Some of the world’s most densely populated river deltas are sinking faster than sea levels are rising, exposing more than 236mn people to growing flood risk, according to a major international study published on January 14.
The research, published in the journal Nature and led by the University of California, Irvine, with partners including the University of East Anglia (UEA), found that human-driven land subsidence, largely caused by groundwater extraction, is now the dominant factor behind the loss of elevation in many deltas.
River deltas occupy just 1% of the Earth’s land surface but host between 350mn and 500mn people and include 10 of the world’s 34 megacities. They underpin global food production, fisheries, ports and major transport networks, making their stability critical to national and international economies. Yet these low-lying landscapes, much of them less than two metres above sea level, are increasingly threatened by a combination of sinking land and rising seas.
“Our study provides the first delta-wide, high-resolution subsidence observations across 40 major river delta systems, revealing not just where land is sinking, but quantifying how much,” said Leonard Ohenhen, an assistant professor of Earth System Science at UC Irvine and lead author of the study, in a press release bne IntelliNews.
Using satellite radar data, the researchers mapped elevation change across 40 of the world’s largest deltas. They found that at least 35% of all delta land is sinking, and in 38 of the 40 deltas studied, more than half of the total area is subsiding.
In 18 deltas, including the Nile, Ganges-Brahmaputra, Mekong, Chao Phraya, Pearl and Yellow rivers, the average rate of land subsidence already exceeds the rate of regional sea-level rise.
“In every delta that we monitored at least some portion is sinking faster than the sea surface is rising,” said Professor Robert Nicholls of UEA and the University of Southampton, a co-author of the study. “In many densely populated deltas, like the Mekong, Chao Phraya and Nile, vast areas are sinking faster than current sea-level rise rates threatening many millions of people.”
Human footprint
All deltas naturally sink over time as newly deposited sediments compact under their own weight, but the study found that human activity has dramatically accelerated this process.
The main drivers are “excessive groundwater extraction, oil and gas exploitation, and land-use changes associated with urbanisation and agriculture”, the authors said.
The researchers calculated that in 35% of the deltas studied, groundwater pumping was the dominant cause of subsidence, as cities, farms and industries draw water from underground aquifers.
Subsidence rates vary widely, from less than one millimetre per year in Canada’s Fraser Delta to more than one centimetre per year in China’s Yellow River Delta, with many areas sinking at more than double the current rate of global sea-level rise.
In the US, the Mississippi River Delta continues to sink rapidly. The study found it is subsiding at an average of 3.3 millimetres per year, while local sea levels along the Gulf Coast are rising at about 7.3 millimetres per year. In some places, however, the land is sinking much faster, by more than 89 millimetres per decade, compounding Louisiana’s long-running land-loss crisis.
Globally, climate change is pushing sea levels higher as polar ice melts and oceans warm. Average global sea levels are now rising by about four millimetres per year. But the researchers warned that in many deltas, sinking land is the more immediate threat.
“The dominance of subsidence persists even when compared to future sea level rise worst-case scenarios,” the study said. “This means that for hundreds of millions of coastal residents, the immediate threat is not just climate change-driven sea level rise alone, but the more immediate threat of the ground sinking beneath their feet.”
“These results give delta communities a clearer picture of what is driving persistent flood risk and overall vulnerability, and that clarity matters,” Ohenhen said. “If land is sinking faster than the sea is rising, then investments in groundwater management, sediment restoration and resilient infrastructure become the most immediate and effective ways to reduce exposure.”
Subsidence overlooked
Nicholls said subsidence is often overlooked until the damage is already visible.
“Subsidence is often ignored until it causes impacts,” he told bne IntelliNews. “A range of options are available including mitigate human-induced subsidence by removing the causes of subsidence e.g., stopping groundwater withdrawal where this is causing subsidence – successfully employed in some cities – notably Tokyo and Osaka but not a universal response as yet.”
Another option is to rebuild land naturally using river sediments, though this is difficult in built-up areas.
“Promote accretion with sedimentation – but this means allowing flooding with sediment-laden water so difficult in urban areas – possible in rural and natural areas – still more of a concept than a real measure,” he said.
In many places, however, societies simply adapt to the sinking land. “Accept the subsidence and adapt to the changes – for relative sea-level rise due to subsidence you can do the same things as you would for sea-level rise – advance, protect, accommodate or retreat. [This is] widely done,” Nicholls said.
He added that the findings highlight the need for more integrated coastal management. “More generally, subsidence shows the need to take a more holistic perspective of managing coasts and considering all the drivers of hazard and risk in management,” he said.
Asia at the epicentre
While all deltas are affected to some degree, the most severe impacts are concentrated in fast-growing developing economies, especially in Asia.
“It affects all river deltas to some degree, but deltas where human-induced changes are fastest see the biggest issues – so mainly developing countries see the big changes,” Nicholls said. “Deltas are also concentrated in east, south-east and east Asia so there is a regional dimension to this issue.”
These regions are home to megacities such as Bangkok, Ho Chi Minh City, Shanghai and Dhaka, where tens of millions of people depend on fragile delta landscapes for housing, jobs, food and water.
Looking further ahead, Nicholls warned that the long-term viability of delta cities is increasingly uncertain.
“To my knowledge, no city has been abandoned to date due to subsidence and enhanced protection has been the norm,” he said. “However, New Orleans has not fully recovered post-Katrina and in Louisiana a lot of people moved vertically up (to Baton Rouge) which is a little higher.”
“Looking to the future your question is a concern and cities in deltas will only remain viable with substantial additional adaptation and efforts to address subsidence as well,” he added.
The researchers say their high-resolution mapping should help governments and planners decide where to focus scarce resources, combining efforts to slow subsidence with long-term climate adaptation as rising seas continue to push against the world’s most vulnerable landscapes.
