How does the synergistic effect of green finance and digital finance affect carbon emission efficiency? An example from the Yangtze River Economic Belt

Green finance (GF) and digital finance (DF) are important enablers for the digitization and greening of the economy, and they are also vital drivers of carbon emission efficiency (CEE). The paper incorporates GF, DF, and CEE into the same research framework to empirically explore the nexus between the synergy of GF and DF and the CEE of the Yangtze River Economic Belt (YREB). The findings are as follows. First, the synergy between GF and DF positively affects the CEE of YREB. This is mainly achieved by facilitating innovation in green technology and lowering carbon emission intensity. Second, the synergy between GF and DF contributes more clearly to CEE for cities in the upper reaches of YREB, cities of high population densities, and cities with high green innovation capacity. Third, there is a single threshold effect of the synergy between GF and DF on the CEE of YREB. When the collaborative development level between GF and DF is larger than 0.6659, the synergy between GF and DF can be more conducive to driving the improvement of CEE. Fourth, the synergy between GF and DF generates a positive spatial spillover effect on the CEE in YREB, which contributes to the CEE of the local cities in YREB and the neighboring cities in YREB. The findings can provide implications for formulating low-carbon development policies.