New research involving the University of East Anglia (UEA) reveals how fast the world's river deltas are sinking and the human-driven causes.
Home to hundreds of millions of people, until now it was unclear what the rate of delta elevation loss is, or what is driving delta subsidence.
In a new study published today in Nature, scientists report that land subsidence caused by humans - through the extraction of groundwater - is the main culprit.
The study, led by the University of California, Irvine and involving researchers from the Tyndall Centre for Climate Change Research at UEA, shows that the sinking of these deltas is placing more than 236 million people at increased flooding risk in the near future.
It also quantified the relative contributions of specific human drivers: groundwater extraction, sediment starvation and urbanization across these deltas, allowing the researchers to identify the dominant cause of the sinking.
The findings could help communities living in deltas better prioritize immediate local interventions alongside climate adaptation efforts.
"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, UC Irvine assistant professor of Earth System Science and lead author of the study.
Co-author Prof Robert Nicholls, of UEA and the University of Southampton, said: "The dominance of subsidence over sea-level rise is striking. In every delta that we monitored at least some portion is sinking faster than the sea surface is rising.
"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."
The team found that, across deltas, land is sinking at an average rate ranging from less than one millimetre per year in deltas like the Fraser Delta in Canada, to more than one centimetre per year in China's Yellow Delta, with many deltaic areas sinking more than double the present rate of global sea-level rise.
In coastal and deltaic regions around the world, the lack of high-resolution elevation change measurements has long hindered efforts to distinguish the severity of land subsidence and sea-level rise.
Using satellite radar data, the researchers analysed surface elevation loss across 40 deltas, which revealed that in 35 per cent of them, extraction of groundwater by humans is the main cause of land subsidence.
In the United States, for example, the Mississippi River Delta has a long-documented history of subsidence, and the new analysis confirms that this trend remains pronounced.
The Mississippi Delta is sinking at an average rate of 3.3 millimetres per year, compared with the regional Gulf Coast sea-level rise of 7.3 millimetres per year – though substantial areas are subsiding faster than this local sea level rise, in some areas more than 89 millimetres (3.5 inches) per decade. These patterns reinforce ongoing land-loss concerns in coastal Louisiana from both the land and the seas.
Climate-driven sea-level rise remains a fundamental long-term threat, melting polar ice and warming ocean temperatures are currently causing global sea levels to rise by four millimetres every year. The subsidence and the rising sea levels are additive leading to increased flooding and submergence of deltaic regions.
The dominance of subsidence persists even when compared to future sea level rise worst-case scenarios. 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," said Dr Ohenhen. "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."
Prof Nicholls added: "Measures to address subsidence are complementary to adapting to rising seas, and dealing with subsidence makes us less vulnerable to rising seas."
Key collaborators on the research included Manoochehr Shirzaei of Virginia Tech, Jim Davis and Austin Chadwick of Columbia University, Philip Minderhoud of Wageningen University and Research, and Julius Oelsmann of Tulane University.
'Global subsidence of river deltas' is published in Nature on January 14.
