Abstract
Urban impervious surfaces area (ISA) and green space (GS), two primary components of urban environment, are pivotal in detecting urban environmental quality and addressing global environmental change issues. However, the current global mapping of ISA and GS is not effective enough to accurately delineate in urban areas due to the mosaicked and complex structure. To address the issue, the hierarchical architecture principle and subpixel metric method were applied to map 30 m global urban ISA and GS fractions for the years 2015 and circa 2020. We use random forest algorithms for retrieval of the Normalized Settlement Density Index and Normalized Green Space Index from Landsat images using Google Earth Engine. The correlation coefficients of global urban ISA and GS fractions were all higher than 0.9 for 2015 and circa 2020. Our results show global urban ISA and GS areas in circa 2020 were 31.19 × 104 km2 and 17.16 × 104 km2, respectively. The novel ISA and GS fractions product can show potential applications in assessing the effects of urbanization on climate, ecology, and urban sustainability.
Highlights
IntroductionGlobal urbanization promotes social–economic development, which profoundly affects ecosystem services and environmental sustainability
The specific strategies are different, such as the kernel density approach and cellular automata model [27], the quantile-based digital number threshold identification [20], and the acquisition of centroid and threshold sites from normalized difference water, vegetation, and built-up indexes [25]. These methods could effectively identify urban extents or built-up areas, we examined the associated datasets and found some disadvantages in generating finer and more accurate urban boundaries retrieved from Landsat images
We confirmed that the hierarchical scale principle and subpixel method perform well in acquiring urban boundaries, as well as impervious surfaces area (ISA) and green space (GS) fractions
Summary
Global urbanization promotes social–economic development, which profoundly affects ecosystem services and environmental sustainability. Humans are the dominant drivers of the land-use/cover change from rural to urban as they seek to improve their well-being [1,2]. Urbanization can influence the biophysical and biochemical processes, i.e., land-cover change, surface radiation and energy changes, hydrologic processes, and materials cycles, among others [6,7,8,9], all of which affect cropland encroachments, urban heat islands, rain flood disasters, biodiversity loss, and climate change in many ways [10,11,12]. New knowledge about global urban land cover is essential to address urban-related issues of environmental sustainability through science–policy interfaces [4,13]
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