18 of Earth's biggest river deltas — including the Nile and Amazon — are sinking faster than global sea levels are rising

Many of the world's biggest river deltas — including the Nile, Amazon and Ganges — are sinking faster than global sea levels are rising, a new study shows.

This means subsidence is becoming the main driver of land loss, coastal flooding and saltwater intrusion at river deltas — surpassing the impacts of sea-level rise from climate change. Researchers also found that groundwater extraction is the biggest cause of subsidence at deltas globally, with urban expansion and declines in rivers' sediment loads contributing to the overall sinking trend.

Deltas are facing a "double burden" of sea-level rise and sinking land that increases the risk of catastrophic flooding and displacement for millions of people in some of the world's biggest cities, the study's authors warn.

"To our knowledge, this is the most comprehensive, high-resolution, delta-wide assessment of contemporary land subsidence ever conducted at the global scale," co-author Manoochehr Shirzaei, an associate professor of geophysics and remote sensing at Virginia Tech, told Live Science in an email. "Across the deltas we analyzed, groundwater storage change emerged as the single most influential anthropogenic factor explaining subsidence patterns in many systems."

Shirzaei and his colleagues used data from the Sentinel-1 satellite to examine subsidence at 40 of the world's largest river deltas between 2014 and 2023. Sentinel-1 captures changes in ground elevation resulting from subsidence, as well as sediment deposition and erosion, according to the study, which was published Wednesday (Jan. 14) in the journal Nature.

Of the 40 deltas, 18 had average annual subsidence rates greater than the current rate of global sea-level rise, which is about 0.16 inches (4 millimeters) per year.

Zooming in, the researchers found that every studied river delta except the Rio Grande Delta was in some places sinking faster than global sea levels are rising. In 38 deltas, more than 50% of the delta area sank during the study period, and in 19 of them — including the Mississippi Delta, the Nile Delta and the Ganges-Brahmaputra Delta — more than 90% of the delta area had subsided.

The worst-affected deltas in the study were the Chao Phraya Delta in Thailand, the Brantas Delta in Indonesia and the Yellow River Delta in China. These showed average sinking rates of about 0.3 inches (8 mm) per year — double the rate of global sea level rise.

Two main points arose from the study, Shirzaei said. "First, land subsidence often exceeds sea-level rise as the dominant driver of relative sea-level rise in river deltas today, meaning that many coastal risks are increasing faster than climate-only projections suggest. Second, there is a profound mismatch between risk and capacity: the deltas sinking fastest are often in regions with the least resources to respond."

River deltas are home to between 350 million and 500 million people around the world. They host 10 of the world's 34 megacities, along with vital infrastructure such as ports, meaning the impacts of subsidence and sea-level rise — such as shoreline retreat and more frequent floods — are immense.

And these huge populations are themselves a driver of subsidence, because cities pile enormous weight onto the land, compressing the soil. Huge populations typically also require massive amounts of water, which exacerbates groundwater pumping. This causes further compaction of the soil.

"In rapidly urbanizing deltas, urban growth can substantially exacerbate land sinking," Shirzaei said. However, groundwater extraction for all purposes, including agriculture and industry, remains the biggest cause of subsidence at deltas, he said. "Groundwater pumping is a well-known local driver of subsidence, but what stood out was how consistently dominant it appears at the global scale, even when compared alongside other major anthropogenic pressures."

Another cause of subsidence is a reduction in the amount of sediment that rivers flush into the ocean due to dams and other river-control strategies. Sediment delivery can offset subsidence and sea-level rise to some extent, but human modifications to natural river flow have disturbed this balance. For example, about 1,900 square miles (5,000 square kilometers) of land has been lost in the Mississippi River Delta since 1932 due to the combined effects of dams, levees and erosion.

The main drivers of subsidence at river deltas are human-made, which presents an opportunity for intervention, Shirzaei said. "One of the most important messages of the study is that subsidence is often manageable," he said.

Alongside efforts to limit climate change, countries should consider reducing groundwater extraction and replenishing aquifers with floodwater or treated wastewater, Shirzaei said. Controlled flooding and sediment diversions can help increase sediment deposition. And restricting heavy infrastructure in the areas most prone to subsidence could also help slow subsidence, he said.

"When combined with flood protection and climate adaptation, these measures can significantly reduce long-term risk," he said.