Science
As the Caspian Sea Recedes, Tectonics May Help Shape Its Coastline
April 3, 2025
Min
Land subsidence and uplift determine where the Caspian Sea’s coastline shifts the fastest.
This article also may be read in Azeri in Eos’s COP29 special edition.
Earth’s largest inland body of water, the Caspian Sea, has been rapidly shrinking for the past 30 years because of a warmer, drier climate and increasing water use upstream. The sea’s retreat—it’s expected to drop up to 18 meters (60 feet) by 2100—is threatening ecosystems, hindering port activity, and spurring local authorities to declare states of emergency.
The speed and shape of changes to the Caspian Sea’s coastline depend on a variety of geologic and anthropogenic forces, according to new research presented at the European Geosciences Union General Assembly 2024 in Vienna. Scientists say their findings will help land managers better prepare human and ecological communities for sea level changes.
“It’s important to know what the local geologic effects are to be able to better plan for these wetlands in the future,” said Eric Fielding, a geophysicist at NASA’s Jet Propulsion Laboratory and coauthor of the new study.
Researchers analyzed data from the European Space Agency’s Sentinel-1 mission, which measured the vertical motion of land between 2014 and 2023.
They focused their analysis on changes to the Gizil-Aghaj State Reserve, a wetland that borders the Caspian Sea in southeastern Azerbaijan. They found that since 2014, the Gizil-Aghaj coastline has receded to expose an additional 218 square kilometers (84 square miles) of land.
The land experienced both uplift and subsidence, and the parts of the reserve that were subsiding saw fewer coastline changes than parts where land was uplifting.
The region’s uplift could be due to some actively deforming folds related to the nearby Caucasus Mountains, Fielding said. Both the Caucasus and the Caspian Sea are within a convergence zone where the Arabian and Eurasian tectonic plates are colliding.
The uplift was surprising since the area is generally very flat, Fielding said. “There’s not much of a ridge at the surface. We wouldn’t look at it and immediately say ‘Well, that’s an active fold,’” he explained.
The flat landscape means there’s likely something else causing the uplift, according to Ian Pierce, a geologist at University of Oxford who was not involved in the new study. If the rates measured by the research team had persisted for thousands of years, as fold deformation typically does, tall landforms would have appeared.
The subsidence rates measured by the team, on the other hand, are in line with what scientists would expect on the basis of measurements from other subsiding coastal regions, said Timothy Dixon, a geophysicist at the University of South Florida who was not involved in the study. Parts of the Mississippi Delta, for example, are subsiding at a rate of 6–8 millimeters (0.2–0.3 inch) per year, he said.
Pierce said he suspects anthropogenic activity—such as water pumping or oil and gas extraction—may have contributed to changes in ground level. “I just don’t think we see those types of magnitudes of rates from just tectonics,” he said.
Understanding how the coastline is changing helps land managers in the Gizil-Aghaj State Reserve predict shifts in habitat conditions and plan their conservation actions accordingly, wrote Bahruz Ahadov, a geophysicist at the Oil and Gas Institute and Institute of Geology and Geophysics in Azerbaijan and coauthor on the new research, in an email.
That could involve preparing both ecosystems and human communities living near the Caspian Sea for the potential impacts of sea level changes, Ahadov wrote. Wetlands located on uplifting land will lose water more rapidly than the other parts of the coast, Fielding added.
The findings are “crucial for adapting to environmental changes and ensuring the long-term viability of these coastal areas,” Ahadov wrote.
NASA’s NISAR (NASA-ISRO Synthetic Aperture Radar) mission, a satellite mission planned to launch in 2024, will provide higher-resolution land deformation data, Dixon and Fielding both said. NISAR will let scientists measure vertical land motion over vegetated landscapes, something that hasn’t been possible with data from existing satellites, they said. “In the areas where we’re not able to exploit Sentinel data, the NISAR data will fill in the gaps,” Dixon said.
“It’s important to keep these vertical land motion measurements going to better understand how the land is moving relative to water levels, in both Azerbaijan and around the world,” Fielding said. “We’re really looking forward to getting this data.”
