Reference
Vinther, B.M., Jones, P.D., Briffa, K.R., Clausen, H.B., Andersen, K.K., Dahl-Jensen, D. and Johnsen, S.J. 2010. Climatic signals in multiple highly resolved stable isotope records from Greenland. Quaternary Science Reviews 29: 522-538.
Background
The authors introduce the report of their new study by writing that "during the past 10 years studies of seasonal ice core δ18O records from the Greenland ice sheet have indicated, that in order to gain a firm understanding of the relationships between Greenland δ18O and climatic conditions in the North Atlantic region, it is important to have not only annually resolved, but seasonally resolved ice core δ18O data."
What was done
Working with 20 ice core records from 14 different sites, all of which stretched at least 200 years back in time, as well as near-surface air temperature data from 13 locations along the southern and western coasts of Greenland that covered approximately the same time interval (1784-2005), plus a similar temperature data set from northwest Iceland (said by the authors to be employed "in order to have some data indicative of climate east of the Greenland ice sheet"), Vinther et al. proceeded to demonstrate that winter δ18O was "the best proxy for Greenland temperatures." Then, based on that determination and working with three longer ice core δ18O records (DYE-3, Crete and GRIP), they developed a temperature history that extended more than 1400 years back in time.
What was learned
In the words of the seven scientists, "temperatures during the warmest intervals of the Medieval Warm Period," which they defined as occurring "some 900 to 1300 years ago, "were as warm as or slightly warmer than present day Greenland temperatures [italics added]."
What it means
As for what this result implies, the researchers conditionally -- and rather amusingly -- state that further warming of present day Greenland climate "will result in temperature conditions that are warmer than anything seen in the past 1400 years." But, of course, their work more directly and unconditionally implies that late 20th-century and early 21st-century weather has not yet been warm enough to confer "unprecedented" status upon Greenland air temperatures. What is more, Vinther et al. readily admit that the independent "GRIP borehole temperature inversion suggests that central Greenland temperatures are still somewhat below the high temperatures that existed during the Medieval Warm Period."