Carbon emissions in the life cycle of urban building system in China—A case study of residential buildings
Introduction
The global climate system is being affected by the emissions of greenhouse gases from human development, especially urbanization activities, of which the most significant is carbon dioxide (CO2) (Prato, 2008). Many countries have set long-term goals for CO2 emission reduction (IPCC, 2007). It is widely accepted that human settlements occupy a small proportion of landmass, while playing a significant role in the change of global carbon cycle (Potere and Schneider, 2007). Accurate analysis of urban carbon cycle and its interactions with other global or regional ecosystems is going to be crucial for predictions of future trajectories of atmospheric CO2 concentrations and global climate change. Recent studies have focused on the carbon fluxes of different land type, such as forest (Churkina et al., 2003), grassland (Torssell et al., 2007) and cropland (West and Marland, 2002, Chakraborty et al., 2006), while neglecting carbon fluxes of urban areas or urbanization regions. As one of the main artificial constructions, urban buildings consume various natural resources, including energy, minerals and water, and release many kinds of pollutants or wastes back to the global/regional ecosystems. These inputs and outputs of mass and energy as a kind of metabolism result in global change presented by air pollution and huge amount of carbon emissions. According to data from the Worldwatch Institute, building sector annually consumes 40% of the stone, sand and gravel, 25% of the timber (Arena and Rosa, 2003). In the member nations of the European Union, buildings consume approximately 50% of the total energy use and contribute almost 50% of the CO2 emissions released to the atmosphere through their life cycles (Dimoudi and Tompa, 2008, Pataki et al., 2009). Besides, compared to other sectors, there is the greatest potential of CO2 emission reduction in the construction sector at a relative low expense, reaching 5.3–6.7 Gt CO2 per year (IPCC, 2007), which makes the studies on CO2 emission reduction from buildings be a hotspot for scientists, publics and policy makers. Although numerous studies have examined several individual phases of a life cycle of buildings, relatively few studies were focused on the carbon emissions over the entire life cycle of buildings. In particular, most studies paid a special attention to energy saving (Buchanan and Levine, 1999, Gustavsson and Sathre, 2006, Dodoo et al., 2009, Li and Colombier, 2009, Kneifel, 2010). Some studies being explicitly dedicated to constructive materials have been done during the recent several years (Seppälä et al., 2002, Nebel et al., 2006), while also neglecting the environmental or ecological performances of the entire building life cycle. The aim of this study is to set up an integrated model (LCCE model) to analyze CO2 emissions over the life cycle of urban building system with two typical architectural structures as masonry-concrete and steel-concrete. The model is intended to serve as an assessment tool to help us identify major sources, fluxes and sinks of carbon cycle in urban building system or urbanizing areas and to improve the building planning, construction and management, finally promote sustainable transition of developmental pattern of urban building system towards a low-carbon one.
Section snippets
LCA-based analyzing method for urban building system
The Life Cycle Assessment (LCA) methodology today represents the main tool used to estimate the positive or negative impacts on various environmental changes and alterations of ecosystems associated to urban building system. Usually, the LCA analysis is presented in the form of aggregation of environmental loads or impacts per unit of building activities, without considering their distributions in time and space (Tukker, 2000). Nowadays, the effective role of LCA methodology in the quantitative
Composition and structure of CO2 emissions model for urban building system
Based on the analysis above, CO2 emissions during the life cycle of urban buildings can be quantified by the formulas as follows:where TE is total amount of CO2 emissions, and i is the ith phase of urban building life cycle, and i is from 1 to 5. We defined this basic model of analysis as the life cycle model of carbon emissions for urban building system, which was abbreviated to LCCE Model.where IEi is CO2 emissions from industrial processes during the
Site investigations and data collection
Urban residential buildings consist of different architectural structures which are made of various types of constructive materials. This analysis takes into account of six main constructive materials of cement, steel, brick, timber, gravel, sand and two types of dominate architectural structures of masonry-concrete and steel-concrete for survey. We investigated about 46 buildings in China, and the main items of building survey include their name, age, storey, architectural structure and area.
Temporal characteristics
The amount of CO2 emissions in the life cycle of residential buildings with masonry-concrete and steel-concrete structures is 329.61 t and 315.79 t per 100 m2, respectively. During the life cycle of typical residential buildings, it was indicated that 85–90% of CO2 emissions were created in the phase of building operation, 7–11% came from constructive materials preparation and 3% came from construction. Compared to the three phases above, building demolition generated fewer CO2 emissions. Besides,
Conclusions
Urban building system is one of the biggest sources of CO2 emissions. Our research introduces a newly developed carbon emissions model (LCCE model) to analyze CO2 emissions of urban building system based on the principle of LCA. A case study from some typical residential buildings of different architectural structures in China was also employed to illustrate how the model help us understand and analyze CO2 emissions in the different phases of a building life cycle and the differences between
Acknowledgements
This research was supported by National Foundation of Natural Sciences of China (70873121) and the State Key Laboratory of Urban & Regional Ecology (SKLURE2008-1-01). This work was also supported by Knowledge Innovation Project of Chinese Academy of Sciences (KZCX2-YW-324) and the Key Supporting Project of Ministry of Science and Technology of China (2007BAC28B04).
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