Carbon Payback Period for Urban Rail Transit: A Life Cycle Assessment of Beijing Capital Airport Express with Scenario Projection

As a large-capacity and efficient public transport system, urban rail transit plays a great role in relieving urban traffic congestion and reducing carbon emissions. Reasonable quantification and maximization of its carbon emission reduction effect is crucial for evaluating external benefits, improving carbon trading theory, and formulating subsidy policies. In this study, based on the perspective of life cycle, a carbon emission payback period calculation model including actual carbon footprint, virtual carbon footprint, and carbon footprint during construction was constructed to quantify the carbon emission reduction effect of urban rail transit. Taking Beijing Capital Airport Express as an example, through scenario projection, it is found that the carbon payback period is different under different scenarios of passenger volume growth and energy structure improvement. Specifically, the carbon payback period under normal growth, rapid growth, and slower growth scenarios of passenger volume is 27, 24, and 28 years, respectively. The carbon payback period under the normal development, rapid development, and slower development of energy structure and energy efficiency level is 27, 26, and 29 years, respectively. The study reveals that large-scale passenger volume, efficient passenger transport intensity, energy structure adjustment, and energy efficiency improvement are the keys to achieve carbon emission reduction in urban rail transit. Accordingly, this study puts forward targeted policy recommendations in terms of management and technical means aimed at accelerating the carbon emission reduction process of urban rail transit.