What was done
A continuous 2200-year proxy temperature record was developed using tree rings cored from 118 trees near the upper timberline in Siberia for the period 212 B.C. to 1996 A.D.  In addition, tree-ring proxy temperatures were obtained for this region during the Holocene Optimum (3300 to 2600 B.C.).  Furthermore, in an effort to assess the probability that temperature trends in this region are a reflection of Northern Hemispheric temperature trends, the authors compared their data to an isotopic oxygen temperature proxy obtained from a Greenland ice core.

What was learned
Fluctuations in average annual temperature from the Siberian record were found to agree well with air temperature variations reconstructed from the Greenland ice core, suggesting to the authors that "the tree ring chronology of [the Siberian] region can be used to analyze both regional peculiarities and global temperature variations in the Northern Hemisphere."  As for trends in the tree-ring temperature record, several warm and cool periods were noted throughout the 2000-year record; a cool period in the first two centuries A.D., a warm period from 200 A.D. to 600 A.D., cooling again from 600 to 800 A.D., followed by the Medieval Warm Period from about 850 A.D. to 1150 A.D., the cooling of the Little Ice Age from 1200 A.D. though 1800 A.D., followed by the recovery warming of the 20th century.  In regard to the warming experienced in the 20th century, the authors note that it is "not extraordinary" and that "the warming at the border of the first and second millennia [1000 A.D.] was longer in time and similar in amplitude."  Reconstructed temperatures for the Holocene, approximately 5000 years ago, revealed an even warmer time period when temperatures averaged 3.3°C higher than the past two millennia.

What it means
It is clear from the data presented in this study that the climatic episodes referred to as the Medieval Warm Period and the Little Ice Age were indeed real - and, likely, global - phenomena and that earth's climate consistently fluctuates between natural warm and cold phases.  Given the author's finding that the 20th century warming is "not extraordinary" when compared to other warmings of their 2200-year record, it stands to reason that the warming of the 20th century is probably nothing more than a natural recovery from the global chill of the Little Ice Age, rather than a response to the buildup of greenhouse gases in our atmosphere.