written 25 October 2020
published 1 November 2020
We may be heading into another drought, but must prepare for the alternative as well.
In November and December of 1861, heavy snowfall had covered the Sierra Nevada mountains of California. Beginning in late December, months of heavy warm rainfall, over 100" in some areas, quickly melted the snow, flooding the entire Central Valley. A shallow inland sea formed, hundreds of miles long by dozens of miles wide, putting Sacramento under 10' of water. During the 6 months it took for the water to subside, the capital moved to San Francisco, and California went bankrupt. Whites had never experienced this kind of flooding in California, but the native people moved out of the valley some weeks before the storms started.
In the last century, climate science developed an understanding of the long period cycles of California weather, which is shaped by our local geography interacting with two large Pacific Ocean patterns: the El Niño-Southern Oscillation (ENSO), and the Pacific Decadal Oscillation (PDO). ENSO arises from conditions in the tropical Pacific, with a period of 6-18 months, and PDO is tied to conditions in the northern Pacific, with a period spanning decades. These two oscillations shift the jet stream, and thus winter storms, now called "atmospheric rivers".
California tends to be wetter than normal during the El Niño phase of ENSO, and dryer during the La Niña phase. The PDO cycle affects California rainfall similarly. Since the two cycles run at different rates, sometimes they compete and cancel each other out, but when they are in synch, we get severe drought or inundation.
Paleoclimatology is the study of climate from before modern record keeping began. Using sediment cores from river floodplains, estuaries, and lakes, the evidence of previous floods and droughts can be determined. We now know an inundation like 1861-2 happens every 150-200 years or so, and some of the previous events were several times larger.
As the details of this scientific investigation come clearer, the full impact is slowly growing among emergency planners. The U. S. Geological Survey Multi Hazards Demonstration Project pulls together scientists, emergency planners, businesses, and governmental agencies to investigate possible natural disaster impacts to better prepare our society. The first project, the ShakeOut Earthquake Scenario, published in 2008 focused on the impact of a 7.5 magnitude earthquake in southern California. In 2010, their second project, ARkStorm (Atmospheric River 1000 Storm), examined the impact of a storm similar to 1861-2, with the following conclusions.
"The Central Valley experiences hypothetical flooding 300 miles long and 20 or more miles wide. Hundreds of landslides damage roads, highways, and homes. Property damage exceeds $300 billion, most from flooding. Agricultural losses and other costs to repair lifelines, dewater (drain) flooded islands, and repair damage from landslides, brings the total direct property loss to nearly $400 billion. Power, water, sewer, and other lifelines experience damage that takes weeks or months to restore. Flooding evacuation could involve 1.5 million residents in the inland region and delta counties. Business interruption costs reach $325 billion in addition to the $400 billion property repair costs, meaning that an ARkStorm could cost on the order of $725 billion, which is nearly 3 times the loss deemed to be realistic for a severe southern California earthquake, an event with roughly the same annual occurrence probability."
"An ARkStorm raises serious questions about the ability of existing federal, state, and local disaster planning to handle a disaster of this magnitude. Responders and government managers at all levels could be encouraged to conduct risk assessments."
Climate change will increase the potential impact of such a storm, because a warmer ocean provides more moisture for a big storm. As we have seen in hurricanes, the damage is not only, or even primarily, from high winds, but from heavy rainfall and flooding.
Large natural disasters are difficult to comprehend in advance, but increased awareness of the possibility is helpful. California has known about big earthquakes for over a century. Evolving building codes and emergency preparedness has reduced the loss of life, the recovery time, and expense. ARkStorm is a first step toward increasing public and governmental agency awareness about this kind of natural disaster.
Information in this article is from "The West Without Water", by Ingram and Malamud-Roam, and from the USGS ARkStorm report.