I highly recommend reading a post at Watts Up With That: Climate and Human Civilization over the last 18,000 years by Andy May. This post examines the earth’s climate and its impact on human civilizations over the last 18,000 years. It is interesting to note that cold periods were accompanied by long-term droughts. Droughts that disrupted civilizations and in many cases promote collapse.
California is mentioned in the discussion. The worst megadrought in California and Nevada area lasted from 832 AD to 1074 AD, right at the end of the Dark Ages. Desprat, et al, 2003 paper, identifies a very cold period from 450-950 AD.
We are coming to the end of the Modern Warm Period, and history has shown that our future will be much colder. The Vostok ice core is a reconstruction of the temperature record for the last 600 million years. Temperatures, today, are lower than they have been for over 250,000,000 years according to the ice core data. So much for the unprecedented global warming claims by the progressive warmers. We are on the cusp of the Next Grand Minimum.