in China’s megacities, tracking the lifecycle of material stock across over a hundred distinct end-use types and 12 categories of materials. Results indicate that annual metabolic flow in these cities has doubled since the early 21st century, reaching 264–737 Mt in 2018, with manufacturing, construction, and transportation as primary drivers. As accumulation intensifies, the material stock’s growth rate diminishes logarithmically, hinting at increased efficiency and a move towards a steady state. Concurrently, scrap flow is on the rise. Driven by population growth, per capita scrap is projected to reach 2.0–4.7 t/cap by 2035, and material stock is expected to rise 1.4–2 fold. Proactive population planning and coordinated development strategies can mitigate the risks associated with this growth and maintain urban system stability.