California baked in two droughts about 1100 years ago, the first lasting 220 years and the second 140 years. Each was much more intense than the mere six-year dry spells that afflict modern California from time to time, new studies of past climates show. It's important to note that this was long before there were SUVs or coal fired power plants.
The findings suggest that relatively wet periods like the 20th century have been the exception rather than the rule in California for at least the last 3,500 years, and that mega-droughts are likely to recur.