Indirect Biophysical Effect of Global Urban Land Expansion on the Surface Climate

Urbanization impacts the Earth’s climate by contributing to buildup of atmospheric greenhouse gases and by modifying surface biophysical attributes. In climate models, the greenhouse gas component is prescribed with emission trajectories embedded in various socioeconomic pathways. However, the biophysical impact is not considered because no models can simulate urban land transitions. Spatial comparisons reveal that cities are typically warmer than adjacent natural landscapes due to contrasts in their biophysical characteristics. Such space-for-time substitutions omit atmospheric feedback processes that often amplify local biophysical signals. Consequently, the omission of biophysical considerations in climate models raises the potential for underestimating future warming projections, particularly in regions with rapid urbanization. Here, we employ a dynamic urban scheme in the Community Earths System Model to qualify the biophysical effect of global urban land expansion under the SSP5 scenario. Using a factorial experiment design, we separate the climate effect of urbanization as direct climate effect arising from altered biophysical properties through land replacement and the indirect effects driven by atmospheric feedback. We find that the biophysical effect of urbanization depends on land aridity. In water-limited region, the biophysical effect leads to a significant increase in air temperature (0.23 ± 0.16 K; mean ± one standard deviation of three model ensembles; p < 0.1) in areas with urban fraction change exceeding 5% by 2070. This warming signal is predominantly driven by indirect effect associated with atmospheric feedback, with the direct impact playing a secondary role. Our findings emphasize the necessity of explicitly integrating dynamic urban schemes into future global climate models and underscore the need for large ensemble simulations to robustly detect urbanization signals.