WEEKLY CLIMATE NEWS

30 March-3 April 2015

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Concept of the Week: Extending the Historical North American Drought Record

Through history, drought has been a major concern for many people as the lack of adequate water can adversely affect agriculture, and in the extreme case, the availability of potable (drinkable) water. The Case in Point for Chapter 9 describes the migration of ancient peoples across the semiarid Southwest due to what may have been major drought conditions. During the last century, the "Dust Bowl" era drought in the 1930s created many problems in this country. The effects of this seven-year long drought were made worse by poor agricultural techniques and land management. The effect of the drought on the nation was also exacerbated by the coincident Great Depression. Drought remains a problem today across Texas and the West Coast as we can see from inspection of the current weekly US National Drought Monitor produced by the National Drought Mitigation Center.

How do the current drought events compare with earlier droughts? A time series of computed Palmer Drought Severity Index (PDSI) values began in 1895 when a sufficiently dense climate network was established. During the 20th century, several episodes of drought have had a major national impact. The exceptional drought that developed in the early 1930s extended across much of the nation resulting in the "Dust Bowl" era. The PDSI time series shows that the 1930s drought was the worst in the last century, with nearly 80 percent of the nation experiencing moderate to extreme drought in 1934. During the 1950s, the southern Plains and the Southwest also experienced a major drought, when 50 to 60 percent of the nation was under drought conditions.

What about farther back in history? Sophisticated tree-ring analysis techniques allow researchers to extend the drought record across a large section of North America farther into the past. In 1998, Edward R. Cook at Columbia University's Lamont-Doherty Earth Observatory along with colleagues at Arizona and Arkansas reconstructed past drought conditions across the nation based upon annual tree-ring data obtained from a network of 388 climatically sensitive tree-ring sites. From these data, time series of annual summertime (June-August) PDSI values were determined back to 1700 at 155 grid points across the nation. These gridded tree-ring chronologies were calibrated with PDSI chronologies generated by instrumental records at selected Historical Climate Network stations commencing in the late 19th century. The researchers found that the 1930s drought was the most severe drought to hit the nation since 1700.

By 2004, the series was expanded to 835 tree-ring sites, primarily across the West, where exactly dated annual tree-ring chronologies were obtained. The new grid covered most of North America with a latitude-longitude spacing of 2.5 degrees. In addition to the 286 grid point PDSI time series, annual contour maps of PDSI were constructed that span much of the continent. This work permitted extension of the spatial and temporal coverage of the drought reconstruction not only into Canada and Mexico, but back 2000 years. From this more recent data set they produced an online "North American Drought Atlas." They found several "megadroughts" in North America were even more severe than the 1930s drought. In addition to being more severe, some droughts extended over several decades, considerably longer than those of the 20th century. One such megadrought was in the 16th century, an event that along with another megadrought into the early 17th century has been implicated by some researchers in the hardships encountered by British settlers in the Virginia area, such as the disappearance of the Roanoke Colony.

Concept of the Week: Questions

(Place your responses on the Chapter Progress Response Form provided in the Study Guide.)

  1. The 1930's Dust Bowl era drought [(does), (does not)] appear to be the most intense across the nation of any in the last two thousand years.
  2. The "North American Drought Atlas" is based on 835 sites where trees in climatically sensitive areas produce [(monthly),(annual),(biennial)] growth rings.

Historical Events:

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Prepared by Edward J. Hopkins, Ph.D., email hopkins@aos.wisc.edu
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