Does the opening of high-speed rails improve urban carbon efficiency? Evidence from a spatial difference-in-difference method

High carbon emission efficiency is an important guarantee for achieving the goals of "carbon peaking" by 2030 and "carbon neutrality" by 2060. Based on the panel data of prefecture-level cities from 2003 to 2018, the spatial difference-in-difference model is used to empirically test the spatial spillover effect of the opening of high-speed rail (HSR) on urban total factor carbon emission efficiency (TFCEE). The results show that the spatial correlation of TFCEE between regions is positive, and the opening of HSR has a significant positive spatial spillover effect on TFCEE. The conclusion is still valid after a series of robustness tests. We further employ a mediating effects model to verify that green technology innovation and labour resource mismatch are two important channels of influence for the opening of HSR to improve TFCEE. The heterogeneity analysis shows that the opening of HSR significantly promotes the higher development level and the provincial capital TFCEE.