Global Urban Land Expansion Tends To Be Slope Climbing: A Remotely Sensed Nighttime Light Approach

The slope climbing of urban land expansion (SE) has become an increasingly prevalent phenomenon, and it has resulted in unique characteristics in terms of urban form, landscape, and governance with challenges for sustainable urban development worldwide. However, relatively little concern has been given to the SE with a long time series worldwide. On the basis of urban entities extracted from Suomi National Polar-Orbiting Partnership Visible Infrared Imaging Radiometer Suite-like (SNPP-VIIRS-like) data, we attempted to quantify and evaluate global SE with the help of slope spectrum analysis from different scales (e.g., global, continental, national, and city scales) during the period of 2000-2020. Our study proves that urban entities not only identify physical scopes but also reflect the intensity differences of socioeconomic development, which can provide an accurate approach to evaluate the SE. We found that global urban entities increased from 157,733 km(2) in 2000 to 470,632 km(2) in 2020, representing a 1.98-fold increase. The global average slope of urban entities was on an upward trend, from 0.85 degrees by 2000 to 0.96 degrees by 2020, an increase of about 0.11 degrees. Asia, Africa, and South America have experienced an evident SE process, while North America has exhibited a slow SE trend. A remarkable phenomenon has been found in China, where rapid SE has occurred due to the country's unprecedented urbanization process. The change value of the upper limit slope of urban land and the average slope climbing index of urban land have also experienced a volatile growth trend at global, continental, national, and city scales. Our study suggests that sustainable urban development should take the SE phenomenon into considerations, and that reasonable governance policies and strategies should be formulated to address its potential impacts.