Researchers at the National Center for Atmospheric Research in the United States have indicated that climate change could double the odds of California experiencing a major flood within the next four decades, signaling a potential leap in the cost and scale of a climate disaster. The findings were published in Science Advances.
Analysis of the large CESM global climate dataset shows that the likelihood of a major flood driven by persistent atmospheric rivers during the cool season increases by roughly 200 to 300 percent. If these extreme events once occurred on a roughly two-century cadence, their frequency could rise to three per century, reshaping the flood risk profile for the state.
Atmospheric rivers are narrow corridors that transport immense quantities of atmospheric water vapor from tropical regions to other climate zones. They behave like concentrated streams of moisture and can deliver rainfall equivalent to long hoses of water stretching thousands of kilometers across the landscape.
California’s mountainous terrain elevates the flood risk and complicates response efforts, while the state’s exposure to wildfires, intensified by warming temperatures, can leave surfaces steep and unstable. When rain falls on charred or degraded ground, runoff increases rapidly, compounding flood hazards and accelerating downstream water movement.
A flood that temporarily submerges California under an inland sea could rank as one of the most costly climate events in history. The economic toll could exceed a trillion dollars, vastly surpassing the cost of past disasters and underscoring the heavy financial burden of climate impacts on a population that now exceeds 39 million residents in the broader region.
California has a historical precedent for mega-flooding. In 1861–1862, the San Joaquin and Sacramento valleys temporarily transformed into a vast inland water body, with several areas reporting water depths approaching nine meters in a matter of weeks. The disaster devastated tens of thousands of acres of farmland, caused widespread property losses, and led to a severe financial crisis for the state. Today’s population makes the potential consequences far more consequential and requires careful consideration of flood risk management and resilience planning.
Across much of the United States, drought conditions have punctuated recent years, yet heavy rainfall and flash floods remain a persistent threat in locales from Eastern Kentucky to St. Louis and even protected areas such as Death Valley. The researchers emphasize that while floods may be unavoidable to some degree, the scale of loss can be substantially mitigated through modernization of flood control, water storage, and emergency response systems. Investment in resilient infrastructure, better forecasting, and proactive land and water management could reduce the damage from future mega-flood events. Advances and NCAR studies.