The question of when California will be “underwater” refers to two fundamentally different scientific threats. The first is the long-term, permanent inundation of coastal areas caused by steadily rising global sea levels due to climate change. The second threat is the risk of massive, temporary inland flooding, a catastrophic event caused by an extreme weather pattern known as a megaflood. Understanding the distinction between these two hazards—permanent coastal rise versus temporary inland catastrophe—provides a science-based perspective on California’s future water risks.
Sea Level Rise: The Long-Term Coastal Threat
California’s coastline is already experiencing the effects of rising seas, driven by the thermal expansion of warming ocean water and the melting of land-based ice sheets and glaciers. The state has seen an average sea-level rise of about eight inches over the last century, but this pace is projected to accelerate significantly after 2050. The rate of future rise is directly tied to global greenhouse gas emissions pathways.
The current scientific consensus suggests sea levels are most likely to rise by about 0.8 feet (9.6 inches) statewide by 2050. State guidance recommends preparing for a rise of 1.67 feet (20 inches) by mid-century, a level that could place approximately $17.9 billion worth of existing coastal buildings at risk. This rise will make current high-tide and storm-surge flooding events much more frequent.
By the year 2100, California is expected to see a rise between 1.6 feet and 3.1 feet under intermediate scenarios. If global emissions remain high, the state must prepare for a possible rise of up to 6.6 feet (2.0 meters). The state’s vulnerability is compounded where local land subsidence, or sinking, increases the rate of relative sea-level rise.
This long-term threat means that low-lying coastal infrastructure and ecosystems face gradual but permanent submergence. Coastal erosion will also intensify, potentially leading to the complete loss of up to two-thirds of Southern California’s beaches under high-end projections by 2100.
The Catastrophic Risk: The Megaflood Scenario
Distinct from the slow, permanent rise of the ocean is the threat of a megaflood, a high-impact, low-frequency event that would temporarily submerge vast inland areas. This scenario is modeled after the devastating Great Flood of 1862, which inundated the Central Valley for weeks. Modern modeling of a similar event is known as the ARkStorm scenario, where ‘AR’ stands for Atmospheric River.
Atmospheric Rivers are concentrated plumes of moisture that transport water vapor across the Pacific Ocean. A megaflood would be triggered by a sequence of extremely intense Atmospheric Rivers stalling over the state for several weeks. This prolonged, intense rainfall would far exceed the capacity of California’s existing flood protection infrastructure, which is designed to handle 100- to 200-year runoff events. The immense volume of water would overwhelm dams, reservoirs, and levee systems across the state.
The ARkStorm scenario predicts the Central Valley would experience flooding up to 300 miles long and at least 20 miles wide, turning the valley floor into a temporary inland sea. While the water would eventually recede, the devastation would be profound, with projected property damage and business interruption costs estimated to be in the hundreds of billions of dollars. This temporary flooding poses a risk to critical infrastructure, agriculture, and millions of residents in inland regions.
Climate science suggests that a warming atmosphere can hold more moisture, which increases the likelihood and intensity of these extreme rainfall events. The risk of a megaflood is projected to have already doubled compared to a century ago, making this catastrophic, temporary flooding a growing concern.
Mapping Vulnerability: Where the Water Goes First
The geographical areas most susceptible to inundation are determined by their elevation relative to the sea and their proximity to major river systems. For long-term sea-level rise, the greatest vulnerability lies in low-lying coastal basins and estuaries. The San Francisco Bay Area is particularly at risk due to its extensive, low-elevation shoreline.
Coastal areas face permanent encroachment, threatening critical infrastructure and sites:
- Key transportation hubs, such as the San Francisco International Airport and Oakland International Airport, which are built on bay fill and face significant flood exposure.
- Hundreds of hazardous industrial sites, including sewage treatment plants and oil refineries, along the San Francisco Bay and Southern California coastal plains, threatening to release contaminants upon inundation.
- Low-lying coastal areas of Los Angeles and Orange County, particularly around the Port of Long Beach and adjacent industrial areas.
The threat from a catastrophic megaflood is concentrated in the state’s interior. The Central Valley, which includes the Sacramento and San Joaquin river basins, is the primary area of concern. Overwhelmed river systems and subsequent failures of the extensive levee network would inundate the agricultural heartland and communities like Sacramento and Stockton. The floodplains and delta counties are the most vulnerable regions to this temporary, yet destructive, inland flooding event.