The nonlinear effect of land freight structure on carbon emission intensity: new evidence from road and rail freight in China

The extensive literature has debated the varying effects of factors on carbon dioxide (CO 2 ) emissions. However, it has paid little attention to land freight structure (FS), including road and rail freight share, which may have different effects on CO 2 emissions. Based on the data from 6 eastern provinces in China during 2005–2019, the panel threshold model is used to explore the dynamic influence mechanism of road and rail freight share on transport carbon emission intensity (CE), respectively. The results show different nonlinear relationships between the share of road and rail freight and transport carbon emission intensity. First, the effect of road freight share on carbon emission intensity is all positive across different stages of trade openness, while such effect goes through a process of increasing and then decreasing with the level of trade openness improving. Second, the driving effect of rail freight share on carbon emission intensity exhibits a “negative–positive-negative” feature as the level of trade openness increases. Third, trade openness generates a double-threshold effect on carbon emission intensity. The differentiated nonlinear effects provide significant evidence of the modal shift from road to rail freight, which would be effective to alleviate transport CO 2 emissions.