Nonlinear mechanisms of CO2 emissions in growing and shrinking cities: an empirical study on integrated effects of aging and industrial structure in Japan

For the first time, this research aims to reveal how population age changes the nexus of industrial structure upgrading-carbon emissions under urban growth and shrinkage using a panel threshold model. Population aging, industrial structure upgrading, and CO2 emission mitigation represent three main challenges for urban development globally. However, only a few studies have integrated these challenges under the same framework, especially in the context of differentiated urban development patterns. Taking Japan as an example, urban development patterns are classified into four types using urban population and population density data from 2010 to 2020, including growth type, potential shrinking type, smart shrinking type, and continuous shrinking type. Each type's CO2 emission patterns regarding energy consumption are then examined. After controlling for several socioeconomic and urban land scale variables, population aging and industry structure were used as the threshold variable and the key explanatory variable in the panel threshold model, respectively. The results showed that growing cities have lower carbon than any of the shrinking cities in terms of CO2 emissions per capita. A single-threshold effect of population aging was detected that changes the nexus of industrial structure upgrading-CO2 emissions in potentially shrinking cities and continuously shrinking cities, while no such nonlinear effect was detected in the growing group. The mitigating effect of industrial structure upgrading on CO2 emissions per capita increases as the aging level crosses 23.71% in potentially shrinking cities. In contrast, the promoting effect of industrial structure upgrading on CO2 emissions per capita weakens as the aging level crosses 20.27% in continuously shrinking cities. This study implies a nonlinear mechanism of CO2 emissions considering the integrated effects of aging and industrial structure, contributing to developing a rational and efficient emission reduction strategy considering differentiated urban development patterns.