The Worldwide Threat of Land Subsidence

By Jim Brace-Thompson

An article in the January 1, 2021, issue of the journal Science maps the global threat of land subsidence. Often, this is caused by depletion of groundwater, which in turn compacts clays, sands, and other sediments holding underground aquifers and thus causes the land above to subside. This groundwater depletion is linked both to natural causes such as drought and to human actions as more and more groundwater is sucked up to satisfy needs of increased population and the agriculture to support ever more people on our planet.

A scientific team led by Gerardo Herrera-García of the Geological & Mining Institute of Spain estimates that nearly 20 percent of the global population faces a threat from “a high probability of subsidence.” The people within this population are concentrated mostly in no less than 15 of the 20 major coastal cities worldwide, which are now ranked with high flood risk due to a double whammy of land subsidence and rising sea level. Areas along the coast of China are ranked at especially high risk.

But the risk is not just along coastal zones. For instance, land is subsiding in the Central Valley of the state of Califonia in the United States and along the foothills of the Himalayas in India, putting both regions at risk of significant flooding during periods of heavy rain and overflowing rivers. The study authors say, “potential subsidence may affect 635 million inhabitants living in flood-prone areas in 2040.” Thus, they urge policy makers to support and increase the analysis of flood risk due to land subsidence and to consider mitigation strategies to ward off future disasters.