Precipitation Volatility in the 21st Century in California

The Challenge

California’s climate is one of extremes, susceptible to rapid shifts between drought and flood. For example, the state experienced a record multi-year dryness between 2012–2016 followed by extreme wetness during the 2016–2017 winter. With global climate change, warming is likely to cause even greater precipitation variability in the future. But patterns of change by region and over time in precipitation extremes in California remain uncertain.    

The Solution

Projected future changes in dry-to-wet precipitation events were quantified using climate model simulations. The research team used specific flood and drought events from California’s history as baselines for exploring the future climatic extremes. By selecting a wide range of wet, dry and rapid dry-to-wet transition events (‘whiplash’) – the team was able to assess the range of potential changes in the frequency of California precipitation extremes caused by increasing atmospheric greenhouse gas emissions. The findings of the study will help inform future climate adaptation strategies as well as water and emergency management practices in 21st century California. 

Results

• Most of California will likely experience a 100–200% increase in the frequency of extremely wet November–March rainy seasons by 2100. These extremely wet seasons will resemble the winter of 2016–2017, when statewide precipitation was 54% greater than average. Extremely wet seasons like this have historically occurred about four times per century. 
• California will also experience a 50–150% increase in the frequency of extremely dry November–March seasons by 2100. These extremely dry seasons will be similar to that of 1976–1977, which have historically occurred only once per century. What is most notable is especially large increases (>140%) across Southern California after 2050, though substantial increases (>80%) extend across most of Northern California as well.   
• A strong pattern of increased precipitation whiplash throughout California is projected, despite only modest changes in mean precipitation. These whiplash increases first emerge in the southern part of the state relatively early (2010–2020) before spreading progressively northward, especially after 2050. As much as 25% and 100% increase in precipitation whiplash is expected by 2100 in Northern and Southern California, respectively. 

Next Steps

The changing characteristics of the wet season alongside increased frequency of seasonal dry and wet extremes will complicate decisions between water capture (water supplies) versus water release (flood protection) in California. Given the new information and discovery from this study, researchers are working with the Los Angeles County Flood Control District, Los Angeles Department of Water and Power, and Metropolitan Water District to examine local and regional water system vulnerabilities and adaptation strategies.  

Additional Outcomes to Date

The study received additional funding from The Nature Conservancy’s NatureNet Science Fellows program, a National Science Foundation grant, and the U.S. Department of Energy Regional and Global Climate Modeling program. 

Publications and Reports

Swain, D. L., Langenbrunner, B., Neelin, J. D., & Hall, A. (2018). Increasing Precipitation Volatility in Twenty-First-Century California. Nature Climate Change, 8(5), 427-433. doi:10.1038/s41558-018-0140-y