California’s terrifying climate forecast: It could face droughts nearly every year (Washington Post)

March 2 2015

Not long ago, scientists at NASA and two major universities warned of an inevitable “megadrought” that will parch the southwestern United States for 35 years, starting around 2050. By then, a new study says, Californians should be fairly accustomed to long, harsh and dry conditions.

Over the past 15 years, temperatures have been rising in the Golden State, resulting in annual periods of extreme and blazing heat, while the cycle of low and moderate precipitation cycles have not changed since 1977. That means that it’s far more likely that extreme heat years will coincide with dry years.

[NASA: A ‘megadrought’ will grip U.S. in the coming decades]

That’s a recipe for drought, the authors said. Mix searing heat with little to no rain and snow, then bake.

Unlike other climate studies that sound an alarm for impact far into the future, the Stanford University study led by associate professor Noah Diffenbaugh pored through historical data from the U.S. National Climatic Data Center to explain current conditionsand concluded that California should get used to it. It was published Monday afternoon in the journal of the Proceedings of the National Academy of Sciences.

Diffenbaugh and two graduate students at Stanford’s School of Earth, Energy an Environmental Sciences explored the role temperature has played in California’s drought for 120 years. Between 1896 and 1994, climate patterns in the state created a 50 percent chance that a year of extremely warm temperatures would merge with a year of moderately dry conditions. But between 1995 and 2014, extreme temperature years were so common that their chance of combining with dry years increased to 80 percent.

The forecast is negative, but not necessarily the outlook, the authors said. California has opportunities to manage its risks with smart water policies that use precipitation to bank ground water so that farms, which use 77 percent of the state’s water, can survive. The statewide water use is similar to what it was 40 years ago, meaning that even though the population has exploded to 33 million, Californians share about as much water now as they did in the 1970s.

State officials can learn from advanced water management practices already in use in the Middle East in nations such as Saudi Arabia and Israel.

“California was already on the cusp, where 100 percent of the years are not only warm but severely warm,” said Diffenbaugh, a senior fellow at Stanford’s university’s Woods Institute for the Environment. “When a low precipitation year occurs with warm conditions, it’s twice as likely to result in drought.”

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As part of the study, the researchers also observed the impact of greenhouse gases from human activity — carbon dioxide, nitrous oxide and methane from power plants, vehicles, factories and other sources — on temperature and precipitation, Diffenbaugh said. Graduate students Daniel Swain and Danielle Touma were co-authors for the study.

“A lot of this paper is about greenhouse gas emissions that have already happened,” Diffenbaugh said. “Really what California is experiencing is the cumulative emissions of greenhouse gases globally. And the United States has been responsible for a large fraction. Historically the United States has been responsible for a quarter of the emissions and the European Union another quarter.”

Even if world governments that are sharply divided over the approach to lowering greenhouse gas emissions somehow managed to reach a consensus, California will still feel the heat well into the future, Diffenbaugh said.

[How dust leaves the Sahara and floats to Amazon forest]

California, the nation’s most populous state, is suffering one of its worst droughts ever, fueled by the exact conditions cited  in the study — record-low precipitation and record-high heat. The lowest calendar year of precipitation on record in the state happened between 2013 and 2014, and 2014 was the hottest year in California history.

Earlier scientific research suggests that the extremely dry and hot period between 2012 and 2014 might be the worst in a millennium, the study said. But even that can’t hold a candle to the droughts expected 35 years from now. Scientists at NASA and at Cornell and Columbia universities said climate models used for a study released two weeks ago show 80 percent chance of an extended drought between 2050 and 2099, lasting more than three decades if world governments fail to act aggressively to mitigate the effects of climate change.
North America has experienced so-called megadroughts before, during the 12th and 13th centuries. But those were caused by natural changes in weather patterns that give megadroughts a 10 percent chance of forming at any time. The harsh future drought will be the result of human-caused warming.

An international panel of leading climate scientists said in 2013 that the planet is warming at an accelerated pace and found with 95 percent certainty that human activity is the cause. The past three decades have been the hottest on the planet since 1850. Carbon concentrations in the atmosphere have increased 40 percent since then, and carbon, methane and nitrous oxide are at levels unprecedented in at least 800,000 years.

“With climate change, the likelihood of a megadrought goes up considerably,” said Toby R. Ault, an assistant professor in the department of Earth and atmospheric sciences at Cornell, one of the co-authors. Benjamin I. Cook of NASA’s Goddard Institute for Space Studies and Jason E. Smerdon of  Columbia’s Lamont-Doherty Earth Observatory were the other authors for that study.

After 2050, there is “overwhelming evidence of a dry shift,” Ault said, “way drier than the megadroughts of the 1100s and 1200s.” The cause, Smerdon continued, “is twofold, reductions in rainfall and snowfall. Not just rainfall but soil moisture … and changes in evaporation that dry out the soil much more than normal.”

Leading to the theorized megadrought, California is likely to experience a series of micro droughts, researchers say.

[West’s historic drought stokes fears of water crisis]

A third study published three years ago had similar findings. The research is newly published, but its findings are not dramatically different from similar studies in the past. Beverly Law, a specialist in global change biology at Oregon State University’s College of Forestry, co-authored a study of megadroughts three years ago.
It showed that a drought that affected the American West from 2000 to 2004 compared to conditions seen during the medieval megadroughts. But the predicted megadrought this century would be far worse. Law said the NASA study confirmed her previous findings.

“We took the climate model . . . and compared” two periods, 2050 to 2099 and 1950 to 1999, she said. “What it showed is this big, red blotch over Southern California.”

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