[Article initialement publié en février 2024]
For several decades, sea level projections have been based mainly on the melting of glaciers and the thermal expansion of the oceans. However, another essential variable is often overlooked: vertical movements of the soil. In California, some coastal areas sink. Others rise under the effect of natural and human processes, locally modifying the relative elevation of the sea.
A study by researchers from the Jet Propulsion Laboratory (JPL) of NASA and the National Oceanic and Atmospheric Administration (NOAA), published in Science Advances, reveals that these variations can considerably amplify the impacts of the rise of water. Thanks to the Sentinel-1 satellites of the European Space Agency (ESA) and the GNSS terrestrial data, scientists have accurately mapped these deformations, highlighting areas particularly vulnerable to floods and the intrusion of salt water by 2050.
Whole cities that sink
The researchers therefore analyzed the vertical soil movements in California between 2015 and 2023 using Insar (Interferometry Radar Interferometry with opening synthesis). By comparing several radar images of the same area taken at different times, this technique detects altitude variations.
In the Bay of San Francisco, cities like San Rafael, Corte Madera, Foster City and Bay Farm Island record subsidence rates of 10 mm per year. This phenomenon is mainly caused by the compaction of sediment. It is a natural process amplified by urbanization and exploitation of underground resources. “” In certain areas such as the artificial land of San Francisco, the subsidence is faster than the elevation of the sea level “, Underlines Marin Goircin. As a result, sea level elevation forecasts in these sectors reach 45 cm by 2050. This represents more than double estimates based solely on swamps.
Other urban areas also undergo a strong subsidge. In Los Angeles and San Diego, the exploitation of hydrocarbons and the pumping of water tables cause irregular soil sagging. This creates uncertainties up to 40 cm in the seabed projections. These variations make planning of coastal infrastructure more complex.
In the central valley of California, intensive extraction of groundwater is a common practice. However, some regions record extreme subsidence rates up to 20 cm per year. This rapid sagging considerably increases the risk of floods. Especially during strong precipitation, by reducing the natural altitude of land compared to rivers and the surrounding coastal areas.
Areas in elevation, but not without risks for California
If some parts of California sink, others experience a opposite phenomenon with a soil uprising. This phenomenon remains particularly visible in the aquifer basin of Santa Barbara. She has experienced a progressive rise since 2018 thanks to the natural recharge of her water tables.
In Long Beach, another mechanism is at work. The extraction and injection of fluids linked to oil operations lead to floor level fluctuations. However, this elevation is not always stable and can be followed by a sudden subsidence when underground pressures vary.
Cartography of land sagging (indicated in blue) and hotlight hot spots (indicated in red). © Nasa Earth Observatory
This variability in vertical movements then poses a major problem for urban planning and adaptation to coastal risks. “” The vertical movements of the soil remain unpredictable and can change its dynamics quickly », Underlines David Ps Bekaert, co-author. If an elevation zone today has a decrease in its recharging in groundwater or a change in extraction operations, it can quickly switch to sagging. It will in fact brutally expose local infrastructure at increased risks of flooding or instability in the field.
In addition, in certain coastal areas, these elevations can interact with erosion and modify exposure to storms and waves. Indeed, a sudden elevation of coastal land could disrupt natural sedimentary balance. This could increase the instability of the cliffs or accelerate the decline in the coast under the effect of the ocean forces.
Between erosion and landslides: increased risks
In addition to the subsidence caused by the exploitation of underground resources, certain sectors of the Californian coast undergo field movements linked to natural processes. In question, in particular landslides and coastal erosion. Thanks to the data, the researchers have highlighted areas particularly sensitive to these phenomena.
In the steep mountains of Big sur and on the Palos Verdes peninsula, slow landslides lead to a progressive sagging of the soil. These movements, often imperceptible to the naked eye, can reach several millimeters a year. They also destabilize the nearby infrastructure. These areas are particularly vulnerable after high precipitation. The latter saturate the water and accelerate their sagging.
Coastal marshes and lagoons are subjected to a subsidence linked to erosion and sedimentary dynamics. In the Bay of San Francisco, the Bay of Monterey and the Estuary of Russian River, the researchers observed a progressive depression of the ground. Coastal erosion, amplified by the rise of water and the intensification of storms, contributes to the gradual disappearance of wetlands. Their natural role as a barrier against floods and the intrusion of salt water is then reduced.
These transformations increase the vulnerability of the ribs in the face of extreme climatic events. “” With the rise in sea level, these areas are increasingly exposed to floods, erosion and intrusion of salt water “Warn Benjamin D. Hamlington, sea level specialist. By reducing the stability of coastal land, these processes may speed up coastal losses. This development will then threaten infrastructure, but also the ecosystems that depend on it.
An essential follow -up to adapt better in California
Faced with these issues, monitoring soil movements becomes a key element in adaptation to climate change. The Opera project (Observational Products for end-users from Remote Sensing Analysis), directed by the JPL, provides detailed maps of these variations to help local authorities better plan regional planning.
With the imminent launch of the Nisar mission (NASA-Isro Synthetic Aperture Radar), data accuracy should further improve. “”These new data will make it possible to better anticipate the impacts of the rise of waters and to adapt our mitigation strategies“Concludes William Sweet, co-author of the study.
By integrating the movements of the soil in the seabed projections, the researchers hope to provide more precise estimates. Above all, they want to avoid undervaluation that can cost the Californian coastal infrastructure dearly.
Source: Marin Govorcin et al., “Variable vertical land motion and its impacts on sea level projections” .Sci. ADV.11, EADS8163 (2025).
With an unwavering passion for local news, Christopher leads our editorial team with integrity and dedication. With over 20 years’ experience, he is the backbone of Wouldsayso, ensuring that we stay true to our mission to inform.
