Across Louisiana and the Mississippi Delta, the ground beneath coastal communities is sinking far faster than the ocean is rising—a revelation that has upended decades of assumptions about how land subsidence affects sea level. Researchers led by Sönke Dangendorf at Tulane University and including Thomas Wahl, an associate professor of civil and environmental engineering at the University of Central Florida, discovered that vertical land motion doesn't happen at a steady rate over time, but rather in sharp, variable phases that dramatically accelerate the relative rise of sea level in vulnerable regions.
For nearly a century, scientists have understood that as coastal ground sinks, the water appears to rise relative to the land, compounding the effects of actual sea level rise. What they didn't realize was how irregular this process could be. In many coastal areas, sea level may be rising one to three millimeters annually, but the land is subsiding 10 to 15 times faster, creating a compounding crisis for infrastructure and residents. The research, published in Nature Geoscience, reconstructed vertical land motion dating back to the early 20th century using data from tide gauges, revealing that human activities and natural phenomena have triggered periods of rapid sinking or rising that previous linear models entirely missed.
The causes are surprisingly specific. Groundwater extraction for growing cities is a major culprit—as cities pump water from aquifers beneath the earth, the ground compacts and subsides. Earthquakes and other natural phenomena also play a role. This matters because it means current projections for future sea-level change across coastal communities are likely underestimating the actual risk. Wahl explains the gravity plainly: "As the sea level goes up and land goes down, you have a bigger problem."
The implications are especially acute for cities that have experienced explosive growth and water demand. But the research also offers a glimmer of hope in the form of a proven solution. Tokyo and Shanghai once faced extreme subsidence—sinking several centimeters per year during the mid-20th century—but dramatically reversed the trend by implementing strict groundwater extraction controls and related land-management policies. If cities can halt the human-caused portion of subsidence, they can at least buy time to adapt to rising seas.
Wahl and his colleagues emphasize that adaptation now is critical. Rather than assuming the problem will solve itself, coastal communities need to plan early and create strategies to manage water and protect infrastructure for as long as possible. Some areas will face harder choices than others, and protection may not be feasible everywhere. Yet there is room for cautious optimism. Wahl points out that technological and strategic solutions we haven't yet imagined may emerge in the coming decades—just as the past 50 to 100 years have brought innovations that previous generations couldn't have predicted. The challenge is formidable, but it is not insurmountable.