Carbon Dioxide (CO2) Emissions in Urban China: Process, Trend and Impact

Urbanization is undoubtedly one of the most significant anthropogenic forces affecting global carbon cycle. Carbon storage and release through anthropogenic (e.g. energy consumption, building, waste) and natural components (e.g. urban vegetation and soil) are intrinsically coupled in urban areas. Both anthropogenic and natural components are equally important for understanding the carbon cycle in urban areas and have to be considered simultaneously. Present studies however mostly one-sided and primarily focus on anthropogenic emissions. Given the substantial scientific gaps, this study aims to build better knowledge on the contributions of urban areas to the increasing atmosphere CO2 emissions at an urban scale, considering both anthropogenic and natural components simultaneously.;First, a process-based definition of urban areas is proposed to capture the inherent dynamics of urban areas, and a threshold technique is developed to map the defined urban areas in this study. Multi-sensor remotely sensed data are used to analyze the dynamic urbanization and related land use/cover conversions. Overall, urban areas have increased by 3.8 times over the studied period of 1985-2010. Croplands and forests are the major sources of the growing urban areas.;Second, taking calibrated nighttime light imagery as a proxy variable, we develop a top-down model to estimate fossil fuel CO2 emissions on the urban scale. Driven by the rapid urbanization in China, the contributions of urban areas to the CO2 emissions have increased substantially. In contrast to the developed counties, per capita CO2 emissions in urban China are higher than the national average, due to higher income, change in lifestyle and easy access to electricity, whereas per capita CO 2 emissions in eastern China is lower than that in western China, due to the diverse scale and structure of local economy. Our analysis also reveals that the booming economy and urbanization are major drivers of the increasing fossil fuel CO2 emissions, while the decoupling effect of energy efficiency reverses in the post-2000 period caused by the booming economy. It is foreseeable that economic reconstruction and energy structure would play a significant impact on carbon reduction if stricter environmental targets are released.;Third, carbon storage and change in natural components of urban areas, in particular, urban vegetation and soils, are also estimated in this study. A stock-change method is applied in this study. This study identifies that the amount of carbon storage in urban areas is comparable to that emitted from fossil fuel burning, and urban soils are the major carbon pools in urban areas. Along with urban expansions, urban vegetation becomes sources of carbon due to loss of biomass, whereas urban soils act as sinks of carbon because increasing urban areas enhance the carbon storage in them. Given the foreseeable urbanization in China, our study has implications for urban managers to enhance carbon storage through urban vegetation and soils, hence offsetting CO 2 emissions from fossil fuel burning.;Finally, a local temperature response to the increasing CO2 in urban areas is analyzed by local Granger causality test. Bidirectional Granger causality presents between surface air temperature and carbon variables in less urbanized regions of China. In the rapid urbanization areas such as eastern coastal regions, only presents the Granger causality from surface air temperature to the fossil fuel CO2 emissions. This is the first attempt to offer insights of local temperature variables response to the increasing CO2 across urban China. Our integrated results are novel in exploring the contributions of expanding urban areas to CO2 emissions across China, including anthropogenic and natural components of urban areas simultaneously. We believe that our findings have clear significance for local governments who strive for constructing low-carbon cities.