From the most prominent and respected expert on climate and weather in Southern California:
Our water supply depends on the snowpack in the northern Sierra and the eastern Rockies. It also depends on the population growth in the Southwest. We share the Colorado with seven states and six Indian nations. Everybody gets an allocation and we leave a couple of drops down at the bottom for Mexico. That’s the background info on water in the West. We capture it, we ship it.
Let’s look back over the last 20 centuries: We’ve seen tremendous droughts in the American West. In the 11th century there was an 80-year drought along the Colorado. This is before global warming by anthropogenic—or man-made—sources. The 20th century, which is when we built our civilization in California, was one of the wettest in 2,000 years. It was an anomaly. We know this from tree ring records. We have built a civilization, which is the sixth- or seventh-largest economy in the world, based on imported water in a wet century. How do you like that?
We have built a civilization in an extremely dry place. The limiting factor in any civilization is primarily water. Look at all the great civilization collapses. The 11th century is when the Anasazis had to disperse because of the 80-year drought on the Colorado. The same is true for many civilizations in Mesoamerica. This story can be told for the civilization that built Angkor Wat in Cambodia. It can be told in the Middle East. One of the primary determinates of human civilization has been drought: natural climate variability. We’ve seen this is our history. The history of the world is written in droughts.
Which is alarming, given the recent behavior of the Pacific Decadal Oscillation. When it's negative we tend to have droughts in California, especially SoCal.