## 2 Literature Review
### Environmental Costs and Carbon Costs  
The global environmental pollution problem has become extremely serious, and the environmental quality is deteriorating [19]. The prominence of environmental problems has expanded the research scope of environmental costs, a concept that has emerged in recent years. Some scholars have defined this concept. [PERSON] [20] stated that environmental costs include direct costs (such as the cost of waste disposal), indirect costs (such as the cost of environmental management systems), uncertain costs (such as fines) and costs that are difficult to determine (such as a company's reputation). [PERSON] [21] developed a life cycle environmental cost analysis (LCECA) model to calculate the environmental cost of wastewater treatment, environmental management, taxes, restoration, energy use, and recycling costs. [PERSON] et al. [22] improved the LCECA model and classified environmental costs into six categories, including resource consumption cost, pollutant emission control cost, pollutant treatment cost, environmental management cost, environmental damage costs and regeneration cycle cost. Other scholars have conducted quantitative research on environmental costs. [PERSON] [23] divided the generalized life cycle cost of buildings into direct costs and environmental costs and established a calculation model for these costs; this model is also the first to quantify the environmental cost of buildings in China. [PERSON] [24] determined the calculation method of different environmental impact cost indicators at each stage by analyzing the stage-specific types and characteristics of the environmental impact cost throughout the life cycle of bridge engineering, resulting in the establishment of an environmental cost analysis model for bridge engineering. With the convening of the Copenhagen Climate Conference at the end of 2009, the concept and practice of a low-carbon economy rose to prominence, and the desire to develop a low-carbon economy caused research on environmental costs to gradually focus on the cost of carbon emissions. Carbon cost is an integral part of environmental cost. Due to the strong externality of environmental cost, carbon cost is difficult to measure. Carbon costs can reflect environmental problems as enterprise costs via carbon pricing mechanisms, which are beneficial for measuring internal enterprise expenditures. Therefore, this paper calculates the carbon cost of bridges to clarify the environmental impact of the bridge construction stage. In addition, the carbon cost calculated in this paper mainly refers to the carbon tax paid by enterprises to the taxation department and the cost of purchasing carbon emission rights in the carbon trading market. [PERSON] et al. [25] and [PERSON] [26] proposed a carbon cost accounting model of buildings under the carbon trading system and carbon tax system. Based on the accounting model, [PERSON] [27] chose a carbon pricing mechanism-a carbon tax mechanism-to monetize the carbon emissions and obtain the carbon cost of the building. [PERSON] [28] divided the building life cycle cost system into two parts: the traditional building life cycle cost and the carbon emission cost, for which the carbon emission cost calculation method was based on the carbon tax policy. The above research shows that using the carbon taxing and trading systems to calculate construction carbon costs is reliable and applicable for calculating bridge carbon costs. However, to date, China lacks special research on bridge carbon costs. Therefore, it is necessary to establish a bridge carbon cost calculation model to clarify the bridge's impact on the environment during the construction phase. First, based on LCA and construction process estimation methods, this paper establishes a carbon emission calculation model for the bridge construction stage. Second, through the carbon taxing and trading mechanisms, the life cycle carbon emissions of bridges are converted into costs, and a bridge life cycle carbon cost calculation model under the two systems is established.