SAN JOSE, Calif. — California’s current drought is being billed as the driest period in the state’s recorded rainfall history. But scientists who study the West’s long-term climate patterns say the state has been parched for much longer stretches before that 163-year historical period began. And they worry that the “megadroughts” typical of California’s earlier history could come again.

Through studies of tree rings, sediment and other natural evidence, researchers have documented multiple droughts in California that lasted 10 or 20 years in a row during the past 1,000 years – compared to the mere three-year duration of the current dry spell. The two most severe megadroughts make the Dust Bowl of the 1930s look tame: a 240-year-long drought that started in 850 and, 50 years after the conclusion of that one, another that stretched at least 180 years.

“We continue to run California as if the longest drought we are ever going to encounter is about seven years,” said Scott Stine, a professor of geography and environmental studies at Cal State East Bay. “We’re living in a dream world.”

California in 2013 received less rain than in any year since it became a state in 1850. And at least one Bay Area scientist says that based on tree ring data, the current rainfall season is on pace to be the driest since 1580 – more than 150 years before George Washington was born. The question is: How much longer will it last?

A megadrought today would have catastrophic effects.

California, the nation’s most populous state with 38 million residents, has built a massive economy, Silicon Valley, Hollywood and millions of acres of farmland, all in a semiarid area. The state’s dams, canals and reservoirs have never been tested by the kind of prolonged drought that experts say will almost certainly occur again.

Stine, who has spent decades studying tree stumps in Mono Lake, Tenaya Lake, the Walker River and other parts of the Sierra Nevada, said that the past century has been among the wettest of the last 7,000 years.

Looking back, the long-term record also shows some staggeringly wet periods. The decades between the two medieval megadroughts, for example, delivered years of above-normal rainfall – the kind that would cause devastating floods today.

The longest droughts of the 20th century, what Californians think of as severe, occurred from 1987 to 1992 and from 1928 to 1934. Both, Stine said, are minor compared to the ancient droughts of 850 to 1090 and 1140 to 1320.

What would happen if the current drought continued for another 10 years or more?

Without question, longtime water experts say, farmers would bear the brunt. Cities would suffer but adapt.

The reason: Although many Californians think that population growth is the main driver of water demand statewide, it actually is agriculture.