Urban Land Expansion Amplifies Surface Warming More in Dry Climate than in Wet Climate: A Global Sensitivity Study

Urbanization changes Earth's climate by contributing to the buildup of atmospheric greenhouse gases and altering surface biophysical properties. In climate models, the greenhouse aspect is prescribed with urbanization and emission trajectories embedded in socioeconomic pathways (SSPs). However, the biophysical aspect is omitted because no models currently simulate spatially explicit urban land transition. Urban land is typically warmer than adjacent natural land due to a large urban-versus-natural land contrast in biophysical properties. The lack of biophysical representation of urbanization in climate models raises the possibility that model projection of future warming may be biased low, especially in areas with intense urban land expansion. Here, we conduct a global sensitivity study using a dynamic urban scheme in the Community Earth System Model to quantify the biophysical effect of urban land expansion under the SSP5-RCP8.5 scenario. Constant urban radiative, thermal, and morphological properties are used. We find that the biophysical effect depends on land aridity. In climate zones where surface evaporation is water-limited, the biophysical effect causes a significant increase in air temperature (0.28 ± 0.19 K; mean ± one standard deviation of nine ensemble pairs; p < 0.01) in areas where urban expansion exceeds 5% by 2070. The majority of this warming signal is attributed to an indirect effect associated with atmospheric and land feedback, with the direct effect of land replacement playing a minor role. These atmospheric feedback processes, including solar brightening, soil drying, and stomatal closure, act to enhance the warming initiated by surface property changes of urban land replacement.