The University of California, Irvine has released new research showing that the world’s major river deltas are experiencing significant land subsidence, with human activity identified as the main cause. The study, published in Nature, provides the first high-resolution observations of elevation loss across 40 major delta systems.
“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 professor of Earth system science and lead author of the study. “We also quantified the relative contributions of specific human drivers: groundwater extraction, sediment starvation and urbanization across these deltas, which allows us to identify the dominant driver of the sinking.”
Researchers found that land in these deltas is sinking at rates ranging from less than one millimeter per year in places like Canada’s Fraser Delta to more than one centimeter per year in China’s Yellow River Delta. Many areas are subsiding at rates more than twice as fast as global sea-level rise.
A lack of detailed elevation data has made it difficult for scientists to assess the extent and causes of land subsidence compared to sea-level rise. Using satellite radar data, Ohenhen’s team measured surface elevation loss and determined that groundwater extraction is responsible for most subsidence in about 35 percent of the studied deltas.
“The dominance of subsidence over sea-level rise was more pervasive than anticipated, and in every delta we monitored, at least some portion is sinking faster than the sea surface is rising,” said Ohenhen. “While this affects less than one percent of the area in deltas like the Rio Grande, in others like the Mekong, Chao Phraya and Nile, vast areas encompassing much of the delta area are sinking faster than current sea-level rise rates.”
In the United States, researchers confirmed ongoing concerns about land loss in Louisiana. “The Mississippi Delta is sinking at an average rate of 3.3 millimeters per year, compared with the regional Gulf Coast sea-level rise of 7.3 millimeters per year – though substantial areas are subsiding faster than this local sea level rise, in some areas more than 89 millimeters (3.5 inches) per decade,” said Ohenhen. “These patterns reinforce ongoing land-loss concerns in coastal Louisiana from both the land and the seas.”
While subsidence currently dominates flood risk exposure for many delta regions, long-term threats remain from climate-driven sea-level rise due to melting polar ice and warming oceans.
Ohenhen noted that these findings should inform communities on how best to adapt: “These results give delta communities a clearer picture of an additional threat, which can cause increased flood exposure, and that clarity on the hazard facing them matters,” he said. “If the 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.”
Collaborators on this project include scientists from Virginia Tech; Columbia University; University of East Anglia; University of Southampton; Wageningen University and Research; and Tulane University. Funding came from NASA and the United States Department of Defense.
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