We reconstructed Southern California rainfall on a yearly basis for the last 9,000 years and during a targeted 8,000 year interval as global climate warmed out of an ice age ~400,000 years ago. We achieved this by using the elemental composition of Santa Barbara Basin sediments as a proxy for river runoff determined by scanning X-ray fluorescence methods which provided 4-7 data points per year.

Santa Barbara Basin is the only location in the NE Pacific that can provide such paleoclimate reconstruction due to its low oxygen bottom waters (minimal sediment blending by organisms), and sediment-laden river discharge following rainfall events. During wet years when river runoff carries significant siliciclastic sediment into Santa Barbara Basin (high aluminum, titanium, silica, iron and potassium concentrations in the sediments) Concentrations of these elements are low during dry years with little river runoff. Wet and drought climate conditions were confirmed by pollen analysis of regional vegetation in the same sediments and intervals when yearly layer counts indicated missing years. Annual cycles of precipitation and runoff are represented by sub-mm-scale cycles in sediment titanium concentration; occurrences of multi-decadal droughts and extreme flood events are clearly discernible throughout the record. The frequency of multi-year droughts and floods determined for key climate intervals provided key information about interannual (El Niño Southern Ocean) to decadal (Pacific Decadal Oscillation) climate modes and their response to climate forcings such as changes in orbital cycles, the position of Intertropical Convergence Zone and the extent of sea ice. The exceptional chronology developed for the record allows detailed correlation to other paleoclimate records in the near and far field. This correlation reveals how changes to the background climate state influence the large-scale atmospheric circulation in the northeastern Pacific, and the formation of “atmospheric rivers” affecting western North America.

      There is a high socio-economic cost to the extreme hydroclimate states described in our manuscript. While California relies on atmospheric rivers to fill its reservoirs, the flood events in our 2,000 year-long record are larger than any historical flood in western North America. The USGS used the smallest flood in the historical sequence (the Winter of 1861-62) to model their worst-case scenario storm in the ArkStorm Project. This historic flood nearly bankrupted California, forcing the state to stop paying employees for 4 months and moving the state legislature to San Francisco. Today California has the 7th largest economy in the world and is home to ~40 million people – such a devastating flood occurring again would have a global impact. Alternatively, if atmospheric rivers fail to form or be steered toward California, severe drought conditions will also cost the state billions of dollars. Understanding the frequency and magnitude of droughts and floods in California will have benefit society by improving climate and flood prediction for policy and planning. Additionally, we extended knowledge of interannual climate variability well beyond instrumental records. These results can be used to assess the skill of Earth System Models at forecasting interannual to decadal climate variability. This will improve prediction of the hydrological cycle response to projected CO₂ forcing, thus advancing climate/weather forecasting.

Multiple graduate students were involved in all aspects of this research, including visiting project collaborators, presenting at national meetings, and publishing papers. The results of this research were disseminated via professional presentations, scientific publications and national data centers.

Last Modified: 12/31/2018
Modified by: Ingrid L Hendy