The Nexus of Urbanization and Carbon Dioxide Emissions Revisited in China: Scale, Distribution, and Urban Form 

Abstract

Urbanization-driven climate change has become a major, pressing issue threatening not only human survival and development but also entire ecosystems. Although many studies have considered the correlation between urbanization and urban CO₂ emissions, they have not reached a consensus on whether the process of urbanization helps or hinders the reduction of urban CO₂ emissions. Thus, in the context of climate change, the impacts of urbanization on the future atmospheric environments of cities throughout the world remain unknown. There are three related challenges. The first challenge lies in the definition of the term “city/urban”, the geographical areas studied to ascertain levels of urbanization and quantities of aggregated urban CO₂ emissions. The second challenge is that the accounting methods and scopes of urban CO₂ emissions applied in related studies are highly inconsistent, resulting in incomparable conclusions on the urbanization–emission nexus. Third, an unfilled challenge for theories and research on the urbanization–emission nexus is to illustrate how the city-size distribution in a national economy impacts the environment. These three challenges call for a comprehensive analysis of the urbanization–emission nexus, allowing us to capture the impact of urbanization on the environment in multiple perspectives.

In response to this call, this thesis proposes an analytical framework for the urbanization–emission nexus. It first emphasizes the necessity of measuring urban areas accurately to properly understand urban CO₂ emissions. Second, this thesis highlights the importance of robust, local-level urban emissions data and comprehensive analysis of sectoral urban emissions. Third, it proposes a multi-perspective analysis of the urbanization–emission nexus, focusing on urban scale, form, and city-size distribution. This thesis uses China as a case study in which to apply the proposed analytical framework.

As the world’s largest CO₂ emitter and second largest economy, China’s significant urbanization potential will probably shape future global CO₂ emissions. To assist China in reconciling the realities of its ambitious goals in urbanization and climate change, this thesis assesses China’s urbanization–emission nexus through four empirical studies. It first benchmarks urban CO₂ emissions in China at the city level, with the goal of understanding such emissions and their heterogenous characteristics. This thesis then examines the influence of urbanization on urban CO₂ emissions and investigates how to reconcile the demand for increased urbanization with the need for massive urban CO₂ emission reductions in China by responding to three questions. (1) Are large cities more efficient in urban CO₂ emissions? (2) What urban form is most conducive to urban CO₂ emission reductions? (3) Should urban populations be concentrated in a few cities or dispersed across a large number of cities to best promote urban CO₂ emission reduction?

The following important findings are obtained from this thesis. First, basing its analysis on comparable international definitions of “urban,” the study finds that urban CO₂ emissions account for roughly 62% of total CO₂ emissions in China, to which the industrial sector (56%) and indirect sector (36%) are major contributors. Furthermore, Chinese cities display great heterogeneity in their urban CO₂ emission structures. Therefore, such cities’ decarbonization must be rated against benchmarks that account for this heterogeneity. Second, a linear scaling effect of urban scale on urban CO₂ emissions is observed. That is, large cities are not more efficient in urban CO₂ emissions than small cities. Third, the decentralized and integral urban form is most conducive to urban CO₂ emission reductions. Therefore, instead of relying on centralized urban form or fragmented urban sprawl, the urbanization process should advocate decentralized urban development and moderate urban sizes to shape a large dominant urban center with several sub-areas. Fourth, urban CO₂ emissions in most Chinese cities are higher than those associated with current city development levels in the normal state. Although large-sized emitting cities produce a large amount of urban emissions, their emissions are proportionately undersized. However, medium-sized emitting cities emit proportionately oversized urban CO₂ emissions, revealing that low-carbon regulations and incentives should be targeted mainly at medium-sized emitting cities and assist them to reach their full reduction potential.

This thesis makes significant contributions to both theoretical and empirical low-carbon urbanization studies and practice. In terms of theoretical and academic contributions, this thesis first enriches existing theories by considering city-size distribution in the urbanization–environment nexus. Second, it benchmarks the influence of urbanization on urban CO₂ emissions based on a definition of “urban” comparable to those used internationally, thereby adding valuable perceptions about China’s urban emissions and its urbanization–emission nexus. Third, this thesis clarifies the scope of urban emissions accounting and investigates the effects of urbanization on various sectoral urban emissions.

This thesis provides significant implications for avoiding the high-carbon risks of China’s urbanization and pursuing a low-carbon growth path. First, the thesis highlights the disconnections between urban and administrative cities in urbanization measures and urban CO₂ emissions accounting. It underlines the need to incorporate urban data into urbanization planning and CO₂ emissions management. Particular, the compiling of robust and comprehensive urban emissions data should be added to the CO₂ emission reduction agenda so that local governments can understand their emission statuses comprehensively and design reduction policies accordingly. In addition, the thesis investigates the heterogeneous mechanisms of the influences of urbanization on sectoral CO₂ emissions, which allows for identifying where emissions from, and piloting deep decarbonization pathways. Second, this thesis considers urbanization policy as a tool to attain urban CO₂ emission-reduction targets, thus filling the gaps in policy implementation to better align urbanization policy context with trends in climate change. The thesis suggests that population and land policies may be used as tools to control urban scale and city-size distribution. It also implies that spatial optimization and urban planning techniques are instrumental to the creation of low-carbon urban forms. Finally, the findings from this thesis show that urban agglomerations should be built and advanced as spatial units in CO₂ reduction management based on three reasons. First, urban agglomerations can reach the required scale (φ=1.2*10^13) for the economies of scale to occur. Second, urban agglomerations can lead to decentralized urban forms. Third, urban agglomerations could effectively drive the development of small and medium cities, assisting them to reach their full potential through coordination development and growth.

The above implications may be generalizable beyond China. That is, a better understanding of China’s low-carbon development could help the global community formulate climate change mitigation policies. Moreover, as low-carbon development is a common task and urbanization is a priority for developing countries, China’s case offers relevant references for developing countries seeking to reconcile the demand for increased urbanization with the responsibility to attain massive CO₂ emission reductions.

Date of Award	23 Aug 2022
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Hon S CHAN (Supervisor) & Ning LIU (Co-supervisor)