Scientists: The American Southwest Faces a “Megadrought”

[Spider Jerusalem]

9. This is apparently historic climatic "normal" for the region.

The Southwest has historically experienced regular periods of extended drought. The 20th century was unusually wet by longterm standards, enough so to represent a climatic anomaly...which is a problem when that coincides with the period of major development and everyone's baseline assumptions of "normal" rainfall.

See for instance here:

California is experiencing its worst drought since record-keeping began in the mid 19th century, and scientists say this may be just the beginning. B. Lynn Ingram, a paleoclimatologist at the University of California at Berkeley, thinks that California needs to brace itself for a megadrought—one that could last for 200 years or more.

As a paleoclimatologist, Ingram takes the long view, examining tree rings and microorganisms in ocean sediment to identify temperatures and dry periods of the past millennium. Her work suggests that droughts are nothing new to California.

"During the medieval period, there was over a century of drought in the Southwest and California. The past repeats itself," says Ingram, who is co-author of The West Without Water: What Past Floods, Droughts, and Other Climate Clues Tell Us About Tomorrow. Indeed, Ingram believes the 20th century may have been a wet anomaly.

http://news.nationalgeographic.com/news/2014/02/140213-california-drought-record-agriculture-pdo-climate/

And here:

The California Department of Water Resources, which had funded some of the research, published the results as an illustrated poster. Beneath a series of stock southwestern postcard shots, the spiky trace of tree-ring data oscillates nervously across the page, from A.D. 762 on the left to 2005 on the right. One photo shows the Hoover Dam, water gushing from its outlets. When the dam was being planned in the 1920s to deliver river water to the farms of the Imperial Valley and the nascent sprawl of Los Angeles, the West, according to the tree rings, was in one of the wettest quarter centuries of the past millennium. Another photo shows the booming skyline of San Diego, which doubled its population between 1970 and 2000—again, an exceptionally wet period along the river. But toward the far left of the poster, there is a picture of Spruce Tree House, one of the spectacular cliff dwellings at Mesa Verde National Park in southwestern Colorado, a pueblo site abandoned by the Anasazi at the end of the 13th century. Underneath the photo, the graph reveals that the Anasazi disappeared in a time of exceptional drought and low flow in the river.

In fact, the tree rings testified that in the centuries before Europeans settled the Southwest, the Colorado basin repeatedly experienced droughts more severe and protracted than any since then. During one 13-year megadrought in the 12th century, the flow in the river averaged around 12 million acre-feet, 80 percent of the average flow during the 20th century and considerably less than is taken out of it for human use today. Such a flow today would mean serious shortages, and serious water wars. "The Colorado River at 12 million acre-feet would be real ugly," says one water manager.

Unfortunately, global warming could make things even uglier. Last April, a month before Meko and Woodhouse published their latest results, a comprehensive study of climate models reported in Science predicted the Southwest's gradual descent into persistent Dust Bowl conditions by mid-century. Researchers at the National Oceanic and Atmospheric Administration (NOAA), meanwhile, have used some of the same models to project Colorado streamflow. In their simulations, which have been confirmed by others, the river never emerges from the current drought. Before mid-century, its flow falls to seven million acre-feet—around half the amount consumed today.

(snip)

Stine found drowned stumps in many other places in the Sierra Nevada. They all fell into two distinct generations, corresponding to two distinct droughts. The first had begun sometime before 900 and lasted over two centuries. There followed several extremely wet decades, not unlike those of the early 20th century. Then the next epic drought kicked in for 150 years, ending around 1350. Stine estimates that the runoff into Sierran lakes during the droughts must have been less than 60 percent of the modern average, and it may have been as low as 25 percent, for decades at a time. "What we have come to consider normal is profoundly wet," Stine said. "We're kidding ourselves if we think that's going to continue, with or without global warming."

http://ngm.nationalgeographic.com/print/2008/02/drying-west/kunzig-text