Abstract Summary
Urbanization as a global phenomenon is a multifaceted process, affecting the Sustainable Development Goals (SDGs) in and around urban areas. Here we do the first global attempt to characterize the complexity of urbanization from 1975 to 2015 in terms of population, built-up structure, and greenness, as well as monitoring urban sustainability indicators at the grid level covering all inhabited areas. We used Global Human Settlement Layer to assess built-up structure, population and land-use efficiency (SDG 11.3.1), combined MODIS/Terra & GIMMS NDVI for long-term greenness, distributed statistical energy consumption by night lights, and population for energy efficiency (SDG 7.3.1), and used near-surface PM2.5 dataset for air quality (SDG 11.6.2). Our results emphasize that the multifaceted nature of urbanization as well as related sustainable challenges vary greatly across regions and times. (1) Increased population density and built-up patch density was dominant in Asia and Africa, while urbanization in Europe and North America took a rather steady pace, combined with widespread greening. (2) According to the urbanization types identified by a self-organizing map (SOM) algorithm, a large proportion of urban and suburban areas experienced two dynamic urbanization types - built-up extension/leapfrog and built-up infill with large population increase (Fig. 1). (3) During different historical periods (1975-1990, 1990-2000, and 2000-2015), annual increases in population and built-up density were slowing coinciding with an increasing greenness – signaling that urbanization processes are becoming less intense, more compact, and “greener” over the most recent period. (4) Land-energy-air SDGs have declined in over 30% of global inhabited grid cells from 2000 to 2015. (5) In land-energy-air sustainability trends, urban areas perform relatively better than rural areas in the Global South, while urban areas in the Global North tend to be less sustainable than their surrounding rural regions. Our findings facilitate a comprehensive understanding of global urbanization and relevant sustainability with many local variations and characteristics. Integrating Earth Observation data is crucial for tracking urbanization and sustainability, and can guide context-specific strategies towards a sustainable and livable future instead of a ‘one-size-fits-all’ policy for cities.
