A new analysis has indicated that sinking ground, known as subsidence, could affect 8 percent of global land surface and 19 percent of the population in the coming decades. Subsidence is triggered by processes such as groundwater depletion and causes the gradual sinking of large areas of the ground over months and years. It can have dangerous and costly effects on areas affected, including increased risk of flooding, causing fissures, permanently reducing the capacity of aquifers, and damaging buildings and infrastructure. It is predicted that groundwater demand will grow in the future due to the growth of the population and economy, exacerbating this process. Subsidence could also be worsened by climate change due to changes in precipitation, frequent and severe floods and droughts, and increased evapotranspiration.
The study, published in Science, started with a large-scale systemic literature review on subsidence worldwide. They found that subsidence caused by groundwater depletion happened at 200 locations in over 34 countries. Subsidence rates in Mexico are some of the highest in the world, as much as 30 centimeters (11.8 inches) per year. Contributing to flooding, subsidence in the Po River Plain threatens 30 percent of the Italian population. Due to unregulated groundwater pumping, Iran has some of the fastest sinking cities, subsiding up to 25 centimeters (9.8 inches) a year. In the past century, Tokyo has seen 4 meters (13.1 feet) of subsidence, and Central Valley, California has seen up to 9 meters (29.5 feet). The Netherlands is 25 percent below the main sea level, primarily due to subsidence. Jakarta, the capital of Indonesia, has seen such severe subsidence that the government is planning to move the capital to the island of Borneo.
The authors of the study proposed a global model that they claim could predict global subsidence susceptibility at a resolution of 1 kilometer (0.6 miles) squared. It does this via statistical analysis of lithology (physical rock characteristics), land surface slope, land cover, and Köppen-Geiger climate classes. It also estimates groundwater depletion probability, identifying areas under water stress or with high groundwater demand.
Results of modeling suggest that 12 million square kilometers (7.5 million square miles) of the global land surface could potentially be threatened by subsidence – 8 percent of the Earth’s surface. Areas at risk were concentrated in and around dense urban areas and irrigated areas. Over 2.2 million square kilometers (1.4 million square miles) were considered high or very high risk. These areas house 1.2 billion people, or 19 percent of the global population. People living in Asia made up 86 percent of the population facing potential subsidence, more than in North America and Europe combined. Egypt and the Netherlands were shown to have the largest populations living in potential subsidence areas below sea level. The results identified 1,596 major cities in potential subsidence areas, with 57 percent in flood-prone areas.
The researchers noted a lack of information as a limit to their approach, as only a third of records in the literature review had data on subsidence extent, and information on impacts and mitigation measures was scarce. Therefore, the global model did not consider the subsidence rate and magnitude. However, the authors say that this model could help identify areas at risk of subsidence for further analysis and intervention.