When Gov. Jerry Brown of California imposed mandatory cutbacks in water use earlier this month in response to a severe drought, he warned that the state was facing an uncertain future. “This is the new normal,” he said, “and we’ll have to learn to cope with it.”  The drought, now in its fourth year, is by many measures the worst since the state began keeping records of temperature and precipitation in the 1800s. And with a population now close to 39 million and a thirsty, $50 billion agricultural industry, California has been affected more by this drought than by any previous one.  But scientists say that in the more ancient past, California and the Southwest occasionally had even worse droughts — so-called megadroughts — that lasted decades. At least in parts of California, in two cases in the last 1,200 years, these dry spells lingered for up to two centuries.  The new normal, scientists say, may in fact be an old one.  Few experts say California is now in the grip of a megadrought, which is loosely defined as one that lasts two decades or longer. But the situation in the state can be seen as part of a larger and longer dry spell that has affected much of the West, Southwest and Plains, although not uniformly. “The California drought is kind of the latest worst place,” said Jonathan Overpeck, a director of the Institute of the Environment at the University of Arizona.  The wider dry spell began after the last strong El Niño, the weather pattern that develops in response to warmer water temperatures in the Eastern Pacific and can bring heavy winter precipitation. That was 17 years ago.  “What we’re seeing is nudging up to being comparable to some of the megadroughts,” said Richard Seager, a climate scientist at Lamont-Doherty Earth Observatory in Palisades, N.Y.  California could get relief as soon as next winter, particularly if another strong El Niño were to occur. But what adds to the worry, scientists say, and what Mr. Brown referred to in announcing the water restrictions, is the potential effect of climate change.  Analysis of tree rings suggests that western states have had many droughts of two decades or longer, including two megadroughts lasting longer than 100 years.  Most climate models show Northern California becoming slightly wetter in the decades to come, but at the same time, average temperatures are expected to increase across the state. And just as record-high temperatures exacerbated the drought this year by causing more evaporation and reducing snowpack, warmer conditions can be expected to make things worse in the future.  A recent study, for example, suggests that in the second half of this century, the warming that will result from rising concentrations of greenhouse gases will greatly increase the risk of a severe long-term drought in the Southwest and Plains that could rival or even exceed some of the ancient ones.  “Climate change is really weighting the dice” in favor of future megadroughts, said Toby R. Ault, a researcher at Cornell University and an author of the study.  Scientists learn about ancient droughts by looking for evidence that can provide clues to temperatures and precipitation at the time. The growth rings of tree trunks are one such proxy — thin rings mean growth stunted by a lack of water. Tree-ring analysis has revealed many historic droughts, including one throughout much of the Southwest around 200 A.D. that lasted for five decades.  While many scientists think they have a good idea why California’s current drought began — it is related to the Pacific water temperature fluctuations, they say — no one is certain what caused the megadroughts of the past. Research “just doesn’t give us the mechanism of steady atmospheric circulation keeping one region dry for decades,” Dr. Overpeck said.  A different kind of analysis led to the discovery of two very long droughts, one that began in the ninth century and lasted about 200 years, and another that began in the 13th and lasted for a century and a half.  In the 1990s, Scott Stine, a professor at what is now called California State University, East Bay, took advantage of a decline in the levels of Mono Lake and other lakes and streams in the eastern Sierra Nevadas to study tree stumps, still rooted in the ground, that had become visible after having been submerged for hundreds of years.  At some point, water levels must have been low enough for long enough for the trees to grow. By dating the stumps using radioactive carbon techniques and noting their elevations, Dr. Stine was able to reconstruct the ancient water levels and thus the drought history of the area.  The extent of these two droughts has been debated — Dr. Stine says they affected areas well beyond California, but other scientists are not so sure.  “The evidence for the existence of sustained severe droughts is pretty convincing,” said Malcolm Hughes, a scientist at the University of Arizona who corroborated some of Dr. Stine’s work using tree-ring analysis. “But the pattern of how that plays out across the continent is what we’re grappling with.”  Still, the two long droughts show that “this is centuries-scale stuff,” Dr. Stine said.  “Equally as important but much easier to forget is that we consider the last 150 years or so to be normal,” he added. “But you don’t have to go back very far at all to find much drier decades, and much drier centuries.”  That raises the possibility that California has built its water infrastructure — indeed, its entire modern society — during a wet period.  Within that century and a half of relative wetness, the period from the mid-1970s to the late 1990s — a time when California’s population soared by about 50 percent — was even wetter, said Dr. Seager of Lamont-Doherty. “All of that growth occurred at a time when more water was available than you’d expect in the long term,” he said.  The California Department of Water Resources studies previous droughts and uses the lessons learned from recent ones to help with preparedness. But Jeanine Jones, interstate resources manager for the department, said that while ancient droughts were indeed bad, California is now such a vastly different place that comparisons have little meaning.  “Asking if we would be planning for a drought that happened in 5000 B.C. is like asking if New York would plan for the next ice age,” she said.  Some research suggests that California would survive, limping along, if a modern drought were to last for decades like the ancient ones.  In 2010, Jay R. Lund, a professor of environmental engineering at the University of California, participated in a study of the economic effects of a 72-year-long hypothetical drought in which the state got about half as much water as normal each year. (They found that their virtual drought did not need to be longer than that because water supply and use eventually reached an equilibrium.)  Not surprisingly, given that it uses about 80 percent of the state’s water, agriculture would suffer greatly, the researchers found. “We’d probably have half the agriculture that we have today in terms of irrigated area,” Dr. Lund said. But farmers would shift to more profitable crops, so with half the water use, profits would only drop about 25 percent, he said.  Still, agricultural towns would suffer as they lost much of their economic base. Urban residents would chafe under water restrictions more draconian than those imposed by Mr. Brown. And the environment would be hit very hard, Dr. Lund said, as reduced stream flows would threaten whole ecosystems.  “But I was actually surprised at how well we’d get through such a drought,” he said. “California would not dry up and blow away. It would be bad but we would still have civilization, so long as we managed it at all well.”
The state, along with much of the West, has gone through worse dry spells lasting decades. Development and climate change complicate predictions.