In recent years, the quest for enhancing urban carbon emission efficiency (UCEE) in China has gained significant momentum, propelled by ambitious national policies geared toward carbon neutrality. While traditional methods such as decomposition analyses have illuminated regional disparities and sources contributing to UCEE variations, they fall short in capturing the intricate temporal and spatial dynamics that unfold within China’s vast urban mosaic. Addressing this gap, contemporary research has leveraged advanced statistical techniques, notably Kernel Density Estimation (KDE), to decode the evolutionary patterns of urban carbon efficiencies with unprecedented nuance and depth. This approach enables a comprehensive understanding of how cities across China’s diverse regions are navigating the complex landscape of carbon reduction efforts.
Kernel Density Estimation, a non-parametric way to estimate the probability density function of a random variable, offers insights beyond mere averages or aggregate indices. Its application to UCEE allows researchers to unravel variations in distribution location, peak morphology, tail behavior, and multimodality—each serving as a proxy for real-world phenomena such as convergence of efficiency, heterogeneity among cities, and emergent polarization. By mapping UCEE distributions across multiple years, analysts can chronicle not just where efficiency stands, but how it evolves, diverges, or converges across China’s urban clusters. This technique uncovers subtle, sometimes counterintuitive, patterns that classical statistical tools might overlook.
The nationwide perspective reveals a heartening trend: the KDE curves for UCEE exhibit a pronounced rightward shift over time. This shift signifies that most Chinese cities have progressively improved their carbon emission efficiencies, aligning well with the aspirations embedded in China’s “dual carbon” strategy, which seeks to peak carbon emissions before 2030 and achieve carbon neutrality by 2060. This broad movement towards greater efficiency reflects multifaceted advancements, including technological adoption, policy implementation, and structural economic shifts. It stands as a testament to the synergy between top-down mandates and localized innovation in addressing climate challenges.
At the regional level, the story unfolds in a more variegated manner. Eastern China and Central China mirror the nationwide progression with substantial rightward shifts in their KDE curves, indicating steady improvement across urban centers. These regions, hosting significant economic hubs and populous cities, have benefited from early access to clean technologies and extensive government support for green initiatives. The confluence of economic vitality and environmental reform initiatives has propelled urban centers there toward more sustainable growth trajectories, enhancing their carbon emission efficiencies dynamically over time.
Contrastingly, Western China initially experienced a leftward shift in its UCEE KDE curves, signaling early challenges and setbacks in carbon efficiency improvements. This countertrend may be attributed to its developmental stages skewed towards resource-intensive industries and infrastructural constraints. Nevertheless, a notable rebound occurs post-2015, with KDE curves exhibiting a rightward trajectory thereafter. This recovery suggests emerging capabilities in adopting cleaner technologies and optimizing energy use, driven by increasing governmental focus on less-developed regions and tailored interventions that address local structural impediments to efficiency.
Northeastern China occupies a more complex and challenging position in this spatial narrative. Its KDE curves have shifted leftward overall, underscoring persistent difficulties in elevating carbon emission efficiency. The region’s industrial legacy, heavily reliant on heavy industries and coal power, appears to have stymied rapid progress. Here, the struggle reflects not only technological and economic inertia but also social and policy complexities, including labor market rigidities and diversified economic structures. The area demands targeted strategies and investments to break through these entrenched barriers and catalyze sustainable urban transformations.
Beyond mere shifts in distribution locations, the KDE analysis illuminates changes in the shape and breadth of primary peaks—indicative of UCEE disparities across cities. Nationwide and within Eastern and Central China, the heightening and broadening peaks signal a convergence trend, wherein disparities among urban carbon efficiencies are diminishing. Such homogeneity hints at the diffusion of best practices and uniformly elevated performance standards across municipalities, fostering a more balanced environmental landscape. These dynamics suggest that policy instruments and market mechanisms are successfully compressing the efficiency gap among cities, promoting inclusivity in environmental progress.
In Western China, the primary peak dynamics are initially characterized by a dip followed by recovery, paralleling its earlier mentioned spatial shifts. This pattern reflects a complex transition phase where UCEE disparities widen temporarily amid structural adjustments before narrowing as new efficiencies take hold. The peaks’ eventual upward movement post-2015 indicates stabilizing and consolidating efficiency gains, emblematic of regional maturation in carbon management practices. Such nuanced insights elevate understanding beyond aggregate metrics, capturing the turbulence and resilience inherent in developmental shifts.
The situation in Northeastern China starkly diverges, with main peaks declining and broadening over time. This phenomenon translates to an expanding divergence among urban centers regarding carbon efficiencies, reflecting uneven adoption of technologies and policy measures. The growing breadth of the peak suggests increasing variation in cities’ capabilities and commitments to carbon reduction. This outcome flags critical concerns, as it may exacerbate socio-economic imbalances and weaken overall policy effectiveness, demanding differentiated support and innovation diffusion mechanisms tailored to the unique challenges of the region.
Another salient feature extracted from KDE curves is ductility—the skewness and tail properties that shed light on the distribution’s extremities. Across China, UCEE distributions exhibit right-skewed tails, indicating the existence of cities markedly outperforming the average in reducing carbon emissions. Nationally, as well as in Eastern and Central China, the tapering of ductility suggests a shrinking divide between the leading cities and the average urban centers, reinforcing the notion of convergence and disseminated excellence in efficiency practices. This movement toward reduced skewness implies more widespread adoption of cutting-edge technologies and efficient energy usage.
However, Western and Northeastern China paint a contrasting picture, with ductility trends diverging rather than converging. Here, persistent low-efficiency cities remain, entrenching disparities in UCEE performance. The long tails denote entrenched outliers with underwhelming progress, signaling systemic challenges. These persistent gaps underline the critical need for strategic interventions focusing on capacity building, infrastructure enhancement, and economic diversification to raise the baseline for lagging cities. Addressing these divergences is crucial for fostering equitable and nationwide transitions towards low-carbon urban development.
The multimodality or number of peaks in KDE distributions further illuminates the spatial carbon efficiency landscape. Nationally, as well as in Eastern and Central China, the UCEE distributions maintain a unimodal profile, reinforcing the narrative of increasing homogeneity and low polarization among cities. This uniformity indicates that most cities reside within a similar efficiency band, reflecting balanced regional development and effectiveness of universal policy measures. Such stability bodes well for cohesive national progress and easier policy calibration without significant regional fracturing.
In contrast, Western China’s KDE curve shows dynamic changes—from dual peaks to a multi-peak configuration—capturing the region’s transition from polarization to a more diversified efficiency spectrum. This morphing suggests that while disparities remain, there is now a broader distribution of cities attaining varying degrees of efficiency success. Such diversification signals complex regional heterogeneity, possibly influenced by localized innovation hubs or discrepancies in industrial structures, mandating nuanced policy designs attuned to local contexts. It hints at emerging sub-regional clusters with distinct carbon management profiles.
Northeastern China’s evolution from single to dual peaks marks growing polarization in urban carbon emission efficiency within the region. The widening gap between peaks reflects bifurcation into notably efficient and inefficient clusters. This spatial polarization poses challenges for regional cohesion and complicates uniform policy implementation. It may exacerbate socio-economic divisions and environmental inequalities, necessitating concerted regional planning and resource allocation strategies that bridge the emerging gaps and foster integrative urban development aligned with carbon goals.
The implications drawn from the KDE-based spatiotemporal analysis resonate deeply with China’s environmental roadmap. The differentiated regional trajectories emphasize that while national momentum is positive and promising, the heterogeneous progress underscores the importance of tailored strategies. Policymakers must embrace region-specific frameworks that address distinctive economic structures, technological capabilities, and socio-political factors to harmonize efficiency gains. Equally, fostering knowledge spillovers between regions can drive overall national advancement, with leading cities serving as incubators and mentors for lagging counterparts.
Furthermore, the revealed convergence trends in Eastern and Central China highlight the success of coordinated policy interventions, technological dissemination, and infrastructural investments. These findings advocate for reinforcing mechanisms that promote cross-city collaboration, incentivize innovation, and ensure equitable access to green technologies. Conversely, the insights on persistent disparities and polarization in Western and Northeastern China call for comprehensive support, encompassing financial, technical, and institutional dimensions. Bridging these divides will be vital for realizing China’s broader climate ambitions and promoting social equity.
These sophisticated KDE analyses contribute valuable methodological advancements in environmental economics and urban studies. By transcending static snapshot statistics and embracing dynamic distributional characteristics, researchers and practitioners gain enriched perspectives on carbon efficiency landscapes. This granular understanding lays the groundwork for adaptive policies responsive to evolving urban realities, facilitating more effective and resilient climate governance within complex and heterogeneous urban systems.
As China accelerates its urbanization and simultaneously battles the climate crisis, integrating such nuanced analytical tools becomes indispensable. The elucidation of temporal and spatial evolution patterns informs targeted investment, capacity-building initiatives, and technological deployment strategies. It also enables real-time monitoring of policy impacts and early identification of emerging carbon hotspots or laggard cities, thereby supporting agile, evidence-based decision-making essential for sustainable urban futures.
In sum, the deployment of Kernel Density Estimation to unravel the dynamic evolution of urban carbon emission efficiency reveals a rich tapestry of progress, challenges, and regional divergences across China. It illuminates paths to convergence and flags areas of persistent disparity, guiding informed policymaking and strategic planning. This research not only enriches the academic discourse but also equips stakeholders with actionable insights critical to steering China toward a low-carbon, sustainable urban era.
Subject of Research: The spatiotemporal evolution and convergence patterns of urban carbon emission efficiency (UCEE) across China’s nationwide urban areas and its major regions.
Article Title: Spatiotemporal evolution and convergence patterns of urban carbon emission efficiency in China.
Article References:
Wu, C., Deng, H., Zhao, H. et al. Spatiotemporal evolution and convergence patterns of urban carbon emission efficiency in China. Humanit Soc Sci Commun 12, 675 (2025). https://doi.org/10.1057/s41599-025-04916-7
Image Credits: AI Generated
