The last three years of drought were the most severe that California has experienced in at least 1,200 years, according to a new scientific study published Thursday.

The study provides the state with breathtaking new historical context for its low reservoirs and sinking water tables, even as California celebrated its first good soaking of the season.

Analyzing tree rings that date back to 800 A.D. — a time when Vikings were marauding Europe and the Chinese were inventing gunpowder — there is no three-year period when California’s rainfall has been as low and its temperatures as hot as they have been from 2012 to 2014, the researchers found.

“We were really surprised. We didn’t expect this,” said one of the study’s authors, Daniel Griffin, an assistant professor in the University of Minnesota’s department of geography, environment and society.

The report, published in the journal of the American Geophysical Union, was written by researchers at Massachusetts’ Woods Hole Oceanographic Institution and the University of Minnesota.

The scientists measured tree rings from 278 blue oaks in central and southern California. Tree rings show the age of trees, and their width shows how wet each year was because trees grow more during wet years.

The researchers compared the information to a database of other tree ring records from longer-living trees like giant sequoias and bristlecone pines, dating back 1,200 years.

Meanwhile, the rain that California received this week provided a promising start to a winter that water managers say needs to be relentless and drenching to break the drought cycle.

‘Good beginning’

“It’s a good beginning,” said Art Hinojosa, chief of hydrology at the state Department of Water Resources. “But we need storm after storm after storm if we have any hope of getting out of the drought this year.”

By April, he said, California needs at least eight more major storm systems like the one this week — as well as many smaller storms — to fill its dangerously low reservoirs and break the drought. Rain and snow this winter needs to be at least 150 percent of average for the reservoirs to fill, Hinojosa said.

Above normal

This week’s storm was the biggest to hit California in roughly two years. Many parts of the state received between 2 and 4 inches of rain, doubling or tripling their totals since July. Through Thursday night, San Jose received 3.79 inches, San Francisco 4.43 inches and Oakland 3.01 inches, bringing each city’s rainfall to above-normal levels for the first time this year.

More important, several of the state’s large reservoirs began to receive moderate amounts of runoff, as the parched ground became saturated. Lake Shasta gained about 6,000 acre-feet through midnight Wednesday, and Oroville Reservoir in Butte County added 17,000 acre-feet. But that new water boosted Shasta’s storage by less than 1 percent, leaving it at only 23 percent full. It added 3 percent at Oroville, which is now 26 percent full, the lowest level in its history for this time of year.

The Sierra snowpack told a similar story. A week ago, it was at 24 percent of the average for this time of year. Thursday, after a week of snow, it was at 39 percent — still far below normal.

Next storm

But more rain and snow is on the way.

In the Bay Area, another cold front will be moving in on Friday and will hang around a couple of days, according to the National Weather Service.

“There will be rain Friday night and into Saturday and then partly clearing on Sunday,” said forecaster Diana Henderson. “Then there will be a few more showers on Monday, and the next system on the horizon will come in at the end of next week.”

The Weather Service issued a report late Thursday saying that because of storms brewing as far away as Hawaii, projections out to Dec. 18 show that “wetter than normal conditions are favored.”

Experts emphasize that a three-year drought cannot be erased in a few days. Not only are reservoirs low, but there are huge “rainfall deficits” built up from the past three years.

San Jose normally receives 42.9 inches of rain in an average three-year period, for example. Between June 2011 and June 2014, it received just 22.8 inches, leaving the city 20 inches short. Similarly, San Francisco is 19 inches behind, Oakland 24 inches.

Overall, 94 percent of California remains in “severe drought,” according to Thursday’s edition of the Federal Drought Monitor, a weekly report from the U.S. Department of Agriculture and other agencies.

It was the tree-ring study showing California suffering its worst drought in 1,200 years, however, that received the most attention Thursday.

The researchers took core samples, which don’t harm the living trees, of oaks as old as 500 years and oak logs dating back more than 700 years, the University of Minnesota’s Griffin said. And they sanded down the wood with extremely fine-grain sandpaper, magnifying the rings 40 times under a microscope and measuring them to within one one-thousandth of a millimeter.

They then compared the findings to the North American Drought Atlas, a detailed collection of other tree-ring data that goes back 1,200 years and includes measurements from ancient trees such as giant sequoias and bristlecone pines. The atlas calculates temperature and rainfall for those years by comparing the ancient tree rings with tree rings from the past 100 years, when modern records were kept.

Although there are 37 times over the past 1,200 years when there were three-year dry periods in California, no period had as little rainfall and as hot of temperatures as 2012-14, the scientists concluded.

With climate change already warming the earth, the last three years in California could become a more recurring event, they said.

“This kind of drought is what we expect to see more of in the future,” said Griffin. “Maybe the future is now.”

Staff writer David E. Early contributed to this report.