In coastal cities with millions of inhabitants around the world, the subsoil sinks rapidly

Coastal cities around the world are sinking rapidly, in some cases by several centimeters per year, as a satellite measurement shows. As a result, millions of people are at greater risk of flooding than simply rising sea levels would suggest.

Asian metropolises such as Jakarta, Taipei and Mumbai are particularly affected, but Tampa in Florida, Istanbul and Auckland also have sharply declining districts.

Coastal areas are among the most densely populated areas on earth, but at the same time they are most threatened by flooding, in two ways. Because, in addition to the rise in sea level, in many places it is endangered by a serious subsidence of the soil, mainly caused by the excessive withdrawal of groundwater. Affected areas include many coasts of Asia, but also California and the east coast of the United States.

More than an inch a year

Pei-Chin Wu and his colleagues at the University of Rhode Island in the United States have now determined which coastal cities are currently most affected by land subsidence using satellite data. To do this, they evaluated synthetic aperture interferometric radar (InSAR) data on the European Earth observation satellite Sentinel-1. For 99 coastal cities around the world, they analyzed bimonthly measurements of subsurface elevation from 2015 to 2020.

The result: “In 33 of the 99 cities studied, at least part of the area is sinking by more than one centimeter per year, five times faster than the average global sea level rise,” the researchers report. Due to climate change and melting ice, the sea level is currently rising by as much as three millimeters per year.

Land subsidence in these coastal cities therefore increases the risk of flooding: “If land subsidence continues at this rate, these cities will be at risk of flooding much earlier than predicted by current sea level models,” the team writes. .

Millions of metropolises in Asia affected

Soil subsidence is strongest in some of Asia’s densely populated metropolitan areas. The subsoil in Jakarta and Semarang in Indonesia, in Tianjin in China and in Chittagong in Bangladesh is sinking by an average of more than three centimeters per year. However, the maximum rate of failure reached up to 30 centimeters at some measurement points, the researchers report.

In addition, more than 20 metropolises are particularly at risk, in which large urban areas, which however are less than five meters above sea level, are sinking at a rapid rate, including Shanghai, Bangkok, Hanoi and Tianjin. In the metropolis of Shanghai alone, 1,700 square kilometers of plains are involved. Mumbai, home to as many as 20 million people, is also sinking up to eight centimeters per year in some places.

But also the western metropolitan areas

But some coastal cities in western industrialized countries are also at serious risk of soil subsidence: in the Tampa Bay region of Florida, a flat area of ​​about 80 square kilometers is sinking up to eleven centimeters per year. In Istanbul, an area on the western outskirts of the city is experiencing rapid subsidence. In Auckland, New Zealand, 80 square kilometers of flat land are still sinking with an average of over an inch per tenant per year.

Scientists warn that millions of people and some of the world’s most important economic centers could be threatened by flooding in the near future if countermeasures are not taken. More than 59 million people live in the four cities badly affected by the subsidence: Chittagong, Karachi, Manila and Tianjin.

Groundwater abstraction is the main cause

Wu and his colleagues see people themselves as the main cause of soil subsidence: “This global problem is largely a consequence of human activity, especially in the form of groundwater abstraction,” they explain. This is demonstrated, among other things, by the fact that the settlement is often particularly large where a particularly large amount of water is drawn from the ground for industrial plants or residential areas. This destabilizes the subsoil and causes it to sink.

However, there are also first positive news: at least in some metropolitan areas it has already been possible at least to slow down the subsidence of the land. For example, the reduction rates in Jakarta and Shanghai were significantly higher about 20 years ago. However, as stricter regulations have been in place for water abstraction since then, the rate has slowed since then. ‘These examples show that such regulation can be an effective tool for stopping failure,’ say the researchers. (Geophysical Research Letters, 2022; doi: 10.1029 / 2022GL098477)

Source: Geophysical Research Letters

This article was written by Nadja Podbregar