Abstract
Dramatic urban land expansion and its internal sub-fraction change during 2000–2020 have taken place in Africa; however, the investigation of their spatial heterogeneity and dynamic change monitoring at the continental scale are rarely reported. Taking the whole of Africa as a study area, the synergic approach of normalized settlement density index and random forest was applied to assess urban land and its sub-land fractions (i.e., impervious surface area and vegetation space) in Africa, through time series of remotely sensed images on a cloud computing platform. The generated 30-m resolution urban land/sub-land products displayed good accuracy, with comprehensive accuracy of over 90%. During 2000–2020, the evaluated urban land throughout Africa increased from 1.93 × 104 km2 to 4.18 × 104 km2, with a total expansion rate of 116.49%, and the expanded urban area of the top six countries accounted for more than half of the total increments, meaning that the urban expansion was concentrated in several major countries. A turning green Africa was observed, with a continuously increasing ratio of vegetation space to built-up area and a faster increment of vegetation space than impervious surface area (i.e., 134.43% vs., 108.88%) within urban regions. A better living environment was also found in different urbanized regions, as the newly expanded urban area was characterized by lower impervious surface area fraction and higher vegetation fraction compared with the original urban area. Similarly, the humid/semi-humid regions also displayed a better living environment than arid/semi-arid regions. The relationship between socioeconomic development factors (i.e., gross domestic product and urban population) and impervious surface area was investigated and both passed the significance test (p < 0.05), with a higher fit value in the former than the latter. Overall, urban land and its fractional land cover change in Africa during 2000–2020 promoted the well-being of human settlements, indicating the positive effect on environments.
Highlights
In recent decades, the rapid economic development and technological progress made great changes in the way of human life, such as the continuously improving infrastructure [1,2] and medical service [3,4] in urban regions
The area ratio of built-up land to the whole continent was always very small. This led to the issue of uneven distribution of random verification points, namely, most of the verification points were distributed in nonurban areas, while there were few verification points in built-up land regions
Gross Domestic Product (GDP) of the whole African continent increased by USD 1.73 × 1012, and we investigated the change in GDP and impervious surface area at the country level and obtained the fitting value of 0.7226 (R2 ) and p < 0.05 during 2000–2020 (Figure 6)
Summary
The rapid economic development and technological progress made great changes in the way of human life, such as the continuously improving infrastructure [1,2] and medical service [3,4] in urban regions. As one of the main global land use change types, urban land area has been showing a continuous and violent expansion process [17,18], and its area by 2030 will be three times greater than that of 2000 [19]. The large-scale assessment and dynamic detection of urban land and its internal land cover changes were conducive to explore their potential impact on the globally eco-environmental issues, such as climate, hydrological system, biodiversity, and biogeochemistry, in the evolution of land use
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