减污降碳视角下空间密度的环境效应——突破行政单元的再讨论

Environmental Effect of Spatial Density from the Perspective of Reducing Pollution and Carbon:A Reassessment Beyond Administrative Units

优化人口空间布局、协同推进减污和降碳,是实现中国式现代化的重要内容。本文突破行政单元约束,以超过一定人口密度阈值的网格单元作为空间观测对象,并构建2000—2020年中国19大城市群栅格数据集,精准探讨空间密度变化对减污降碳的影响及作用机制。研究发现,网格单元空间密度对PM2.5与人均碳排放的影响均呈“倒U”型趋势,拐点值分别为3718人/km2和4128人/km2。这主要受四大作用机制影响,即城市用地变迁、生态空间演变、生产方式优化和交通设施变化。进一步分析空间密度的规模效应与结构效应表明,规模效应的主导地位造就了密度与环境的非线性关系,这为相关研究争论提供了新的解释。本文也为如何从空间布局层面推动减污降碳协同增效提供了政策启示。

Optimizing the spatial distribution of the population and collaboratively advancing the reduction of pollution and carbon are essential components of achieving Chinese modernization.As various production factors gradually transcend the spatial constraints of individual urban administrative units to cluster,flow,and integrate on a larger scale,urban agglomerations are becoming the regions with the highest spatial density in China.How to move beyond the conventional perspective of administrative units to more accurately identify the environmental effect of spatial density,thereby seeking a win-win path for pollution and carbon reduction,is a major theoretical and practical issue that urgently needs exploration.This paper transcends the constraints of administrative units by using grid units with a population density exceeding 100 persons/km2 as spatial observation units.It constructs a gridded dataset for 19 major urban agglomerations in China from 2000 to 2020 to precisely explore the impact of spatial density changes on pollution and carbon reduction and the underlying mechanisms.The findings are in four aspects.Firstly,the impact of spatial density in grid cells on PM2.5 and per capita carbon emissions is nonlinear,following an“inverted U”shape,with turning points at 3718 persons/km2 and 4128 persons/km2,respectively.Secondly,the nonlinear impact of spatial density is driven by four mechanisms:urban land change,ecological spatial evolution,production mode optimization,and transportation infrastructure change.In particular,the increase in spatial density leads to a rise and then a fall in urban construction land,resulting in an“inverted Ushaped”curve for pollutants and carbon emissions from the construction sector.It also causes ecological spaces to shrink initially and then recover,thereby creating a U-shaped pattern for a city’s ecological self-cleaning and carbon reduction capabilities.Additionally,the higher spatial density initially promotes rough production methods which then become optimized,causing pollution and carbon emissions from the production sector to follow an“inverted U-shaped”curve.The increased spatial density also initially causes transportation infrastructure to expand and then contract,producing an“inverted U-shaped”curve for pollution and carbon emissions from the transportation infrastructure sector.Thirdly,there is heterogeneity in the environmental effect of spatial density.In terms of development stages,mature urban agglomerations exhibit more pronounced“dual turning point”characteristics in the impact of spatial density on PM2.5 and per capita carbon emissions,with lower turning points compared to developing and emerging agglomerations.Fourthly,the increase in spatial density involves both the“scale effect”and the“structure effect”,with the dominance of the scale effect creating a nonlinear relationship between spatial density and the environment.This paper proposes several policy suggestions.Firstly,density-based zoning control is implemented to promote coordinated environmental governance.Due to the differences in spatial density distribution,the conventional“one-size-fitsall”approach to urban spatial management is insufficient for accurately addressing pollution reduction and carbon mitigation issues across varying spatial densities.Drawing from the experience of cities like Shenzhen,Dongguan,and Hong Kong,it is necessary to establish differentiated density zoning management mechanisms.Secondly,for urban agglomerations at different stages of development such as mature,developing,and emerging,categorizing and formulating spatial density optimization strategies and priority lists for pollution and carbon reduction are essential.Thirdly,we should focus on key areas such as urban-rural development,ecological construction,industrial structure,and transportation infrastructure to achieve synergistic efficiency in pollution and carbon reduction.Fourthly,we should promote simultaneous regulation of urban scale and structural adjustment.By delineating urban growth boundaries,it is possible to promote efficient concentration of population,capital,technology,and information within established spatial units towards core areas.This paper’s primary innovation lies in using grid units within urban agglomerations with specific density thresholds as spatial research objects,rather than administrative units,to accurately reflect the scope and intensity of actual economic activities.In contrast,previous studies based on administrative units such as provinces,prefecture-level cities,and counties often include vast areas of low population density and non-urban development zones,potentially expanding the space for real economic activities,thereby affecting the accuracy of results.Another innovation is that this paper simultaneously explores the effects of spatial density changes on environmental pollution and carbon emissions at different stages.Particularly,this paper holds certain value for future research extensions:it reveals the dimensional differences in spatial density metrics,which can enrich studies on density;the constructed data architecture primarily using remote sensing grids and combining micro and macro perspectives is advantageous for improving the precision of future regional and urban studies.