From Co-Evolution to Conversion: Reassessing the Digital–Green Nexus for Urban Sustainability in Major Chinese Cities

Digitalization and green development have advanced together across major Chinese cities, but aggregate co-movement does not necessarily imply a robust positive association within cities. Using a balanced panel of 100 major Chinese cities (2012–2023), we construct multidimensional digitalization (DIG) and green development (GDI) indices and combine spatial and distributional analysis with a common five-specification comparison, covariance decomposition, and robustness checks. Both index means increase, while DIG dispersion widens and GDI dispersion narrows. The correlation between the annual mean DIG and GDI is 0.991 but falls to 0.517 after linear detrending, and to 0.130 for annual changes. Persistent city differences and common-year movement account for 48.04% and 51.67% of pooled covariance, respectively, whereas the two-way residual accounts for 0.30%. The two-way fixed-effects estimate is negative but statistically imprecise (β = −0.181, p = 0.150). Dimension-specific and alternative weight estimates are not uniform, cautioning against interpreting the composite result as evidence that every environmental outcome responds identically. We describe this pattern as a co-evolution–conversion gap: strong system-level co-movement coexists with weak or uncertain average within-city alignment. The findings support assessing energy, pollution, and ecological outcomes directly rather than inferring green performance from digital capability alone. Urban policymakers should evaluate digital investment alongside direct measures of energy use, emissions, pollution, and ecological amenities, while tailoring complementary actions to local industrial and energy conditions.