What was done
Decadal surface air temperatures were derived for the last two millennia from ice core and tree-ring data acquired at five locations on the Tibetan Plateau.

What was learned
The late 20th century was found to be the warmest period in the past two millennia at two of the sites (Dasuopu, ice core; Dunde, ice core); but such was not the case at the other three sites (Dulan, tree ring; South Tibetan Plateau, tree ring; Guilya, ice core).  At Guilya, the data indicated it was significantly warmer than it was in the final two decades of the 20th century for most of the first two centuries of the record, which comprise the latter part of the Roman Warm Period.  At the South Tibetan Plateau it was also significantly warmer over another full century near the start of the record; while at Dulan it was significantly warmer for the same portion of the Roman Warm Period plus two near-century-long portions of the Medieval Warm Period.

What it means
For the majority of the data sets investigated, there were several times over the past two millennia when it was significantly warmer, and for longer periods, than it was during the late 20th century.  These observations do not bode well for the climate-alarmist claim that the late 20th century experienced temperatures that were unprecedented over the past two millennia, just as it also does not bode well for their refusal to recognize the existence of the millennial-scale climatic oscillation that sequentially brought the earth the Roman Warm Period, Dark Ages Cold Period, Medieval Warm Period, Little Ice Age and the Modern Warm Period.