Mapping the Thermal Footprint of Urbanization: A Long-Term Perspective based on Local Climate Zone Dynamics

Compounding with global warming induced by carbon emissions, urbanization is exerting an additional warming effect, further threatening local urban residents. However, due to the heterogeneity within urban areas, the risk among residents in the same city varies significantly. Researchers have explored various methods to describe urban surfaces, among which the Local Climate Zone (LCZ) framework has proven to be an effective tool for characterizing diverse urban morphologies globally. Although global LCZ mapping products are available, inter-annually comparable LCZ time series remain scarce in the current literature. Additionally, the evaluation of thermal characteristics of urban morphology often relies on comparing median or average Land Surface Temperature (LST) between LCZ types, which can introduce substantial bias due to spatial autocorrelation caused by terrain differences, uneven human activities, and other factors. Thus, there is an urgent need for a dynamic monitoring and impact evaluation paradigm for urban morphology.

In this study, we present an annual LCZ time-series mapping framework and generate time series from 2000 to 2020 for three major Chinese urban agglomeration: Jingjinji, the Yangtze River Delta, and the Greater Bay Area. Comparing with baseline (supervised classification directly), LCZ time-series generated by our framework ensured the consistency between years. Furthermore, by integrating the LCZ time series proposed in this study with MODIS Land Surface Temperature (LST) datasets, we developed a time-series analysis method to quantify LST changes induced by different urban morphology transformations. The mapping results reveal that high-rise buildings are the primary distinguishing feature between urban areas of different sizes. Over the past two decades, the composition of urban morphology has been converging between urban areas of varying sizes but diverging within intra-city land use zones. Moreover, urban morphology patterns differ significantly between urban expansion areas and urban renewal areas. Our findings indicate that the impact of urban morphology changes varies significantly. Specifically, urban renewal, predominantly characterized by vertical development, exerts an asymmetric effect on urban temperatures: it mitigates urban warming during the day but intensifies it at night. In contrast, the effect of urban expansion on urban warming is more pronounced during the day than at night. At the city scale, changes in urban morphology generally contribute to a warming effect, both diurnally and nocturnally. Urban expansion is identified as the primary driver of rising city temperatures. However, the divergent impacts of vertical development, which is likely to dominate future urbanization, must not be underestimated.