Demographic transition and environmental governance are the most prominent focal points of global concern in the 21st century. We quantitatively evaluate the distinct carbon emission impacts of the global demographic transition by regression models based on C–D production function. Our study highlights that while demographic transition boosts per capita economic growth, it adversely affects overall economic output and aggregate economic growth, highlighting significant reductions in carbon emissions as a result of these demographic changes. However, it has a negative contribution to overall economic output and economic growth. Meanwhile, demographic transition eventually reduces carbon emissions to some extent. The relationship between population urbanization and carbon emissions mainly conforms to an inverted U-shaped curve, while some of it does to a linear growth pattern. However, the relationship between population aging and carbon emissions mainly conforms to an inverted U-shaped pattern. The impacts of demographic transition on carbon emissions confirm the universality of the EKC law in the particular production function.

China’s rapid urbanization has spurred economic growth and posed environmental challenges. We investigate the relationship between multidimensional urbanization and environmental pollution by a fixed effect model based on the panel data of 70 cities in three major urban agglomerations in the Yangtze River Economic Belt during 2005–2020. Overall, environmental pollution aggravates and then decreases in three major urban agglomerations, which is closely related to China’s environmental pollution control policies. Environmental pollution shows obvious spatial heterogeneities by five levels in three major urban agglomerations, which have been gradually changed from high-value levels to low-value levels. In the Yangtze River Economic Belt, environmental pollution is dominated by “High–High” and “Low–Low” clusters. Among them, “High–High” clusters move eastwards, while “Low–Low” clusters gradually concentrate southeastwards. Multidimensional urbanization helps to alleviate regional environmental pollution. Economic, social, and land urbanization reduces environmental pollution in three urban agglomerations to a certain extent. Population urbanization has a non-significant effect on environmental pollution. Social urbanization has improved environmental pollution in the Chengdu–Chongqing urban agglomeration (CC) and the Yangtze River Delta (YRD), which is increased by population urbanization in the middle reaches of the Yangtze River (MYR) and is improved by comprehensive urbanization in CC.