China’s Cities are Subsiding

Zurui Ao et al., A national-scale assessment of land subsidence in China’s major cities.Science384,301-306(2024).DOI:10.1126/science.adl4366

Robert J. Nicholls, Manoochehr Shirzaei , Earth’s sinking surface.Science384,268-269(2024).DOI:10.1126/science.ado998

The article below is excerpted from the peer reviewed research study and related invited perspective cited above.

 Subsidence is the lowering or sinking  of the Earth’s land surface through natural or manmade processes. It is a widespread and sometimes dramatic phenomenon. Such sinking is caused by a range of factors, including groundwater withdrawal, which is generally considered the most significant driver. However, at present our understanding of subsidence is not complete, though the phenomenon has been observed for more than a century.

 Earth’s surface experiences natural uplift and subsidence due to various geological processes such as glacial retreat or earthquake. As mentioned above groundwater withdrawal almost always promote subsidence. Subsidence most often occurs on coastal sedimentary plains and deltas, - inland sedimentary plains often show similar behavior. In coastal areas, subsidence also contributes to relative sea level rise.

Over the past decade, advances in satellite imaging technology have enabled scientists to measure down to millimeter-scale changes in land level over days to years. Using measurements from the satellite Sentinal-1 Interferometric Synthetic Aperture Radar (InSAR) and ground-based GPS data, Zurui Ao and colleagues examined 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 ground surface and can detect changes in elevation down to millimeters per year.  

In the study cited above 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. This is a concern because the impacts of subsidence in urban areas include direct damage to buildings and foundations, infrastructure, drains, and sewage systems. It also exacerbates the occurrence and effects of flooding, especially in coastal cities, compounding climate change.

Ao et al were not able to attribute the measured subsidence to specific physical causes (building weight, groundwater withdrawal, landfill etc.) because they lacked data and models of the various processes that can or do contribute to subsidence. Predicting or slowing future subsidence requires an understanding of all the causes, including human activities and climate change, and how they might change with time. 

from Ao et al

The authors speculate on what some of the causes might be based on previous work in subsidence. The first factor is the geological setting beneath the city. Closely related with the geological setting is the weight of buildings-China has had an extraordinary speed of urban construction in the past 3 decades. Ao et al found that the later the building was constructed, the faster the subsidence tends to be, and counter intuitively  they found that heavier buildings tend to subside more slowly.  The authors speculated that heavier buildings are anchored on deeper bedrock and are therefore less prone to subside. A major factor identified is groundwater loss, which decreases pore pressure and leads to subsurface compaction. This has been observed worldwide including the coastal plain of the eastern united states.