Tracking and predicting sea level rise involves more than measuring the height of our oceans: Land along coastlines also inches up and down in elevation. Using California as a case study, a NASA-led team has shown how seemingly modest vertical land motion could significantly impact local sea levels in coming decades.
By 2050, sea levels in California are expected to increase between 6 and 14.5 inches (15 and 37 centimeters) higher than year 2000 levels. Melting glaciers and ice sheets, as well as warming ocean water, are primarily driving the rise. As coastal communities develop adaptation strategies, they can also benefit from a better understanding of the land’s role, the team said. The findings are being used in updated guidance for the state.
“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.
Scientists mapped land sinking (indicated in blue) in coastal California cities and in parts of the Central Valley due to factors like soil compaction, erosion, and groundwater withdrawal. They also tracked uplift hot spots (shown in red), including in Long Beach, a site of oil and gas production.
The new study illustrates how vertical land motion can be unpredictable in scale and speed; it results from both human-caused factors such as groundwater pumping and wastewater injection, as well as from natural ones like tectonic activity. The researchers showed how direct satellite observations can improve estimates of vertical land motion and relative sea level rise. Current models, which are based on tide gauge measurements, cannot cover every location and all the dynamic land motion at work within a given region.
Local Changes
Researchers from JPL and the National Oceanic and Atmospheric Administration (NOAA) used satellite radar to track more than a thousand miles of California coast rising and sinking in new detail. They pinpointed hot spots — including cities, beaches, and aquifers — at greater exposure to rising seas now and in coming decades.
To capture localized motion inch by inch from space, the team analyzed radar measurements made by ESA’s (the European Space Agency’s) Sentinel-1 satellites, as well as motion velocity data from ground-based receiving stations in the Global Navigation Satellite System. Researchers compared multiple observations of the same locations made between 2015 to 2023 using a processing technique called interferometric synthetic aperture radar (InSAR).
Homing in on the San Francisco Bay Area — specifically, San Rafael, Corte Madera, Foster City, and Bay Farm Island — the team found the land subsiding at a steady rate of more than 0.4 inches (10 millimeters) per year due largely to sediment compaction. Accounting for this subsidence in the lowest-lying parts of these areas, local sea levels could rise more than 17 inches (45 centimeters) by 2050. That’s more than double the regional estimate of 7.4 inches (19 centimeters) based solely on tide gauge projections.
Not all coastal locations in California are sinking. The researchers mapped uplift hot spots of several millimeters per year in the Santa Barbara groundwater basin, which has been steadily replenishing since 2018. They also observed uplift in Long Beach, where fluid extraction and injection occur with oil and gas production.
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The scientists further calculated how human-induced drivers of local land motion increase uncertainties in the sea level projections by up to 15 inches (40 centimeters) in parts of Los Angeles and San Diego counties. Reliable projections in these areas are challenging because the unpredictable nature of human activities, such as hydrocarbon production and groundwater extraction, necessitating ongoing monitoring of land motion.
Fluctuating Aquifers, Slow-Moving Landslides
In the middle of California, in the fast-sinking parts of the Central Valley (subsiding as much as 8 inches, or 20 centimeters, per year), land motion is influenced by groundwater withdrawal. Periods of drought and precipitation can alternately draw down or inflate underground aquifers. Such fluctuations were also observed over aquifers in Santa Clara in the San Francisco Bay Area, Santa Ana in Orange County, and Chula Vista in San Diego County.
Along rugged coastal terrain like the Big Sur mountains below San Francisco and Palos Verdes Peninsula in Los Angeles, the team pinpointed local zones of downward motion associated with slow-moving landslides. In Northern California they also found sinking trends at marshlands and lagoons around San Francisco and Monterey bays, and in Sonoma County’s Russian River estuary. Erosion in these areas likely played a key factor.
Scientists, decision-makers, and the public can monitor these and other changes occurring via the JPL-led OPERA (Observational Products for End-Users from Remote Sensing Analysis) project. The OPERA project details land surface elevational changes across North America, shedding light on dynamic processes including subsidence, tectonics, and landslides.
The OPERA project will leverage additional state-of-the-art InSAR data from the upcoming NISAR (NASA-Indian Space Research Organization Synthetic Aperture Radar) mission, expected to launch within the coming months.