Impacts of China's Sponge City projects on urban thermal environments: a synthetic control approach

Various green development initiatives aiming at ecosystem restoration and natural disaster adaptation often highlight ancillary urban cooling effects as key co-benefits. However, the empirical literature on these cooling co-benefits and their underlying mechanisms remains limited. This study aims to fill this gap by focusing on China's Sponge City initiative as a case example. In particular, we examine the urban cooling effects associated with the initiative across a sample of 45 pilot cities over an 11-year period (2010 – 2020). Using a difference-in-differences design with 23 treated cities and 22 control cities, our group-level analysis reveals that the estimated cooling effects remain statistically insignificant overall. To account for potential intra-group heterogeneity, we further employ the synthetic control method, focusing on the 8 treated cities that demonstrate a satisfactory pre-treatment model fit. These city-specific results are consistent with our aggregate findings: in most cases, Sponge City projects do not yield statistically significant reductions in urban air temperature. The only exception is Xiamen, where summer-time urban air temperature declined by 0.18°C during the post-intervention period. A follow-up case study suggests that the urban cooling co-benefits of a given Sponge City project partly depend on consistent and systematic planning interventions for effective project scale-up and management. Given these findings, reaping the heat-mitigation potential of proposed Sponge City design concepts and principles seems to require increased efforts to promote best practices and mutual learning at multiple levels.