A group of international scientists led by Nanyang Technological University, Singapore (NTU Singapore), has found that many densely populated coastal cities around the world are at risk from rising sea levels because large parts of their land are sinking.
Using a cloud-based technique called Interferometric Synthetic Aperture Radar (InSAR), the research team analysed satellite photos of 48 cities from 2014 to 2020. They indicate that a rise in industrial operations, such as the extraction of groundwater, oil, and gas, and the rapid construction of buildings and other urban infrastructure may be contributing to this susceptibility.
Global sea levels are increasing due to the melting of ice sheets and the expansion of warmer seawater. According to scientists, though, sinking land or land subsidence can exacerbate the issue. Land subsidence varies on a neighbourhood and even a block-by-block basis, but the team discovered a median sinking speed of 16.2 millimetres (mm) per year across 48 cities, with some sinking at 43 mm per year. The current mean worldwide sea-level rise is 3.7 mm per year.
The findings are an illustration of research that supports the NTU 2025 strategic plan, which aims to address humanity’s great issues on sustainability and speed the translation of scientific discoveries into products that lessen the human impact on the environment.
This research also contributes to the Singapore National Sea Level Programme (NSLP), which is financed by Singapore’s National Research Foundation and National Environment Agency. The objective of the research programme is to provide policymakers with the knowledge necessary to protect Singapore’s coastlines.
Sinking ground causes higher sea levels and a greater risk of flooding in coastal areas. The findings enable impacted communities and governments to determine which locations are particularly vulnerable to high levels of land subsidence and to take measures to mitigate their respective coastal risks.
This work underscores the importance of high-resolution satellite data for a better understanding of this issue; because subsidence rates can fluctuate rapidly over small areas, land-based measurements frequently fail to capture the real scope of the issue.
By figuring out how much and how quickly these heavily populated coastal cities are sinking, the study helps limit projections of coastal flooding in the coming decades. Scientists expect more land to be flooded as sea levels rise and land sinks, so they need to know how much more land will be flooded.
The 48 cities were chosen based on a minimum population of five million by 2020 and a maximum distance of fifty kilometres from the shore. Comparing coastal cities across the globe, the researchers found that the fastest rates of relative local land subsidence are centred in Asia, particularly Southeast Asia.
Because InSAR enables reliable measurements of coastal subsidence to a tenth of a millimetre, the researchers elected to employ it. InSAR utilises satellite-collected radar pictures of the Earth’s surface to map the deformation of the ground. The InSAR datasets are larger and more precise because, unlike visible or infrared light, the radar waves utilised by INSAR penetrate most weather clouds and are equally effective in the dark.
A common cause of rapid land subsidence is groundwater extraction. This is worrisome in Asia, where many coastal cities have become economic hubs and there is a significant need for groundwater extraction to meet the water requirements of rising populations.
A combination of rising oceans, big populations residing on low-lying coastal lands, and sinking lands will have severe effects on many Asian cities in the absence of significant mitigation initiatives.
The report emphasises that while this is a global issue, in many instances the remedy must be local. Slowing the extraction rate of groundwater to a sustainable level should be a primary concern for all coastal towns. The researchers want to advance their investigation by forecasting the rates of sinking land, considering climate and weather-related variability and sensitivity.