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California repeats a history of drought: The new normal may in fact be an old one

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, 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 past 1,200 years, these dry spells lingered for as long as two centuries.

The new normal, scientists say, may in fact be an old one.

Few experts say California is 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 Nino, the weather pattern that develops in response to warmer water 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 Nino were to occur. But what adds to the worry, scientists say, and what Brown referred to in announcing the water restrictions, is the potential effect of climate change.

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.

“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,” 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, 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 – 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 Stine’s work using tree-ring analysis. “But the pattern of how that plays out across the continent is what we’re grappling with.”

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.

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,” Lund said.

But farmers would shift to more profitable crops, so with half the water use, profits would drop only 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 Brown. And the environment would be hit very hard, 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.”