What was done
The authors conducted an extensive review of Medieval Warm Period-Little Ice Age climatic conditions as revealed in a variety of proxy records obtained throughout western North America and the tropical Pacific.

What was learned
Graham et al. concluded that the great balance of evidence they analyzed pointed to "generally arid conditions across much of the western and central US from as early as 400 A.D. until about 1300 A.D., followed by a rapid shift towards a wetter regime resembling modern climate." The heart of this Medieval Climate Anomaly (MCA), as they describe it (to emphasize that something in addition to temperature was involved, primarily precipitation), "lasted from about 800-1250 A.D. and included episodes of severe centennial-scale drought," which "affected regions stretching from northern Mexico, California and central Oregon, eastward through the Great Basin and into the western prairies of the central US." In addition, they report considerable direct evidence for "La Niņa-like conditions in the tropical Pacific during the MCA," although they describe this evidence as "not as compelling as that for arid conditions in the western US."

What it means
The eleven researchers say the findings of the many studies they reviewed show that "medieval times witnessed a distinctive pattern of climate change in many regions around the planet," and "as such, the findings suggest the evolution [our italics] of the concept of an Atlantic-European 'Medieval Warm Period' into a surprisingly sharp instance of Holocene climate change with near-global [our italics] manifestations." Or as they rephrase it in the final paragraph of their paper, "the near-global scale of MCA climate change seems to be becoming more apparent." In fact, it is becoming ever more apparent with every passing week, as evidenced by each and every week's new addition to the ever expanding data base that comprises our Medieval Warm Period Project.