The sky grew darker, and when the clouds finally let loose, the water stood an inch deep on the level driveway and streamed into the window wells. Later, in the basement, among the ruined boxes of books and whatnot, I saw a little plastic bear, long forgotten, bobbing gently against the wall and floating on his back looking up at the floor boards. It was hard to imagine that anyone anywhere lacked water.

But tell that to people on the West Coast, particularly Californians. Rainfall record keeping in California began in July 1849 in San Francisco. In those 165 years, never has there been a year as dry as 2013, but 2014 might top it. A recent LA Times headline blared “California drought will only get worse, experts say”; currently 99.8% of the state is under “Severe” drought conditions. Much of California’s water comes from winter precipitation that accumulates in the Sierras. Melt water is stored in reservoirs, but most of California’s major reservoirs are below 50% of capacity; some are well below that, and with the hot, normally dry summer months upon us, the most reasonable hope for rain in California is not until early next winter.

Drought is nothing new. As a child, I was taught about ancient civilizations that dwindled and faded: Egypt’s Old Kingdom, Bronze Age cultures around the eastern Mediterranean, and the Akkadians, who gave rise to the Assyrians and the Babylonians, in the Fertile Crescent region. That’s all ancient history. They disappeared and nobody’s quite sure why. But drought has always been a likely hypothesis.

Recent research sheds light on what may have happened to those peoples and another civilization that also flourished and faded at about that time, the Harappan culture. The Harappans were Bronze Age people who thrived 4,000 years ago in the Indus Valley in what is now northwestern India and eastern Pakistan. They had two well-developed cities with efficient municipal sewer systems, standardized units of weights and measurements, and a system of writing that has not been decoded.

Paleoclimatological evidence published last February and based on oxygen isotope deposits in the shells of fresh-water snails dug from stratified layers of lake bed sediment suggests an “abrupt weakening” of the Indian summer monsoon preceded a prolonged drought that occurred about 4,100 years ago. The authors contend that this caused “deurbanization” of the Harappan culture and perhaps those other civilizations too. That drought lasted some 200 years.

The most extensive and long lasting drought during the past 300 years in North America was from 1930 until 1940 – the Dust Bowl drought, chronicled in the Grapes of Wrath and by the grainy images of “Black Blizzards” that were created by air-borne soil and that obscured the sky and sun sometimes for days. During the worst year of that drought, 70% of the USA was affected.

In the course geological time, 300 years is the blink of an eye. Regardless of the records the current California drought is breaking, tree-ring data used to infer the historical climate tell us that the last century in California has been unusually wet. California has been much drier in the past, and its droughts have lasted for decades and sometimes centuries. Within the last 1,200 years, there have been two prolonged droughts, lasting between 140 and 200 years.

So how about the current California drought? No one knows whether this drought will last another year or another century. Our understanding of what causes drought is incomplete. Computer models designed to predict and simulate drought are not sufficient to forecast the weather or climate 100 years hence.

Surface water temperature of the eastern Pacific Ocean seems to be one important variable – almost certainly one among many. Data from tree rings, ancient mollusks, and such likely demonstrate a correlation between several historical megadroughts and cool surface waters in the eastern Pacific.

When cooler sea temperatures persist, ridges of high pressure form over the northeastern Pacific Ocean (such as the one that’s been in place since 2012). These force the jet stream and mid-latitude (California) storms to track north and effectively block moisture-laden storms from reaching California and bringing winter rain and snow. This same weather pattern spawned the advance of the infamous “Polar Vortex” that chilled much of the country last winter.

These cooler sea temperatures are linked to a pattern of Pacific climate variability called the Pacific Decadal Oscillation (PDO), similar to the more familiar El Niño pattern but lasting 20 to 30 years rather than 6 to 18 months. The eastern Pacific has been in a cold phase of PDO for 15 years or so.

What if California’s drought does persist for a decade or 100 years? Would California be deurbanized and begin to resemble Saudi Arabia’s Empty Quarter? Workers at University of California-Davis Center for Watershed Sciences studied this question and concluded that an apocalypse would not ensue. Their published report says that, while it wouldn’t be pretty, if well- managed, an extreme drought, such as the 72-year one they modeled, would cause little damage to California’s economy “…with a statewide cost of only a few billion dollars a year out of a $1.9 trillion-a-year economy.”

To be sure, there would be cataclysmic social dislocation for some, and agriculture, wildlife, and some ecosystems and areas such as the Central Valley, where much of North America’s vegetables are grown, would be severely affected. During the drought, reservoirs and lakes would never refill. But “California has a very flexible water supply system that can support a large population and economy under extreme adverse circumstances – provided it is well-managed.” Read the whole thing.