Carbon cycle of urban system: Characteristics, mechanism and theoretical framework

Abstract

Since 1990's, researches on carbon cycle were mostly focused on natural processes, such as farmland ecosystem, forest ecosystem and soil, but the anthropogenic carbon process in urban system was always ignored. Presently, under the pressure of climate change and international carbon emission reduction, how to coordinate the relationship between economic development and urban environmental protection, promote carbon emission reduction and develop low-carbon economy has been a hot multi-disciplinary topic. Urban system is an ecosystem that is profoundly impacted by human activities on the earth, in which land use and land cover changes drastically. With much human energy consumption and carbon emission, urban carbon cycle will inevitably have a profound impact on global carbon cycle and climate change. So, urban carbon cycle plays an important role in regional and global carbon cycle processes. Research on urban carbon cycle is not only the important basis for establishing low-carbon urban strategies, but also the theoretical reference for choosing low-carbon city pattern. Firstly, the concept of "natural-social dualistic carbon cycle" of urban system was put forward and analyzed, and the circulation process map of urban system was established. It shows that the carbon cycle process of urban system was quite different from that of natural ecosystem. Further, the main characteristics of urban carbon cycle were discussed in this paper. Generally, urban system is a complex system that includes natural and anthropogenic processes, horizontal and vertical processes, surface and underground processes, economic and social processes. Secondly, the mechanism of carbon cycle in urban system was discussed from several aspects, such as: the main processes of carbon storage and carbon input & output fluxes, life cycle of carbon storage, carbon fluxes and carbon circulation processes, carbon input & output types of urban system, etc. In this section, the processes of carbon storage and carbon fluxes of urban system were emphasized in details for better understanding the process of urban carbon cycle. The life cycle analysis of carbon storage, carbon flux and carbon circulation processes gave us a clear idea on how to control the important carbon emission reduction sections. Thirdly, the research framework of carbon cycle in urban system was established according to systematic division and carbon circulation process analysis. Based on the analysis of carbon processes in urban system, the urban system was divided into several social-economic subsystems (such as industrial, commercial, residential, transportation and office subsystems) and natural subsystems (such as water body, farmland, forest, pasture land and urban greenbelt, etc.). Furthermore, the carbon circulation processes between urban system and external system, and the carbon processes between urban natural ecosystems and social-economic systems were analyzed, and the theoretical framework of urban carbon cycle research was established. Finally, the future research trends of urban carbon cycle were put forward, such as: building systematic urban carbon cycle model, establishing urban carbon emission factor database and calculation criterion, promoting contrast studies on carbon cycle and circulation mode of cities under different developing levels, strengthening control and management of key processes in carbon cycle of urban system.