The sinking of the land is linked to the extraction of underground resources and rapid urbanization
An international team of scientists has found that many densely populated coastal cities around the world are at high risk of extreme relative sea level rise as the land is sinking due to groundwater extraction and other industrial processes.
The team of scientists, led by Nanyang Technological University, Singapore (NTU Singapore), University of New Mexico, ETH Zürich, and NASA’s Jet Propulsion Laboratory, operated by the California Institute of Technology, processed satellite images of 48 of the largest coastal cities. Worldwide from 2014 to 2020 to measure Earth subsidence rates using a cloud-based processing system. The study found that cities such as Tianjin (China), Ho Chi Minh City (Vietnam), Chittagong (Bangladesh), Yangon (Myanmar), and Jakarta (Indonesia) are experiencing rapid land subsidence.
As land ice sheets melt and sea water expands, sea levels rise globally. However, land subsidence can occur much faster than sea level rise, exacerbating the problem of coastal flooding, according to scientists.
The results of the study entitled Land subsidence increases sea level rise: InSAR analysis of 48 major coastal citiesIt was published in the magazine Nature Sustainability in September
“By estimating how much and how quickly these densely populated coastal cities are receding, our study helps constrain projections of coastal flooding in the coming decades as we expect more land to be inundated by rising sea levels and land subsidence,” said lead author Cheryl. Tay, Ph.D. Student at Asian School of Environment NTU.
Eric Lindsay, Associate Professor in the United Nations University’s Department of Earth and Planetary Sciences, specializes in analyzing satellite data and monitoring Earth’s subsidence rates. He is one of the co-authors of the study, which began when he was a research fellow at NTU’s Earth Observatory in Singapore. Lindsay has spent several years in Southeast Asia studying the movement of the Earth’s surface to monitor subterranean hazards that can lead to disasters such as floods in Jakarta, Indonesia.
“This study highlights the value of high-resolution satellite observations to better understand this problem — subsidence rates can vary rapidly across small regions, which means that ground-based measurements often do not capture the true scale of the problem,” Lindsey said.
To obtain accurate measurements of Earth’s subsidence, Lindsay and co-authors combined data collected by the Sentinel-1 satellites operated by European Space Agency (European Space Agency). Since the launch of the first Sentinel-1 satellite in 2014, they have been orbiting the Earth using radar to measure subtle changes in surface elevation. This technology is known as synthetic aperture radar, or InSAR. Each Sentinel-1 satellite takes 90 minutes to complete one orbit.
However, due to the Earth’s rotation, it can take up to 12 days for a satellite to pass over the same location twice. This results in twice-monthly observations of the Earth’s surface for every point on the planet. The European Space Agency (ESA) collects, stores and provides free global access to raw data that is sent back to Earth.
To further his research on Earth subsidence, Lindsay collaborated with UNM Advanced Research Computing Center (CARC) to host and process data that will help researchers determine the actual state of aquifers around the world with a focus on New Mexico.
To create Earth landing maps, Lindsey first collects all satellite images collected over an area. This means that it routinely downloads hundreds or thousands of high-resolution images covering roughly 24,000 square miles each. This data is collected and processed together to form a record of Earth’s slow motion toward or away from the satellite, with a resolution of centimeters or better. However, the large number of images required in this process consumes tens of terabytes of data. This huge amount of data can be problematic and require huge storage capacities.
“For this study, we used a cloud processing system that gave us access to a lot of processing power and nearly unlimited storage,” Lindsay said. “However, we’ve found that this flexibility comes at a much higher cost than if we could host the data locally.”
To help solve these storage issues, Lindsey was recently awarded a National Science Foundation (NSF) grant to create a new data storage contract at UNM CARC. This increased storage, combined with CARC’s high-speed parallel computing capabilities, will enable it to process hundreds of terabytes of data needed to update these coastal land subsidence maps as well as examine land subsidence caused by groundwater extraction across the southwestern United States.
The 48 cities surveyed as part of the NTU study were selected based on a minimum population of five million in 2020 and located within a maximum of 50 kilometers from the coast. A comparison of coastal cities around the world shows that the fastest relative local land subsidence speeds are concentrated in Asia, especially Southeast Asia.
Land subsidence varies at neighborhood level and even at block level but across the 48 cities surveyed, the team found an average sinking velocity of 16.2 millimeters (mm) per year, while some have land sinking at 43 millimeters per year. The current global average sea level rise is 3.7 mm/year. Coastal cities experiencing significant land sinking at a rate of more than 20 mm per year include Tianjin (China), Ho Chi Minh City (Vietnam), Chittagong (Bangladesh), Yangon (Myanmar), Jakarta (Indonesia), and Ahmedabad (India).
The findings are an example of groundbreaking research that seeks to address humanity’s major challenges to sustainability and accelerate the translation of research discoveries into innovations that mitigate human impact on the environment.
“The rapid sinking of the land is often caused by the extraction of groundwater,” Tay said. “This is concerning in Asia where many coastal cities are now centers of growth, and their demand for groundwater extraction is high to meet the water needs of a growing population.”
“Our study highlights the fact that while this is a global issue, in many cases the response must be local,” Lindsay said. “Slowing the rate of groundwater extraction to a sustainable level should be a top priority for all municipalities in coastal areas.”
This research will contribute to the National Sea Level Program in Singapore with support from the National Research Foundation, Singapore, and the National Environment Agency of Singapore.
Cut image: A girl plays in the flood waters caused by the subsidence that left her neighborhood below the high tide line. Jakarta (Indonesia) is home to more than 20 million people on the island of Java, and is one of the many cities now facing this problem. Image credit: Eric Lindsay, University of New Mexico.