Promoting or Inhibiting? The Nonlinear Impact of Urban–Rural Integration on Carbon Emission Efficiency: Evidence from 283 Chinese Cities

In the context of global climate governance and China’s ‘Dual Carbon’ strategy, enhancing carbon emission efficiency (CEE) is a critical pathway toward high-quality development. Urban–rural integration (URI), reshaping urban–rural structures and resource allocation, has significant environmental implications. However, the mechanisms through which URI influences city-level CEE remain underexplored. Using panel data from 283 Chinese prefecture-level cities (2005–2022), we employ a Spatial Durbin Model to investigate URI’s direct and spatial spillover effects. First, spatiotemporally, URI demonstrates an imbalanced pattern, with higher levels in eastern coastal regions and lower levels in central and western areas. Conversely, CEE exhibits a north–south divide, with higher efficiency in the south. URI advancement has been sluggish with persisting imbalances, whereas CEE has demonstrated a consistent upward trend. Second, the relationship between URI and CEE is characterized by nonlinearity and spatial dependence. The direct effect follows a U-shaped curve, initially inhibiting but later promoting local CEE once a threshold is surpassed (URI = 0.103). The spatial spillover effect follows an inverted U-shaped trajectory (threshold URI = 0.179), suggesting that inter-regional dynamics evolve from synergistic promotion to potential competition. These findings underscore the necessity of phased, adaptive policies to unlock the potential between URI and CEE, providing a scientific basis for coordinating urban–rural development with carbon neutrality objectives.