Another strand of research uses the term urban metabolism rather metaphorically. Thesestudies employconcepts and methods from political science, sociology, social geography or ethnography but usually do not aim at quantifying the biophysical processes at the core of SMR46,47; for a recent review see

Urban metabolismstudies focus onmaterial and energy flows within urban systems,accumulation of material stocks,and theexchange processes of urban areas with theirhinterlands. This tradition waspioneered among others by AbelWolmanand Stephen Boyden(Fig2)36,37, and indeed avant la lettreby Heinrich von Thünen3

Urban Metabolism 40(UM) is an interdisciplinary concept that studies how cities interact with the environment and more 41specifically how they use resources and emit pollution flows as well as the associated societal, 42economic and environmental challenges around these flows.