Satellite data reveals what is driving land loss across river deltas worldwide.
By Chloe Baul
In some of the world’s most crowded coastal regions, the ground is dropping faster than the sea is rising, putting millions of people at risk, researchers say.
In a study published Wednesday in Nature, scientists say many of the planet’s largest river deltas are losing elevation more quickly than local sea levels are rising, dramatically increasing the risk of flooding, saltwater intrusion and long-term land loss.
Researchers analyzed 40 deltas across five continents, including the Mississippi, Mekong, Nile and Ganges–Brahmaputra systems. In 18 of those deltas, land subsidence already exceeds sea-level rise, putting more than 236 million people at heightened near-term risk.
Using satellite radar, the research team created a detailed, delta-wide map of elevation change. Each pixel represents about 75 square meters of land, allowing scientists to track how different parts of a delta are rising or sinking over time.
“In many places, groundwater extraction, sediment starvation, and rapid urbanization are causing land to sink much faster than previously recognized,” Leonard Ohenhen, the study’s lead author and an assistant professor at the University of California, Irvine, said in a press release.
Virginia Tech geoscientists Manoochehr Shirzaei and Susanna Werth led the research. Shirzaei said the scale of the problem was larger than many scientists expected.
“Our results show that subsidence isn’t a distant future problem — it is happening now, at scales that exceed climate-driven sea-level rise in many deltas,” Shirzaei, director of Virginia Tech’s Earth Observation and Innovation Lab, said in the press release.
River deltas naturally compact over time as sediments slowly settle. But human activity has sped up that process in many parts of the world. Shirzaei said the biggest differences between deltas come down to how people manage water, land and rivers.
Heavy groundwater pumping removes water from underground sediments, causing the land to compress and sink. Dams and river engineering trap sediment that would otherwise replenish deltas. Growing cities add weight to soft ground and increase demand for water, which often leads to even more pumping.
“When groundwater is over-pumped or sediments fail to reach the coast, the land surface drops,” Werth said in the press release. “These processes are directly linked to human decisions, which means the solutions also lie within our control.”
Some of the fastest sinking areas are not just in big cities.
Shirzaei said one of the biggest surprises was how widespread subsidence is across entire delta regions, including rural and agricultural areas.
“One of the most striking surprises was how widespread and severe subsidence is, even outside major cities,” he said in an email. “We expected large cities on deltas to be sinking rapidly — but we were surprised to see entire delta regions, including rural and agricultural areas, sinking at similar or even faster rates.”
That sinking can turn into real flooding problems quickly. In some places, land is dropping by several millimeters to more than a centimeter each year.
“Within a decade, drainage systems stop working properly,” Shirzaei said. “Floods that used to happen once every few decades can become annual or even seasonal.”
Saltwater can also creep into freshwater supplies and farmland before permanent flooding becomes visible.
The implications are serious for major food-producing regions. The Mekong Delta in Vietnam, one of the world’s most important rice-growing areas, is especially vulnerable.
“In the Mekong Delta, the situation is more urgent,” Shirzaei said. “High subsidence rates driven largely by groundwater extraction mean that land loss and flooding could accelerate rapidly, threatening food security, freshwater supplies, and livelihoods for tens of millions of people.”
The Mississippi Delta faces a different, but still costly, future if sinking land continues without large-scale sediment restoration.
Despite the risks, Shirzaei said subsidence is one of the few major climate-related threats that can still be slowed through local action.
“If we had to name one: better groundwater management,” he said. “Reducing excessive groundwater pumping — through regulation, alternative water supplies, managed aquifer recharge, and more efficient water use — can significantly slow or even halt subsidence in many deltas.”
He said the stakes are high because sinking land often goes unnoticed until it becomes a crisis.
“Subsidence is happening now, and for hundreds of millions of people living on deltas, it is already reshaping daily life, infrastructure, and safety,” Shirzaei said. “The good news is that because subsidence is largely driven by human decisions, it is also one of the most solvable coastal challenges we face — if we act early enough.”
