People have settled on river deltas for millennia, drawn by fertile soil, fresh water, and abundant food sources. Today, these areas are among the most densely populated in the world, but the hundreds of millions of people who live there face a growing threat.

Coastal flooding is disproportionately affecting river delta populations, forcing them to contend with infrastructure damage, saltwater intrusion, and erosion. One reason for this is climate-driven sea level rise, but a study published Wednesday in the journal Nature finds that many of the world’s river deltas are actually sinking faster than sea levels are rising.

The findings point to heightening near-term flood risk for more than 236 million people, but river delta flooding is an issue of global food security as well. “Billions of people rely on the food that is produced from these deltas,” co-author and Virginia Tech geoscience professor Manoochehr Shirzaei told Gizmodo.

Rethinking the drivers of river delta flooding

The researchers, led by former Virginia Tech graduate student Leonard Ohenhen, who is now an assistant professor at the University of California, Irvine, used satellite data to create the first map of vertical land motion—or subsidence—for 40 major river deltas worldwide.

Their satellite data was gathered using InSAR (Interferometric Synthetic Aperture Radar), a powerful remote sensing technique that uses multiple radar images of Earth’s surface to map tiny elevation changes over time. The map offers a high-resolution view of delta-wide subsidence spanning five continents and 29 countries.

Deltas experiencing the highest rates of subsidence include the Mekong in southern Vietnam, the Nile in northern Egypt, the Chao Phraya in southern Thailand, the Ganges-Brahmaputra in eastern India, the Yellow River in northern China, and the Mississippi River in the Gulf of Mexico. These deltas are home to some of the most densely populated cities in the world, including Bangkok, Cairo, and Kolkata.

These deltas are sinking about 4 millimeters per year on average, faster than current estimates of global sea level rise. “This is a really fast rate,” Shirzaei said. A change of 4 mm per year may not sound like much, but over time, it drastically increases the risk of flooding, erosion, and saltwater intrusion (saline contamination of soils and fresh water), he explained.

While both land subsidence and sea level rise contribute to flood risk, the fact that these deltas are sinking faster than sea levels are rising means that subsidence is the primary driver of their near-term flood risk, according to Shirzaei.

“Sea level rise exacerbates the effect of land subsidence, not the other way around,” he said. “If you want to come up with an adaptation strategy or create a resiliency plan, this is the data you should look at, rather than projections of sea level rise to 2100.”

The causes of land subsidence and solutions

The good news is that land subsidence is much easier to address in the near term than sea level rise. Whereas sea level rise is driven by global climate change, land subsidence is driven by regional human activities such as excessive groundwater pumping or underground mining.

If communities mitigate these activities and take steps to reverse the damage, they can halt subsidence and rapidly regain elevation. “We have very good engineering solutions for that,” Shirzaei said. One of the best is managed aquifer recharge, which involves pumping water back into depleted underground aquifers to lift the land surface and replenish the groundwater supply.

To help river delta communities address land subsidence, Shirzaei and his colleagues hope to tailor their observations for decision-making. Future studies will need to account for infrastructural differences to fully assess the subsidence-driven flood risk across various river deltas, he explained.

Shirzaei also hopes researchers will eventually be able to map land subsidence across the entire world—similar to Google Earth, but for land deformation. “This is possible,” he said. “The satellite data that we use are publicly available data, we don’t pay for it, and we have the technology to process the data and turn them into such useful measurements at [high] precision and accuracy. The only thing we need is real investment.”

Access to this data will become increasingly important as climate change continues to compound the effect of land subsidence across the world’s coastlines. “Having this data processed and made publicly available—I think that’s a right that everybody has,” Shirzaei said.