Sinking land will intensify the effects of rising sea levels

New research reveals that subtle shifts in land elevation can considerably increase or decrease the local impacts of sea level rise along coastlines. 

Even though these changes usually amount to fractions of an inch per year, they may alter flood risk, wave exposure, and saltwater intrusion for nearby communities.

Looking beyond ocean heights

Accurately predicting future sea level rise involves more than measuring how high the oceans climb. It also requires determining whether coastal lands are subsiding or uplifting. 

A NASA-led study focused on California demonstrates how slow vertical movement of the land could have serious implications for areas already dealing with the challenges of a warming planet.

Sea levels along California’s coastline are projected to rise by 6 to 14.5 inches (15 to 37 centimeters) above year 2000 levels by 2050, mainly due to melting glaciers and ice sheets, as well as the thermal expansion of warming ocean waters.

However, the recent study emphasizes that a better grasp of land elevation trends is crucial for refining local sea level forecasts.

“In many parts of the world, like the reclaimed ground beneath San Francisco, the land is moving down faster than the sea itself is going up,” said lead author Marin Govorcin, a remote sensing scientist at NASA’s Jet Propulsion Laboratory in Southern California.

Tracking land motion in detail

The new research also highlights that the pace and scale of vertical land motion vary widely, shaped by both natural processes – such as tectonic activity – and human endeavors like groundwater pumping or wastewater injection. 

Previous regional models relying on tide gauges cannot capture all these dynamic changes or produce the detailed local picture that direct satellite observations now enable.

Scientists from NASA’s Jet Propulsion Laboratory (JPL) and the National Oceanic and Atmospheric Administration (NOAA) used satellite radar to study over a thousand miles of California’s coastline. 

The experts identified multiple “hotspots,” including urban centers, beaches, and aquifers, where the sinking or rising of land could interact with ongoing sea level rise in significant ways.

Shifting land and rising sea levels

By analyzing radar measurements from European Space Agency’s Sentinel-1 satellites and velocity data from ground-based Global Navigation Satellite System stations, the researchers pinpointed areas experiencing subsidence of more than 0.4 inches (10 millimeters) per year.

Regions like San Rafael, Corte Madera, Foster City, and Bay Farm Island in the San Francisco Bay Area are gradually sinking due to sediment compaction, among other factors. 

As a result, projected sea levels in these localities could exceed 17 inches (45 centimeters) by 2050 – over twice the general regional estimate of 7.4 inches (19 centimeters) indicated by tide gauge data.

Certain areas, however, are rising. The Santa Barbara groundwater basin has been slowly lifting since 2018 thanks to aquifer replenishment, while Long Beach exhibited uplift driven by fluid extraction and injection linked to oil and gas activities.

In parts of Los Angeles and San Diego Counties, human-driven changes in land motion increase sea level projections by as much as 15 inches (40 centimeters). Groundwater use, hydrocarbon extraction, and other unpredictable land-use practices raise uncertainties that demand vigilant, real-time monitoring.

Dowmward-drifting terrain

Farther inland, the Central Valley experiences rapid sinking – sometimes up to 8 inches (20 centimeters) annually – because of widespread groundwater withdrawals. 

Scientists also recorded cyclical patterns of rising and falling land in Santa Clara, Santa Ana, and Chula Vista, correlated with fluctuations in rainfall and drought. 

In geologically active coastal regions like Big Sur and Palos Verdes, slow-moving landslides are causing portions of terrain to drift downward. Around San Francisco and Monterey bays, subsiding marshes and lagoons highlight erosion’s role in land changes.

Changes in the coming decades

Policymakers, the public, and scientists can keep an eye on coastal land motion through the OPERA (Observational Products for End-Users from Remote Sensing Analysis) initiative, led by JPL. 

This project details surface elevation changes across North America, including subsidence zones, tectonic shifts, and landslide-prone areas. 

The data will become even more precise once the next-generation NISAR (NASA-Indian Space Research Organization Synthetic Aperture Radar) mission is launched in the coming months.

By incorporating satellite-based measurements of land elevation changes into broader sea level rise models, researchers hope to provide more accurate and actionable forecasts. The goal is to help coastal communities develop more robust strategies for coping with climate change in the decades to come.

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