What was done
Gridded reconstructions of summer (June-August) Palmer Drought Severity Index (PDSI) over the continental United States were developed "from annual proxies of drought and wetness provided by 426 climatically sensitive tree-ring chronologies."

What was learned
It was determined that the greatest twentieth-century moisture anomalies across the United States were the 13-year pluvial over the West in the early part of the century, and the epic droughts of the 1930s (the Dust Bowl years) and 1950s, which lasted 12 and 11 years, respectively.  In comparing these events to earlier wet and dry periods, the authors make the following points.

The 13-year pluvial from 1905 to 1917 had three earlier analogs: an extended 16-year pluvial from 1825 to 1840, a prolonged 21-year wet period from 1602 to 1622, and a 10-year pluvial from 1549 to 1558.  The 11-year drought from 1946 to 1956, on the other hand, had at least twelve earlier analogs in terms of location, intensity and duration; but the Dust Bowl drought was greater than all of them ? except for the sixteenth-century "megadrought," which lasted some 18 years and was, in the words of the authors, "the most severe sustained drought to impact North America in the past 500 to perhaps 1000 years."

What it means
Climate alarmists claim that global warming will bring extremes of both floods and droughts to the world, and that the earth is now warmer than it has been in the entire past millennium.  If these two claims were correct, we should clearly be seeing wet and dry periods that exceed the worst of the past half-millennium or more.  In this extensive and well-calibrated record from the United States, however, we don't.  Ergo, something is rotten in more than Denmark.