Major deltas worldwide sinking faster than sea levels are rising, risking lives of millions

The world's major river deltas, including the Ganga-Brahmaputra, are sinking faster than sea levels are rising, heightening flood risk in surrounding areas and potentially impacting the lives of 236 million people, a study has found.

Major causes for delta subsidence are groundwater withdrawal, a reduced river sediment supply, and urban expansion, researchers, including those from the US's Virginia Polytechnic Institute and State University, said.

Using satellite data gathered during 2014-2023, the team created a map of surface elevation change across 40 river deltas in five continents. Some portion of nearly every delta was found to be sinking faster than the sea is rising.

The Ganga-Brahmaputra in India, the Mekong in Vietnam, the Nile in Africa, the Chao Phraya in Thailand and the Mississippi in the US are among the deltas with 90 per cent area affected by subsidence, findings published in the journal Nature show.

"Our analysis shows that current average subsidence rates exceed geocentric SLR in 18 of the 40 deltas, including the Nile, Mekong, Red River, Ganges-Brahmaputra, Brahmani, Mahanadi, Chao Phraya, Ciliwung, Brantas and Yellow River deltas, affecting approximately 236 million people -- a population about 50 per cent larger than those residing in deltas in which the current rates of geocentric SLR outpace the subsidence rates (156.9 million)," the authors wrote.

Co-author Manoochehr Shirzaei, associate professor of geophysics and remote sensing at the Virginia Polytechnic Institute and State University, said, "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."

Deltas around the world face a "double burden" of climate change-induced sea-level rise and sinking land, which together drive a relative sea-level rise at rates exceeding global averages, the researchers said.

While sea-level rise reflects global-scale processes and progresses at a uniform rate, subsidence occurs at a local to regional scale and reflects local natural and human processes, they added.

The study provides the first high-resolution, delta-wide assessment of elevation loss across 40 river deltas worldwide, the team said.

Groundwater depletion was found to be the strongest overall predictor of delta sinking, though the dominant driver can vary regionally.

"When groundwater is over-pumped or sediments fail to reach the coast, the land surface drops. These processes are directly linked to human decisions, which means the solutions also lie within our control," author Susanna Werth, assistant professor of hydrology and remote sensing, said.