California is not projected to be fully submerged, but the threat of it going “underwater” is complex, rooted in two distinct scientific threats. These are the gradual, long-term process of climate-driven sea level rise and the potential for sudden, catastrophic geological events. The scientific consensus is that the state faces a certainty of chronic, worsening flooding in specific areas, alongside a lower-probability, high-impact risk of rapid land loss. This combination of rising water and potentially sinking land defines the hazard California must manage.
Gradual Sea Level Rise Projections
The most consistent threat to California’s coastline is the accelerating rise of the global ocean, driven primarily by two physical processes. One is the thermal expansion of seawater, where the ocean absorbs excess heat, causing water molecules to occupy a greater volume. The other factor is the addition of water from melting land-based ice sheets and glaciers, particularly those in Greenland and Antarctica.
The rate of rise is not uniform globally, and specific factors increase the local projection for California. Ice loss from the West Antarctic Ice Sheet has a disproportionately large effect on the California coast due to gravitational changes. This means local sea levels may rise approximately 25% more than the global average for ice melt in that region. The state has experienced about eight inches of sea level rise over the last century, and this pace is expected to accelerate significantly after 2050.
State guidance recommends preparing for scenarios by 2100 ranging from 1.6 feet up to a high-end projection of 6.6 feet, assuming continued high greenhouse gas emissions. The intermediate scenario suggests a rise of about 0.8 feet by 2050, which is enough to seriously impact coastal infrastructure. As sea levels rise, the occurrence of combined flooding—where storm runoff meets high groundwater and elevated ocean levels—will become more frequent and severe. This will cause widespread damage even before the highest projections are reached.
Tectonic Activity and Sudden Subsidence
The other major threat of inundation comes not from rising water, but from the land sinking, a process known as subsidence. This is tied to the state’s active tectonic setting, which includes the San Andreas Fault and the Cascadia Subduction Zone. A major earthquake can cause land elevation to drop suddenly, allowing the ocean to rush into newly lowered coastal areas.
The Cascadia Subduction Zone, running offshore from Northern California up to Canada, is a particular concern. When this zone experiences a large earthquake, the land on the North American plate can instantaneously drop by 1.6 to 6.6 feet. This sudden vertical change in elevation dramatically heightens the risk of coastal flooding and tsunami inundation immediately following the seismic event.
Research suggests that major quakes on the Cascadia zone may trigger seismic activity on the San Andreas Fault within hours or days, creating a catastrophic chain reaction. This sequence would combine the sudden drop in elevation with widespread structural damage and liquefaction in low-lying areas. This geological mechanism bypasses the decades-long timeline of climate change. It presents a low-probability but high-impact scenario for rapid land loss.
Mapping California’s Most Vulnerable Geography
The combination of gradual sea level rise and potential sudden subsidence is not a uniform threat across California. It is focused on specific low-lying regions and critical infrastructure hubs. One of the most vulnerable areas is the Sacramento-San Joaquin River Delta, a vast network of levees protecting farmlands and communities often below sea level. The Delta serves as a major hub for the state’s freshwater supply, and a breach of its aging levee system would be a major disaster.
The San Francisco Bay Area faces extensive risk due to its expansive, low-elevation shoreline. Counties like San Mateo and Marin, along with cities built on former wetlands, are vulnerable to increased tidal and storm surge flooding. Projections show that even a moderate sea level rise scenario could put billions of dollars worth of buildings and critical infrastructure at risk of inundation.
In Southern California, sandy beaches and coastal wetlands face erosion and permanent inundation within this century. Major transportation corridors and wastewater treatment plants are frequently situated at the lowest elevation. These facilities are directly exposed to rising water and groundwater levels. The vulnerability is concentrated in areas where the land is already flat or was historically filled in.
Scientific Timelines and Probability Assessment
California faces two distinct timelines of risk, rather than a single apocalyptic moment. The threat of gradual coastal inundation from sea level rise is a certainty, with effects becoming chronically severe by the middle of the century. By 2050, localized chronic flooding is expected to become the norm in many low-lying coastal and bay areas, requiring massive and costly adaptation efforts.
The possibility of a sudden inundation event is tied to the timelines of major earthquakes, which operate on geological scales. The probability of a magnitude 8-plus earthquake on the Cascadia Subduction Zone is low in any given year, but the consequences are extremely high. This includes a rapid land drop of several feet. This scenario represents a high-impact, low-probability threat of rapid coastal transformation that could occur with almost no warning.
The state is planning for severe long-term damage and significant economic cost, not complete disappearance. The majority of the state will remain well above sea level. However, key population centers, infrastructure, and agricultural lands in low-lying regions face a near-certain future of increasing flood exposure. The overall risk is a combination of a slow-motion catastrophe and the possibility of a rapid geological disaster.