Coastal Floods & Sinking Cities: A Global Crisis

Coastal flooding is worsening globally due to a combination of rising sea levels and land subsidence, with cities across Asia and the United States particularly vulnerable. In Jakarta, sea levels have already surpassed land elevation in some areas, necessitating concrete barriers to prevent inundation.

Sea Level Rise and the Moon’s Orbit

Eko Prasetyo, Director of Maritime Meteorology at BMKG, stated that the increased sea levels are primarily triggered by the decreasing distance between the Earth and the Moon, particularly when the Moon is at its closest point, known as perigee. The impact of the year’s second-largest supermoon in early December is still being felt in Indonesian coastal areas, with seawater breaching embankments and flooding river systems.

Land Subsidence: A Major Factor

However, the Indonesian Environmental Forum (Walhi) emphasizes that land subsidence plays a more significant role in coastal flooding, especially in Jakarta. Land subsidence is caused by spatial exploitation, unplanned urban development, and excessive groundwater extraction.

Sinking Cities: A Global Challenge

A recent report by the World Economic Forum (WEF), titled ‘Resilient Economies: Strategies for Sinking Cities and Flood Risks,’ highlighted the lowering of urban land elevation as a global challenge that has not received sufficient attention. The report warns that land subsidence “can threaten the well-being and livelihoods of cities worldwide.”

The WEF report explains that unsustainable human behavior, including groundwater extraction and rapid urbanization, are primary causes of land subsidence, increasing the risk of flooding, infrastructure damage, and population displacement. When combined with rising sea levels and extreme weather, land subsidence can create existential threats.

Globally, approximately 6.3 million square kilometers of land – an area equivalent to India, Argentina, and Japan combined – is experiencing land subsidence, affecting an estimated two billion people.

Global Hotspots and Rates of Subsidence

Analysis of 99 coastal cities showed that 33 experienced land subsidence at a rate up to five times faster than global sea level rise. Shanghai has areas subsiding at up to 10 millimeters per year, while the fastest rates are found in South Asian, Southeast Asian, and East Asian cities.

In some areas, subsidence rates are 10 to 20 times faster than sea level rise. Jakarta, Semarang, and Tianjin are among the cities experiencing this extreme subsidence. Northwest Jakarta is sinking at a rate of up to 280 millimeters (almost 0.3 meters) per year, while Semarang experiences subsidence of 60–120 millimeters annually.

The phenomenon is not limited to Asia. Around 20 percent of urban areas in 28 major U.S. cities experienced land subsidence between 2015 and 2021, potentially affecting approximately 34 million people. Cities like Houston, Dallas, Fort Worth, Chicago, New York, and Detroit are affected, with over 70 percent of their areas impacted. Over 90 flood events have occurred in eight U.S. cities with subsidence rates exceeding 3 millimeters per year, placing around 29,000 buildings at high risk.

Mexico City is sinking at a rate of 350-450 millimeters per year, while the San Joaquin Valley in the U.S. experiences subsidence of 150-300 millimeters annually. Iran’s Rafsanjan Plain is subsiding up to 300 millimeters per year, and Tehran’s rates vary from 50-250 millimeters per year. Other affected cities include Ho Chi Minh City (40-70 millimeters per year), Bangkok (9-30 millimeters per year), Lagos (2-87 millimeters per year), Yangon (10-110 millimeters per year), and Beijing (15-138.5 millimeters per year).

In the last three decades, the rate of global sea level rise has more than doubled to around 3.3 millimeters per year, with projections indicating a potential rise of up to 16.9 centimeters in the next 30 years.