China’s major cities show considerable subsidence from human activities

The land under nearly half of China’s major cities is undergoing moderate to severe subsidence, affecting roughly one-third of the nation’s urban population, according to a systematic national-scale satellite assessment. The findings suggest that within the next century, 22 to 26% of China’s coastal land will have a relative elevation lower than sea level, putting hundreds of millions of people at elevated risk of flooding due to sea-level rise. Over the last several decades, China has experienced one of the most rapid and extensive urban expansions in human history. This massive wave of urbanization may be threatened by land subsidence – a gradual sinking of an area of land. To date, instances of subsidence have been increasingly reported in major Chinese cities. However, a comprehensive understanding of the scale and speed of subsidence in China’s cities remains unclear. Using measurements from the spaceborne Sentinal-1 Interferometric Synthetic Aperture Radar (InSAR) and ground-based GPS data, Zurui Ao and colleagues performed a national-scale scale evaluation of land subsidence in 82 of China’s major cities from 2015 to 2022. InSAR uses highly precise radar pulses to measure the change in distance between the satellite and the grounds surface and can detect even relatively small changes in elevation on the order of millimeters per year. Ao et al. found that 45% of the studied urban land area is subsiding faster than 3 millimeters per year (mm/year), and as much as 16% is subsiding at a rate of 10 mm/year or more. These sinking lands contain 29% and 7% of China’s urban population, respectively. According to the authors, this subsidence is associated with a range of anthropogenic factors, including groundwater extraction and the weight of the built environment. The findings suggest that due to this subsidence and projected sea-level rise, roughly a quarter of China’s coastal lands will have an elevation lower than sea level, presenting a considerable risk of flooding for large populations unless adequate protective measures to mitigate city subsidence are implemented and maintained. “One major challenge is to move from measuring subsidence to thinking systematically about its implications,” write Robert Nicholls and Manoochehr Shirzael in a related Perspective. “Ideally, this will guide immediate and long-term strategic actions, analogous to strategies that have emerged for coastal areas threatened by sea level rise.”