Spatial spillovers and nonlinear drivers of water-supply carbon emissions in China

Water supply systems are critical components of urban infrastructure and significant contributors to global carbon emissions. These systems face an emerging challenge in balancing the increasing demands of water security with international climate mitigation goals. To combat water scarcity, many regions have transitioned toward energy-intensive water sources such as inter-basin water transfer and desalination, which significantly increase electricity-dependent indirect emissions. Concurrently, the global shift toward clean energy in electricity generation has provided a crucial mechanism for mitigating these emissions. However, the complex interactions among shifting water-source mixes, energy transitions, and socioeconomic drivers remain poorly understood, often obscuring the effectiveness of decarbonization strategies. Existing quantification frameworks frequently overlook the spatial spillover effects of economic development and the risk that new water security strategies will offset decarbonization gains. Here we show that China's carbon emissions from water-supply processes rose to 228 Mt CO₂ yr^-1 by 2022, despite initial declines driven by clean energy expansion. Using a three-stage quantification-decomposition-attribution framework, we find that while economic development generally suppresses emission in neighboring regions via technology diffusion, it exhibits a national U-shaped relationship with carbon output. Crucially, central China displays an inverted U-shaped pattern, suggesting a localized risk of high-carbon lock-in as industries and water demands shift. These findings reveal a critical paradox in the water-energy-carbon nexus where water security measures may inadvertently undermine climate targets. Our results advocate for integrated regional governance and differentiated policy interventions to safeguard both water and climate stability in rapidly developing regions.