THE DROUGHT afflicting California—now heading into its third year—has taken a turn for the worse. It seems that 2014 is shaping up to be the driest in nearly a century. Back in January, Governor Jerry Brown declared a state of drought emergency, and urged Californians to cut their water use by 20%. In February, with a good deal of political capital at stake, President Obama visited Fresno, the heart of the state’s agriculture belt, with an offer of $183m in federal aid (see “The drying of the West”, February 22nd 2014).

At the time, there was still hope that the weather pattern causing the prolonged drought—what meteorologists had dubbed a “ridiculously resilient ridge” of high pressure parked off the coast of the Pacific North-west—would break down by late spring. The polar jet stream could then veer south and drive ocean storms spawned in the Gulf of Alaska towards California, instead of pushing them east, to dump snow on Canada and divert frigid air into central and eastern parts of the United States (the source of last winter's brutal weather there).

It all depended on an El Niño emerging off the coast of South America. Such events occur when the temperature of the surface waters in the eastern part of the southern Pacific Ocean becomes higher than usual, and the air-surface pressure in the western Pacific is above average. When that happens, the ocean tilts upwards by a couple of feet (60cm) towards the east.

The opposite effect, the boy-child's sister known as La Niña, occurs when the surface waters off South America become cooler and the air pressure falls in the western Pacific, causing the ocean to tilt in the opposite direction. The resulting rocking motion as the ocean flips between the two states is known as the El Niño Southern Oscillation (ENSO). Extremes in this oscillation cause floods and droughts around the Pacific and elsewhere. For California and the American south-west, a vigorous El Niño is a blessing, bringing much-needed rain to the region in autumn and winter.

As recently as June, the Climate Prediction Centre in College Park, Maryland, put the chance of a drought-busting El Niño at 80% or more. Such hopes have begun to fade. The centre’s climatologists are now distancing themselves from their earlier predictions.

Even if an El Niño does emerge this autumn, it is no longer thought likely to pack the punch needed to bring relief to California. Certainly, no-one expects it to be anything like the “Godzilla of El Niños” that doubled the region’s rainfall in 1997. “The great wet hope is going to be the great wet disappointment”, Bill Patzert, a climatologist at the Jet Propulsion Laboratory in Pasadena, told the San Gabriel Valley Tribune recently.

But while El Niño and La Niña events alternate every couple of years or so, a more mysterious long-term effect known as Pacific Decadal Oscillation (PDO) is at work as well. Like the ENSO, this switches between two different states—one cooler, the other warmer—but does so every 25 to 30 years.

Until a decade or so ago, the PDO was positive, maintaining the ocean off the west coast of the United States slightly warmer than normal—and thus favouring the chance of an El Niño. Since then, the PDO has turned negative, making coastal waters cooler overall. Combining the two effects, the omens for California do not look good.

With 2013 the driest year on record for much of California, and last winter’s snowpacks on the Sierra Nevada and Trinity mountains down by 80%, the region now seems to be in for a prolonged drought. State-wide precipitation for the year ending June 30th was 12.4 inches—not even half of the historical annual average of 25.3 inches.

Meanwhile, the latest data from the National Drought Mitigation Centre in Lincoln, Nebraska, show the whole of California to be now experiencing a “severe” drought or worse. In fact, over 80% of the state is suffering “extreme” or “exceptional” conditions. The Lincoln centre categorises droughts from abnormally dry (D0) to exceptional drought (D4). The monitoring centre had never before recorded D4 conditions since it started collecting data in 1999.

How long might such an exceptional drought last? Lynn Ingram, a geography professor at the University of California, Berkeley, believes it could persist for a decade or more. If so, it would be one of the worst in the past 500 years. Dr Ingram points to paleo-climate data that correlate the thickness, or absence, of growth-rings in trees with annual precipitation. Such records show California has suffered two droughts over the past 1,200 years that lasted for 120-200 years.

To see what impact such conditions would have on the state, Jay Lund, director of the Centre for Watershed Sciences, has simulated the effects of a 72-year drought, with rivers and streams flowing at 50% their average rates. The model built by Dr Lund and his colleagues at the University of California, Davis, sought to show whether, under such dire circumstances, California's water management system could continue to supply enough for people to drink, grow crops and still provide a habitat for fish and other wildlife.

As expected, the simulated drought proved catastrophic for some ecosystems and farming communities, with the greatest impact being felt in the agricultural heartland of the Central Valley. But the researchers were surprised by how little damage the simulated mega-drought did to the economy. The overall cost to the state was a few billion dollars a year. In hindsight, it is not difficult to see why. Despite California being the world’s fifth-largest supplier of food and agricultural commodities, farming accounts for little more than 2% of the state’s $2 trillion economy.