What was done
The authors documented characteristic time scales of variability in seven climate indices (three historical, three tree-ring, one glacial) whose time-dependent behavior is sensitive to some aspect of the El Niño/Southern Oscillation (ENSO) phenomenon, five of which indices sampled at least portions of the Medieval Warm Period (MWP).

What was learned
In the words of the authors, "within the time interval encompassed by the Medieval Warm Period, only the Nile River flood-deficit record and the northern Patagonia summer temperature index appear to show extraordinary behavior."  In the former case, they note that "much lower incidences of Nile flood deficits are recorded, whereas for the latter, warmer summers are indicated."  They also note that tree-ring data from the Sierra Nevada, USA "indicate warmer summer during the MWP" and that "the ð¹⁸O record [from Peru's Quelccaya ice cap] also exhibits a marked dip (cooler temperatures) during the core of the Little Ice Age."

What it means
Palaeoclimate data from several locations around the world bear testimony to the global nature of both the Little Ice Age and earlier Medieval Warm Period.  In addition, the authors say that "given the limited sampling available during the MWP, the results of our study suggest that there were not major differences in the climatic expression of the ENSO phenomenon during the last millen[nium], a period encompassing a warm epoch and a generally colder period."  Hence, they conclude that "ENSO may have operated in a substantially similar fashion over the past thousand years as it has during the past century."