What was done
Working with stalagmite SV1 from Grotta Savi -- a cave located at the southeast margin of the European Alps in Italy (45°37'05" N, 13°53'10" E) -- the authors developed a 17,000-year record of speleothem calcite δ¹⁸OC data, which they calibrated against "a reconstruction of temperature anomalies in the Alps" that was developed by Luterbacher et al. (2004) for the last quarter of the past millennium.

What was learned
This work revealed -- among several other things (due to the great length of time involved) -- the occurrence of the Roman Warm Period and a Medieval Warm Period that was broken into two parts by an intervening central cold period. With respect to both parts of the Medieval Warm Period, the five researchers say they were "characterized by temperatures that were similar to the present," while with respect to the Roman Warm Period, they say its "temperatures were similar to those of today or even slightly warmer [italics added]."

What it means
We have here yet another example of the millennial-scale oscillation of climate that has characterized the earth throughout glacial and interglacial periods alike; and we note that it provides three good examples of times when temperatures were equally as warm as they are presently, or even warmer, in spite of the fact that the air's CO2 content was much lower at those earlier times than it is today. These facts call into serious question the great warming role that climate alarmists have assigned to the air's current CO2 content, in that something other than elevated CO2 had to have been responsible of the significant warmth of those earlier periods, which further suggests that something other than elevated CO2 may well be responsible for the significant warmth of our day.

Reference
Luterbacher, J., Dietrich, D., Xoplaki, E., Grosjean, M. and Wanner, H. 2004. European seasonal and annual temperature variability, trends, and extremes since 1500. Science 303: 1499-1503.