River deltas occupy just one per cent of Earth’s land surface, yet they are home to between 350 and 500 million people. They support agriculture, fisheries, ports, transport networks, and entire national economies. Cities such as Dhaka, Bangkok, Shanghai, Kolkata, and Alexandria all sit on deltas. These areas are naturally low-lying and fragile, making them especially vulnerable to environmental pressures.

Traditionally, sea-level rise caused by melting glaciers and warming oceans has been seen as the main danger. But scientists say that in many deltas, sinking land known as subsidence is worsening conditions much faster and in more localised ways, according to research published in Nature on January 14.

Land is sinking faster than the sea is rising

Using satellite radar data collected between 2014 and 2023, researchers measured changes in land height across 40 of the world’s major river deltas. The findings were stark. Every delta studied showed signs of land subsidence. In more than half of them, the average sinking rate exceeded 3 mm per year. 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.

In some places, the difference is dramatic. Parts of Thailand’s Chao Phraya delta and Indonesia’s Brantas delta are sinking at more than twice the pace of rising seas. When land sinks, even small increases in sea level can cause larger floods, push saltwater into farmland and drinking supplies, and damage buildings and infrastructure.

Overall, scientists estimate that more than half of the world’s total delta land area is currently sinking. South and Southeast Asia are the most affected regions, accounting for the largest share of sinking land and population exposure. Major cities built on these deltas are often sinking at the same or even higher rates than surrounding rural areas.

What makes subsidence particularly hazardous is that it occurs unevenly. In the same delta, there could be regions sinking quickly into the water while other parts are relatively stable or even rising slightly. This makes some areas extremely hazardous due to sudden flooding and loss of land.

Human activities are causing the problem

All deltas sink over time as a result of loose sediments compacting due to their own weight. But scientists say human activity has sharply accelerated the process. The strongest driver is groundwater extraction. As water is extracted from underground aquifers by cities and agricultural areas, the land above sinks. This has been seen in many Asian deltas, where groundwater is extensively extracted for drinking, irrigation, and industrial use.

Other significant causes include the reduction of sediment discharge in rivers, which is often due to the construction of dams and flood control embankments, and the rapid growth of cities. Deltas need new sediment deposits to compensate for the loss of land due to erosion and compaction. When sediment is trapped upstream, deltas become delta-starved. At the same time, cities exert pressure on already unstable land.

The analysis shows that in 10 of the 40 deltas studied, groundwater loss was the single-most important factor behind sinking land. In others, subsidence was driven by a combination of groundwater pumping, reduced sediment supply, and urban growth. In wealthier deltas, such as those in Europe or North America, sediment loss and infrastructure played a larger role. In lower-income regions, groundwater extraction emerged as the dominant issue.

Crucially, scientists found that in most deltas, subsidence now contributes more to relative sea-level rise than climate-driven ocean rise. This means that even if global emissions were reduced tomorrow, many deltas would still face worsening flood risks unless subsidence is addressed directly.

The human cost is already severe. Of the roughly 76 million people living in delta areas less than one metre above sea level, more than 80 per cent live on sinking land. In several regions, especially South and Southeast Asia, population growth continues in these vulnerable zones, increasing exposure year after year.

A crisis that can still be slowed

Unlike global sea-level rise, which depends on long-term climate trends, subsidence is a local process and, in many cases, preventable. Scientists say this is 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. Many of the most affected deltas are in low- and middle-income countries with limited resources and weaker institutions. According to global adaptation rankings, nearly two-thirds of the deltas studied fall into a high-risk category, facing both rapid relative sea-level rise and low capacity to respond.

By contrast, deltas in countries with stronger governance, such as the Netherlands or parts of China, are better equipped to manage risks, though even they struggle. The Mississippi Delta in the United States, for example, has lost thousands of square kilometres of land despite decades of awareness.

Scientists warn that focusing only 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 sinking land continues unchecked, researchers say, the world could see accelerated displacement, food insecurity and economic disruption across some of its 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.