Supply-chain transmission and shared responsibility for multi-gas footprints across Chinese urban agglomerations

: Urban agglomerations connect the production, consumption, and governance of greenhouse gas emissions through interregional supply chains, yet existing studies often separate footprint accounting, emission drivers, supply-chain transmission, and responsibility sharing. This study examines 18 Chinese urban agglomerations comprising 204 cities using an integrated framework of environmentally extended multiregional input–output analysis (EE-MRIO), exact structural decomposition across all 120 factor-order permutations (SDA), structural path analysis (SPA), multi-gas emission-state diagnostics and a Shapley cooperative game. Consumption-based footprints of carbon dioxide, methane, nitrous oxide and aggregate greenhouse gases were assessed for 2012, 2015 and 2017. Per capita final demand increased emissions across all gases and periods, while the main mitigation source shifted from improvements in resource and emission intensity in 2012–2015 to changes in production structure in 2015–2017. For nitrous oxide, the −51.98% intensity effect in the earlier period was substantially offset by production restructuring and an expansion in final demand. Supply-chain pathways were gas-specific: carbon dioxide and aggregate greenhouse gases were transmitted mainly through electricity–industry–investment chains, whereas methane and nitrous oxide were more closely associated with agriculture, fuel exploitation and waste treatment. In 2017, eastern final demand induced 22.7–36.6% of the corresponding source-side footprints in western China, with the highest share for methane. Strong inter-gas coupling did not necessarily imply coordinated mitigation. Under the baseline Shapley allocation, five leading agglomerations accounted for 50.66% of total responsibility, while cross-scenario tests identified a stable four-agglomeration core and a policy-sensitive boundary. These findings support a differentiated, gas-specific and adaptive framework for shared supply-chain responsibility.