Sinking Ground, Rising Danger: The Hidden Crisis Threatening Coastal Cities
A groundbreaking global study of major river deltas has uncovered a startling reality: human-driven land subsidence is now overtaking climate change as the biggest flood risk for millions of people living in coastal urban areas. This research, published in Nature on January 14, 2026, shifts the focus from rising sea levels to the rapid sinking of land, a phenomenon exacerbated by human activities that demands urgent attention.
The Accelerating Threat of Land Subsidence
For years, warnings about coastal flooding have centered on rising sea levels due to climate change. However, mounting evidence indicates that land subsidence—the sinking of the ground—is moving faster and posing a more immediate danger in densely populated regions. Across dozens of major river deltas, the land is sinking at rates that often exceed the pace of sea-level rise, creating a compounded risk that could lead to devastating floods, land loss, and long-term instability.
River deltas, which cover just one per cent of Earth's land surface, are home to between 350 and 500 million people. These areas support critical infrastructure, including agriculture, fisheries, ports, and transport networks, with cities like Dhaka, Bangkok, Shanghai, Kolkata, and Alexandria all situated on deltas. Their naturally low-lying and fragile nature makes them exceptionally vulnerable to environmental pressures, and now, human-induced subsidence is pushing them closer to the brink.
Key Findings from the Global Analysis
Using satellite radar data collected between 2014 and 2023, researchers measured land height changes across 40 of the world's major river deltas. The findings were stark and alarming:
- Every delta studied showed signs of land subsidence, indicating a widespread and pervasive issue.
- In more than half of the deltas, the average sinking rate exceeded 3 mm per year, with some areas experiencing much faster declines.
- In 13 deltas, including the Nile, Mekong, Yellow River, and Chao Phraya, the land is sinking faster than the current global rate of sea-level rise.
- Parts of Thailand's Chao Phraya delta and Indonesia's Brantas delta are sinking at more than twice the pace of rising seas, highlighting extreme localised risks.
Overall, scientists estimate that more than half of the world's total delta land area is currently sinking, with South and Southeast Asia being the most affected regions. This uneven subsidence can lead to sudden flooding and land loss in specific areas, making some zones extremely hazardous.
Human Activities Driving the Crisis
While all deltas naturally sink over time due to loose sediments compacting under their own weight, human activity has sharply accelerated this process. The primary drivers include:
- Groundwater extraction: As cities and agricultural areas pump water from underground aquifers, the land above sinks. This is particularly prevalent in Asian deltas, where groundwater is extensively used for drinking, irrigation, and industrial purposes.
- Reduced sediment discharge: The construction of dams and flood control embankments traps sediment upstream, starving deltas of the new deposits needed to compensate for erosion and compaction.
- Rapid urban growth: The expansion of cities exerts additional pressure on already unstable land, exacerbating subsidence rates.
The analysis revealed that in 10 of the 40 deltas studied, groundwater loss was the single-most important factor behind sinking land. In others, a combination of groundwater pumping, reduced sediment supply, and urban growth drove subsidence. Notably, in wealthier deltas, such as those in Europe or North America, sediment loss and infrastructure played larger roles, while in lower-income regions, groundwater extraction emerged as the dominant issue.
The Human Cost and Urgent Need for Action
The human cost of this crisis is already severe. Of the approximately 76 million people living in delta areas less than one metre above sea level, more than 80 per cent reside on sinking land. In regions like South and Southeast Asia, population growth continues in these vulnerable zones, increasing exposure year after year. Scientists warn that subsidence now contributes more to relative sea-level rise than climate-driven ocean rise, meaning that even if global emissions were reduced immediately, many deltas would still face worsening flood risks unless subsidence is addressed directly.
Unlike global sea-level rise, which depends on long-term climate trends, subsidence is a local process and, in many cases, preventable. This presents both a challenge and an opportunity. Measures such as regulating groundwater extraction, restoring sediment flows, recharging aquifers, and planning cities more carefully could slow or even halt sinking in some areas. However, the ability to act varies widely, with many affected deltas in low- and middle-income countries having limited resources and weaker institutions.
Moving Forward: From Global Conversations to Local Actions
Scientists emphasise that focusing solely on rising seas misses the bigger picture. Subsidence is not just an added problem; in many places, it is the main driver of risk. Addressing it requires shifting climate adaptation from a global conversation to local action, tailored to how people use land and water. If left unchecked, unchecked sinking could lead to accelerated displacement, food insecurity, and economic disruption across some of the world's most important regions.
The message is clear: saving coastal cities will require more than fighting climate change. It will also mean stopping the ground beneath them from giving way. By prioritising local interventions and sustainable practices, we can mitigate this hidden crisis and protect millions from its devastating impacts.
