It’s hardly news that global sea levels are rising due to climate change, a threat facing coastal communities around the world, including New York City, but some cities are at greater risk than others. Coastal cities in Southeast Asia, including Bangkok, Jakarta, and Manila, face the most immediate threat.
According to a 2022 study led by US Geological Survey research geophysicist Tom Parsons, “The most rapid subsidence is occurring in South, Southeast, and East Asia. However, rapid subsidence is also happening in North America, Europe, Africa, and Australia,” the team wrote in their study. “Human activity – primarily groundwater extraction – is likely the main cause of this subsidence.”
Tianjin, China, was found to be sinking the most at 5.22 cm (2.06 inches) per year, followed by Semarang, Indonesia at 3.96 cm (1.56 inches) per year, and Jakarta at 3.44 cm (1.35 inches) per year. The only US city to appear in the top 10 was Houston, sinking at a rate of 1.95 cm (0.77 inches) per year.
However, the city undoubtedly sinking the fastest is Mexico City, Mexico. A similar study in 2021 showed that the city has been sinking at a fairly constant rate of 50 cm (19.69 inches) per year since 1950. The subsidence is not the result of groundwater pumping, but because of compaction of the clay-rich lake bed on which Mexico City was built.
The main points of the study show that the team measured the subsidence rate for 99 coastal cities around the world between 2015 and 2020 using the PS Interferometric Synthetic Aperture Radar method and Sentinel-1 data.
In most cities, part of the land is subsiding faster than sea level is rising and will be challenged by flooding sooner than expected.
Coastal nations must institute continuous monitoring and policy interventions to reduce subsidence rates and minimize their consequences.
The InSAR (Interferometric Synthetic Aperture Radar) study from 2015 to 2020 reveals that various key cities globally are experiencing significant subsidence, which outpaces sea level rise. This phenomenon poses severe economic and human impacts due to the high value and dense population of these areas. Notable examples include:
Istanbul, Turkey’s capital with 15 million residents, has a 5 × 20 km western section sinking over 2 mm/year.
Lagos, Nigeria’s capital and Africa’s most populous metropolitan area with over 24 million people, has a central 5 × 10 km area subsiding over 2 mm/year.
Taipei, the capital of Taiwan, home to 2.7 million people, is experiencing widespread subsidence exceeding 2 mm/year. Mumbai, India, with a population of around 20 million, has significant portions sinking faster than 2 mm/year. Auckland, New Zealand’s largest city with 1.6 million residents, has a substantial area, over 80 km², subsiding more rapidly than 2 mm/year. The Tampa Bay Area in Florida, USA, with over 3 million people, has an 800 km² northwest section, including a 25-km-long coastal area, subsiding faster than 2 mm/year.
The study highlights that the fastest subsidence rates are predominantly in Asian cities. Jakarta is notably recognized for rapid subsidence. Other Asian cities like Chittagong (Bangladesh), Tianjin (China), Manila (Philippines), and Karachi (Pakistan), with a combined population of 59 million, also exhibit alarming subsidence rates. Tianjin’s maximum subsidence rate surpasses 40 mm/year, Chittagong and Manila’s over 20 mm/year, and Karachi’s over 10 mm/year. These rates are significantly higher than average sea level rise. The subsidence in these areas, often residential or industrial, is likely due to excessive groundwater extraction.
The significant subsidence rates in cities like Istanbul, Lagos, Taipei, Mumbai, Auckland, and the Tampa Bay Area necessitate urgent and comprehensive measures to ensure future safety and sustainability, including the potential relocation of populations. As these cities sink, they face an increased risk of flooding, exacerbated by rising sea levels, which can lead to widespread damage to infrastructure, including buildings, roads, and bridges. This situation also poses a threat of water contamination, as saltwater intrusion into freshwater sources becomes more likely, particularly in coastal areas.
To address these challenges, a multifaceted approach is required. Infrastructure must be adapted and strengthened to withstand subsidence and flooding, involving the construction of elevated buildings and improved drainage systems. Sustainable water management practices are essential, including reducing groundwater extraction and exploring alternative sources like rainwater harvesting. Investment in flood defense systems, such as sea walls and levees, is crucial.
Urban planning must be revised to account for subsidence risks, avoiding construction in high-risk areas and incorporating green spaces for water absorption. However, in some cases, these measures may not be sufficient, and relocating populations inland may become a necessary strategy. This relocation would involve significant social, economic, and cultural considerations, as communities may need to leave ancestral homes and livelihoods behind. Therefore, it’s essential to plan these moves carefully, ensuring adequate infrastructure and economic opportunities in the new locations to support the displaced populations.
Community preparedness is key, with education on risks and responses to subsidence and flooding, along with effective emergency response plans. International collaboration and funding will be important in supporting research, technology transfer, and infrastructure development. Continuous monitoring and research into subsidence rates and mitigation strategies will help in adapting to changing conditions and improving response strategies.
Overall, addressing the challenges posed by subsidence requires a comprehensive approach that combines infrastructure development, sustainable resource management, community preparedness, international cooperation, and, when necessary, the planned relocation of populations to safer areas.