Assessment on regional carbon dioxide mitigation efficiency of bicycle-sharing in China: based on SE-SBM-Malmquist-Tobit model considering undesirable output

China's urban bicycle-sharing is crucial in promoting carbon mitigation owing to its inherent green and low-carbon intention. However, the real carbon mitigation potential is overshadowed by the reality of regional differences and associated environmental and social problems; therefore, an objective evaluation is urgent. Based on the SE-SBM model considering undesirable outputs, this paper combines the Malmquist index to measure carbon mitigation efficiency. The study found that the four major categories of cities show continuous W-shaped fluctuations of decreasing and then increasing, and only ten cities have higher relative efficiency, with 13.89% of DMU being effective. The carbon reduction performance shows a regional difference pattern of west-middle-east stepped growth, and the evolutionary characteristics of different types of cities have noticeable spatial and temporal inertia and spatial aggregation characteristics. Additionally, the area positioning for carbon reduction was determined by a three-dimensional matrix analysis, and Tobit regression was used to identify the significant variables influencing the effectiveness of carbon reduction.