Two worst North American droughts in past 1000 years were 1934 and 1580 per NASA study through 2005. 1934 drought covered 71.6% of western N. Am., 2012 drought only 59.7%

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10/15/14, "1934 Drought in Dust Bowl Days Was Worst in Thousand Years for U.S.: NASA," nbcnews.com, Gil Aegerter

"The drought of 1934 wasn’t just bad, it was the worst. That’s the finding of a reconstruction of North American drought history over the past 1,000 years, done by scientists from NASA and Columbia University’s Lamont-Doherty Earth Observatory. Their study, to be published in the Oct. 17 issue of Geophysical Research Letters, concludes the drought of 1934 during the Dust Bowl years in the North American Plains was 30 percent more severe than the next worst, which occurred in 1580, NASA said. 

 

The scientists used tree ring records from 1000 to 2005 along with modern observations. They found that the 1934 drought extended across 71.6 percent of western North America, compared with 59.7 percent during the 2012 drought. "It was the worst by a large margin," said Ben Cook, climate scientist at NASA's Goddard Institute for Space Studies in New York and lead author of the study. 

 

The scientists found two main reasons: a winter high-pressure system over the West Coast that blocked precipitation and spring dust storms that suppressed rainfall."

 

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From NASA link above:

 

10/14/14, "NASA Study Finds 1934 Had Worst Drought of Last Thousand Years," nasa.gov, Ellen Gray

"A new study using a reconstruction of North American drought history over the last 1,000 years found that the drought of 1934 was the driest and most widespread of the last millennium.

Using a tree-ring-based drought record from the years 1000 to 2005 and modern records, scientists from NASA and Lamont-Doherty Earth Observatory found the 1934 drought was 30 percent more severe than the runner-up drought (in 1580) and extended across 71.6 percent of western North America. For comparison, the average extent of the 2012 drought was 59.7 percent.

"It was the worst by a large margin, falling pretty far outside the normal range of variability that we see in the record," said climate scientist Ben Cook at NASA's Goddard Institute for Space Studies in New York. Cook is lead author of the study, which will publish in the Oct. 17 edition of Geophysical Research Letters.

Two sets of conditions led to the severity and extent of the 1934 drought. First, a high-pressure system in winter sat over the west coast of the United States and turned away wet weather – a pattern similar to that which occurred in the winter of 2013-14. Second, the spring of 1934 saw dust storms, caused by poor land management practices, suppress rainfall....

"What you saw during this last winter and during 1934, because of this high pressure in the atmosphere, is that all the wintertime storms that would normally come into places like California instead got steered much, much farther north,” Cook said. “It's these wintertime storms that provide most of the moisture in California. So without getting that rainfall it led to a pretty severe drought."

This type of high-pressure system is part of normal variation in the atmosphere, and whether or not it will appear in a given year is difficult to predict in computer models of the climate. Models are more attuned to droughts caused by La Niña's colder sea surface temperatures in the Pacific Ocean, which likely triggered the multi-year Dust Bowl drought throughout the 1930s. In a normal La Niña year, the Pacific Northwest receives more rain than usual and the southwestern states typically dry out.

But a comparison of weather data to models looking at La Niña effects showed that the rain-blocking high-pressure system in the winter of 1933-34 overrode the effects of La Niña for the western states. 

This dried out areas from northern California to the Rockies that otherwise might have been wetter. As winter ended, the high-pressure system shifted eastward, interfering with spring and summer rains that typically fall on the central plains. The dry conditions were exacerbated and spread even farther east by dust storms.

"We found that a lot of the drying that occurred in the spring time occurred downwind from where the dust storms originated," Cook said, "suggesting that it's actually the dust in the atmosphere that's driving at least some of the drying in the spring and really allowing this drought event to spread upwards into the central plains."

Dust clouds reflect sunlight and block solar energy from reaching the surface. That prevents evaporation that would otherwise help form rain clouds, meaning that the presence of the dust clouds themselves leads to less rain, Cook said.

"Previous work and this work offers some evidence that you need this dust feedback to explain the real anomalous nature of the Dust Bowl drought in 1934," Cook said.

Dust storms like the ones in the 1930s aren't a problem in North America today. The agricultural practices that gave rise to the Dust Bowl were replaced by those that minimize erosion. Still, agricultural producers need to pay attention to the changing climate and adapt accordingly, not forgetting the lessons of the past, said Seager. "The risk of severe mid-continental droughts is expected to go up over time, not down," he said."

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Much dust over US comes from China:  "On some days, almost a third of the air over Los Angeles and San Francisco can be traced directly to Asia."...

7/20/2007, "Huge Dust Plumes From China Cause Changes in Climate," Robert Lee Hotz, Wall St. Journal

Image caption: "A satellite view from 2001 shows dust arriving in California from Asian deserts. Concentrations of dust are visible to the south, near the coastline (lower right); To the west the dust is mixed with clouds over open ocean. This dust event caused a persistent haze in places like Death Valley, California, where skies are usually crystal clear."

"Courtesy SeaWiFS Project, NASA/Goddard Space Flight Center and ORBIMAGE"
 

"One tainted export from China can't be avoided in North America-- air. An outpouring of dust layered with man-made sulfates, smog, industrial fumes, carbon grit and nitrates is crossing the Pacific Ocean on prevailing winds from booming Asian economies in plumes so vast they alter the climate. These rivers of polluted air can be wider than the Amazon and deeper than the Grand Canyon.
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"There are times when it covers the entire Pacific Ocean basin like a ribbon bent back and forth," said atmospheric physicist V. Ramanathan at the Scripps Institution of Oceanography in La Jolla, Calif.

On some days, almost a third of the air over Los Angeles and San Francisco can be traced directly to Asia. With it comes up to three-quarters of the black carbon particulate pollution that reaches the West Coast, Dr. Ramanathan and his colleagues recently reported in the Journal of Geophysical Research....

 

Asia is the world's largest source of aerosols, man-made and natural. Every spring and summer, storms whip up silt from the Gobi desert of Mongolia and the hardpan of the Taklamakan desert of western China, where, for centuries, dust has shaped a way of life. From the dunes of Dunhuang, where vendors hawk gauze face masks alongside braided leather camel whips, to the oasis of Kashgar at the feet of the Tian Shan Mountains 1,500 miles to the west, there is no escaping it....

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Once aloft, the plumes can circle the world in three weeks. "In a very real and immediate sense, you can look at a dust event you are breathing in China and look at this same dust as it tracks across the Pacific and reaches the United States," said climate analyst Jeff Stith at the National Center for Atmospheric Research in Colorado. "It is a remarkable mix of natural and man-made particles.
 
This spring, Dr. Ramanathan and Dr. Stith led an international research team in a $1 million National Science Foundation project to track systematically the plumes across the Pacific. NASA satellites have monitored the clouds from orbit for several years, but this was the first effort to analyze them in detail.
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For six weeks, the researchers cruised the Pacific aboard a specially instrumented Gulfstream V jet to sample these exotic airstreams. Their findings, to be released this year, involved NASA, the National Oceanic and Atmospheric Administration and nine U.S. universities, as well as the National Institute for Environmental Studies in Japan, Seoul National University in Korea, and Lanzhou University and Peking University in China.
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The team detected a new high-altitude plume every three or four days. Each one was up to 300 miles wide and six miles deep, a vaporous layer cake of pollutants. The higher the plumes, the longer they lasted, the faster they traveled and the more pronounced their effect, the researchers said."...

 

  

Image caption: "A satellite image from 2005 shows a plume of dust flowing from China to the north of the Korean Peninsula and over the Sea of Japan. Such plumes can cross the Pacific and scatter dust across the Western U.S." "Courtesy SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE."

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Ed. note: I copied the above 2007 WSJ article including maps several years ago. As of 2014 the maps may no longer appear in the archived article. Susan
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