For decades, discussions surrounding coastal risk have focused primarily on climate change and sea level rise. However, a significant new global study reveals an even more urgent threat: land subsidence, affecting hundreds of millions of people living in delta regions, including urban hubs like New Orleans and Bangkok.

In various locations around the world, land is sinking at rates that often surpass the rising sea levels.

Utilizing satellite radar technology to monitor minute changes in the Earth’s surface, researchers have discovered that over half of the world’s deltas—low-lying areas where major rivers converge with the ocean—are currently sinking. This gradual subsidence, in conjunction with sea level rise, poses the most significant flood risk in many densely populated delta regions on Earth.

“This is truly a declaration of war,” stated Professor Robert Nicholls, co-author of the study and coastal scientist at the University of Southampton. The findings were reported in BBC Science Focus. “Until now, no one had taken a global perspective on delta subsidence. This study highlights the breadth of the issue and underscores the urgency of addressing it.”

The survey results can be found in the journal Nature.

Global Problems Hidden in Plain Sight

Delta regions comprise only 1% of the Earth’s land area but are home to approximately 350 to 500 million people, including some of the world’s most significant cities and productive agricultural zones. These areas serve as economic powerhouses, environmental hotspots, and essential food sources, yet they are inherently fragile.

Deltas are formed by loose, water-saturated sediments deposited over millennia. In their natural state, these sediments compact under their own weight and gradually sink.

Historically, natural subsidence was balanced by periodic flooding that replenished the land with fresh sediment, but modern interventions have disrupted this equilibrium.

The recent study analyzed satellite measurements across 40 major delta regions from 2014 to 2023, creating the first high-resolution global image detailing land elevation changes.

The findings were alarming: currently, at least 35% of delta regions have subsided, with over half of the land surface subsiding in most deltas.

In 18 of the 40 river deltas examined, land is sinking faster than local sea level rise, revealing hotspots where subsidence dominates over regional and global sea level increases.

A similar pattern is evident across continents—Asia, Africa, Europe, and the Americas—where relative sea levels rise due to both ocean expansion and land subsidence.

“From a risk perspective, it doesn’t matter if sea levels rise or land sinks,” Nichols explained. “The ultimate effect is the same, but the responses to those threats may differ.”

What is Causing the Sinking?

The study identified three primary causes of anthropogenic land subsidence: groundwater extraction, reduced sediment supply, and urban expansion. Among these, groundwater pumping is the most significant predictor.

When groundwater is extracted, the soft surrounding sediments collapse and compact, a process that is nearly irreversible. Once the sediment is compacted, it will not return, even if water levels recover.

In 10 out of the 40 delta regions studied, groundwater depletion was the main factor driving land subsidence. Additionally, reduced river sediment caused by damming and flood defenses, combined with the weight of growing cities built on soft soils, contribute to this crisis.

As a result, what was once a slow geological phenomenon has transformed into an urgent environmental crisis.

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US Case: Mississippi Delta

The Mississippi River Delta in New Orleans and Louisiana exemplifies this issue in the United States.

The analysis confirms widespread subsidence across the delta, with over 90% of the region experiencing subsidence at an average rate of 3.3 mm per year. Some localized areas even sink much faster.

While this rate may seem minimal, it accumulates significantly over decades, especially alongside the threats posed by rising sea levels and hurricanes.

The Mississippi Delta has lost thousands of square kilometers of coastal wetlands over the last century, resulting in catastrophic damage. An area the size of a soccer field is lost to open water every 100 minutes.

The lack of fresh sediment is a critical issue. Levees and dams prevent flooding and the natural deposition of new sediments that help rebuild the land. Additionally, drainage systems, oil and gas extraction, and decades of groundwater pumping exert further stress on fragile soils.

While some delta areas display resilience, one proposed solution is relocating populations away from these vulnerable regions. For instance, New Orleans has seen a steady population decline since the 1960s.

“In the United States, people tend to accept the idea of relocation,” Nichols noted, emphasizing that societal mobility and favorable land-use policies make this transition more politically feasible than in parts of Europe and Asia, where long-term protective measures are generally favored.

Warning to Major Cities

While North America grapples with these challenges, the most extreme subsidence rates can be found in parts of South and Southeast Asia, where population density is high and dependence on groundwater for agriculture, industry, and drinking water prevails.

Regions such as the Mekong River (Vietnam), Ganges and Brahmaputra rivers (Bangladesh and India), Chao Phraya River (Thailand), and Yellow River (China) are sinking faster than current global sea level rise in some areas by over a centimeter per year.

Mega-cities like Bangkok, Dhaka, Shanghai, and parts of Jakarta are built on these subsiding foundations.

The good news is that, unlike global sea level rise—which unfolds over centuries—human-induced land subsidence can respond swiftly to policy changes. A notable success story is Tokyo.

In the mid-20th century, unchecked groundwater extraction caused parts of Tokyo to sink more than 4 meters. However, rigorous regulations on groundwater use and investments in alternative water sources resulted in a swift decrease in subsidence rates.

“Authorities have enacted legislation to ensure sufficient alternative water supplies and eliminate groundwater extraction,” Nichols remarked. “And almost overnight, this led to stabilization.”

Additional solutions include managed flooding in agricultural areas to replenish soil sediments. “Sediment is often deemed a pollutant,” Nichols points out. However, when rivers overflow, they deposit valuable materials that built the delta, a process sometimes referred to as “brown gold.”

Urban areas can be fortified with effective engineering solutions such as sea walls, levees, and storm surge barriers. “Addressing subsidence complements efforts to adapt to sea level rise and reduces vulnerabilities,” Nichols added, as reported here.

Shifting Attitudes Towards Coastal Risk

The study’s authors emphasize that land subsidence has been dangerously overlooked in global climate risk strategies, largely viewed as a local rather than a global issue.

However, local does not equate to minor. Even under severe climate scenarios, land subsidence is expected to remain the primary driver of relative sea level rise in numerous delta regions for decades to come.

Financial and institutional barriers often hinder large-scale interventions in many areas, but deferring action only exacerbates the costs and challenges of future adaptations.

Once land subsides, initiating new urban developments is not feasible, leaving communities to face tough decisions about relocation.

As Nichols succinctly states, “The first crucial step is to acknowledge that a problem exists.”

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