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Rising seas threaten to consume the coastal areas of major metropolises around the world. Now those risks are compounded by an accelerating danger: Most of those cities are also sinking.

That means flooding and other disruptions sharpened by future sea-level rise could hit those urban centers far sooner than expected, according to a study in Geophysical Research Letters.

Using satellite data to measure subsidence rates in 99 coastal cities, the researchers found that many of those metropolises are sinking faster than sea levels are rising. Coastal subsidence is happening globally, but the biggest problem is in Asia—where rapidly urbanizing areas are increasing demand for groundwater.

The study shows that in Jakarta, the dense Indonesian capital that’s teeming with an estimated 11 million people, land subsided nearly 15 times as fast as global mean sea-level rise between 2015 and 2020—making it one of the fastest-sinking cities on Earth.

Some of the sinking is tied to natural processes, but it’s greatly accelerated by human activity. The research finds that the main driver of the accelerated subsidence is likely groundwater extraction as booming city populations put more pressure on underground aquifers for washing, cooking and bathing. Oil and gas production and new construction also contribute to the problem.

The financial and human costs stand to be significant even in cities where only certain areas are sinking faster than seas are rising, the researchers say. That is true of cities such as Lagos, Nigeria, Africa’s most populous metropolitan area; Taiwan’s capital, Taipei; and Mumbai, India, the world’s seventh-most populous city.

In addition to Jakarta, four other cities in Asia—Chittagong, Bangladesh; Tianjin, China; Manila in the Philippines; and Karachi, Pakistan—are all rapidly subsiding, putting a combined 59 million people at risk of increased flooding and related impacts. In Tianjin, a major port city near Beijing, maximum subsidence rates are almost 20 times greater than mean sea level rise.

Some cities have taken beneficial steps to curb subsidence. Parts of Jakarta were sinking at up to 280 millimeters a year, according to a 2011 study in the journal Natural Hazards, until the government put in place regulations aimed at reducing groundwater extraction.

Officials in Shanghai, Houston, and Silicon Valley in California also took measures to improve groundwater management, highlighting the role regulation can play in addressing land subsidence where human activity is to blame.

Other measures have been costly and ineffective, in part because they don’t account for the role subsidence plays.

Plans to build a giant sea wall around Jakarta to protect it from sea-level rise have only been partially fulfilled and have been complicated by rapidly accelerating impacts due to climate change, such as more severe storms.

Jakarta is still sinking in some places by 20 to 30 millimeters a year, and the government has plans to move the capital to a seaport on the eastern coast of Indonesian Borneo, raising fresh concerns among environmentalists that its development could threaten water supplies and natural ecosystems.

Reprinted from E&E News with permission from POLITICO, LLC. Copyright 2022. E&E News provides essential news for energy and environment professionals.

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