What was done
The authors analyzed the alluvial stratigraphic evidence of an episode of major channel incision in tributaries of the upper Republican River between c. 1100 and 800 ¹⁴C yr BP.  This major channel incision was compared with drought-related proxy records from 28 sources (based on lacustrine, Aeolian and geomorphological evidence) throughout the Great Plains of the United States and surrounding regions to demonstrate its geographical extent.

What was learned
Channel incision in the Republican River between AD 900 and 1200 correlated well with a multi-centennial episode of widespread drought, which in the words of the authors, "coincides with the globally recognized Medieval Warm Period."  No attempt, however, was made to investigate the underlying climatic cause or causes of this mega medieval dry period.

What it means
Once again we have additional evidence for one of the regional hydrological expressions of the "globally recognized" Medieval Warm Period that climate alarmists, radical environmentalists, and numerous politicians and governmental functionaries of many nations refuse to recognize, because acknowledging its existence pulls the rug from under the house of cards (the hockeystick temperature history of Mann et al. 1998) they proffer as the basis for their claim of dramatic late 20th-century global warming, which they contend is unprecedented over the past two millennia and, therefore, attributable to the concomitant late 20th-century increase in the atmosphere's CO2 concentration.

Reference
Mann, M.E., Bradley, R.S. and Hughes, M.K.  1998.  Global-scale temperature patterns and climate forcing over the past six centuries.  Nature 392: 779-787