Abstract
Deterioration of the urban thermal environment, especially in megacities with intensive populations and high densities of impervious surfaces, is a global issue resulting from rapid urbanization. The effects of landscape patterns on the urban thermal environment within a single area or single period have been well documented. Few studies, however, have explored whether the effects can be adapted to various cities at different urbanization stages. This paper investigated the variations of these effects in the five largest and highly urbanized megacities of China from 1990 to 2020 using various geospatial approaches, including concentric buffer analysis, correlation analysis, and hierarchical ridge regression models. The results indicated that the effects of landscape patterns on the urban thermal environment were greatly variable at different urbanization stages. Although landscape composition was more important than landscape configuration in determining the urban thermal environment, the standard coefficients of composition metrics continuously decreased from 1990 to 2020. However, configuration metrics, such as patch density, edge density, and shape complexity, could affect the land surface temperature (LST) to a larger extent at the highly urbanized stage. The urbanization process could also affect the cooling effect of urban green space. At the initial stage of rapid urban expansion in approximately 2000, urban green space explained the most variation in LST, with a value as high as 10%. To maximize the cooling effect, the spatial arrangement of urban green space should be highlighted in the region that was 10–15 km from the city center, where the mean LST experienced a significant decline. These results may provide deeper insights into improving the urban thermal environment by targeted strategies in optimizing landscape patterns for areas at different urbanization stages.
Highlights
A megacity is generally considered a metropolitan area with a total population of more than 10 million [1]
This paper found that the urban expansion of the five megacities presented similar characteristics, which could be divided into three stages: low-speed expansion around the central area from 1990 to 2000, high-speed expansion in the suburban area from 2000 to 2010, and low-speed expansion in the suburban area from 2010 to 2020
The results indicated that there was a significant decreasing trend of Relative LST (RLST) considering that the percentage of landscape (PLAND) of different landscape types both had a strong correlation with the urban thermal environment along the urban gradient (Figures 5 and 6), which was observed in other cities [39,62]
Summary
A megacity is generally considered a metropolitan area with a total population of more than 10 million [1]. As an important part of the urban eco-environment, the urban thermal environment is greatly affected by rapid urbanization, especially in megacities [9]. A well-documented consequence of the urban thermal environment change is the formation of urban heat island (UHI), referring to the phenomenon that the atmospheric and surface temperatures in urban areas are higher than those in the surrounding rural areas [10]. The urban thermal environment has a profound effect on the local climate, human settlement, energy consumption, biodiversity, and ecosystem functions [11,12,13,14]. Excess heat will greatly increase the energy consumption for building refrigeration, leading to economic burdens on urban development [17]. Urban warming in hot climates exerts heat stress on organisms and may reduce biodiversity and affect ecosystem functions [18]. It is of special importance to investigate the factors that affect the urban thermal environment
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