Instructions: In this task, you are given a text of many news articles seperated by special token "|||||". Your task is to summarize them.
Input: In the 1930s, the southwestern Great Plains suffered a series of severe droughts. Overfarming and overgrazing had destroyed prairie grasses, making the topsoil even more vulnerable to strong winds. NASA scientists now say that one of those drought years, 1934, was the driest and most widespread in a millennium. 
  
 An estimated 100 million acres were turned into wasteland during the Dust Bowl, which contributed to the Great Depression's bank closures, business losses, unemployment and other physical and emotional hardships, as vividly chronicled in John Steinbeck's classic novel, The Grapes of Wrath. The first drought to hit the U.S. in 1930-31 created 14 dust storms in 1932, followed by 38 in the next year. Farmers, however, continued plowing the land, believing that rain would return. Instead, they got the 1934 drought, which severely impacted 27 states. 
  
 As it turns out, the successive dust storms were making the droughts even worse. 
  
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 Using a tree-ring-based drought record from the years 1000 to 2005 and modern records, scientists from NASA and the Lamont-Doherty Earth Observatory have discovered that the 1934 drought was 30% more severe than the runner-up drought (in 1580) and extended across 71.6% of western North America. By way of comparison, the average extent of the 2012 drought was 59.7%. 
  
 According to NASA: 
  
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 "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. 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. "In combination then, these two different phenomena managed to bring almost the entire nation into a drought at that time," said Richard Seager, professor at the Lamont-Doherty Earth Observatory of Columbia. "The fact that it was the worst of the millennium was probably in part because of the human role." According to the recent Fifth Assessment Report of the Intergovernmental Panel on Climate Change, or IPCC, climate change is likely to make droughts in North America worse, and the southwest in particular is expected to become significantly drier as are summers in the central plains. Looking back one thousand years in time is one way to get a handle on the natural variability of droughts so that scientists can tease out anthropogenic effects— such as the dust storms of 1934. "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, Seager says, agricultural producers need to pay attention to the changing climate and adapt accordingly, not forgetting the lessons of the past. "The risk of severe mid-continental droughts is expected to go up over time, not down." ||||| Walter Ranzini/NY Daily News/Getty 
  
 Farms failed and livestock starved in the central United States during the Dust Bowl drought of the 1930s. The event was not just the region’s worst dry spell in modern memory — it was the worst in North America over the past millennium, researchers report in Geophysical Research Letters1. 
  
 “Not only did 1934 [the first year of the Dust Bowl] stand out in terms of extent and intensity, but it was the worst by a fair margin,” says Benjamin Cook, a climate scientist at the NASA Goddard Institute for Space Studies in New York and a co-author of the study. The drought takes its name from a period in April 1934, when winds blew dust from the US Great Plains as far east as North Carolina and as far south as Florida. 
  
 Cook and his colleagues used the North American Drought Atlas, a 2,005-year record derived from tree-ring chronologies that reconstructs drought and precipitation patterns. They found that the 1934 drought covered more than 70% of western North America and was 30% more intense than the second most severe drought in the region, which happened in 1580. 
  
 The researchers also looked for causes behind the 1934 drought. An earlier study2 led by Siegfried Schubert of the NASA Goddard Space Flight Center in Greenbelt, Maryland, had pegged the Dust Bowl’s origins on sea-surface temperatures, which were marginally cooler in the Pacific and warmer in the Atlantic. 
  
 But in the latest analysis, Cook and his colleagues say that this event had a minor role in the drought. They pin the blame instead on a change in atmospheric circulation: a high-pressure ridge centred over the west coast of North America during the autumn and winter of 1933–1934 that blocked wet weather from California and the Northwest. 
  
 A similar, but more persistent, atmospheric pattern was at work off the California coast this past winter, and moved storms north. Cook and his colleagues found that similar ridges preceded some of the worst west coast dry spells, including the 1976 California drought — a two-year event marked as the most severe in California’s recorded history. 
  
 “Whenever you see drought, there is always a ridge. But last year’s ridge was a record,” says Simon Wang, a climate scientist at Utah State University in Logan. “The question is what’s causing it to amplify?” Wang and his colleagues have found that the atmospheric ridge in California last winter can be traced to human-made warming of the western Pacific Ocean3. 
  
 Previous studies have also identified a human role in the Dust Bowl. Sparse rainfall and poor land-use practices helped to kick up dust and spread it across the Midwest and eastern United States during the historic drought. In an earlier study, Cook and his colleagues found that airborne dust particles amplified the drought by blocking the Sun’s energy, which reduced evaporation, cloud formation and rainfall over the region4. 
  
 In the latest analysis, Cook and colleagues “make a strong case that the most famous drought in American history was aggravated by human activity, by testing an old idea with climate models and empirical analysis,” says David Stahle, director of the Tree-Ring Laboratory at the University of Arkansas in Fayetteville. ||||| 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. 
  
 [image-50] 
  
 "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. 
  
 [image-113]"In combination then, these two different phenomena managed to bring almost the entire nation into a drought at that time," said co-author Richard Seager, professor at the Lamont-Doherty Earth Observatory of Columbia University in New York. "The fact that it was the worst of the millennium was probably in part because of the human role." 
  
 [image-69]According to the recent Fifth Assessment Report of the Intergovernmental Panel on Climate Change, or IPCC﻿, climate change is likely to make droughts in North America worse, and the southwest in particular is expected to become significantly drier as are summers in the central plains. Looking back one thousand years in time is one way to get a handle on the natural variability of droughts so that scientists can tease out anthropogenic effects – such as the dust storms of 1934. 
  
 “We want to understand droughts of the past to understand to what extent climate change might make it more or less likely that those events occur in the future," Cook said. 
  
 The abnormal high-pressure system is one lesson from the past that informs scientists' understanding of the current severe drought in California and the western United States. 
  
 "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." 
  
 [image-77] 
  
 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. 
  
 › Read the paper at Geophysical Research Letters 
  
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Output:
If you've lately found yourself wondering when the worst drought North America has suffered occurred, here's your answer: 1934. At least, as far as the last millennium goes. In a new NASA study, scientists say that a combination of atmospheric conditions (a high pressure ridge off the West Coast that blocked wet weather) and dust storms (the product of grass-eviscerating activities like overfarming and overgrazing that left topsoil susceptible to wind) culminated in the most severe drought in the past 1,000 years. In fact, the Dust Bowl drought of 1934 was so bad that it was 30% worse than the next most severe drought, which occurred in 1580, reports Nature. Scientists drew this conclusion using an analysis of tree rings between the years 1000 and 2005. The 1934 drought extended across 71.6% of Western North America, while the extent of 2012's drought didn't quite reach 60%. As io9 reports, drought led to 14 dust storms in 1932 and 38 in 1933, but farmers kept working the land, expecting rain to return. "As it turns out, the successive dust storms were making the droughts even worse." NASA explains why: "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." While the agricultural practices that led to those dust storms are no longer used today, scientists say the risk of droughts is only going up, and that agricultural producers will need to adapt accordingly. (The latest drought, meanwhile, is actually causing the West to rise.)