Background
The infamous "hockeystick" temperature history of Mann et al. (1998, 1999) depicts a Northern-Hemispheric temperature increase over the last two decades of the 20th century that climate alarmists claim is unprecedented over the past millennium, while the expanded version of Mann and Jones (2003) depicts a late 20th-century global temperature increase that is said to be unprecedented over the past two millennia.  It is extremely important, therefore, that every study that can contribute to the evaluation of these strident claims be given careful consideration, which is why we address the subject so frequently.

What was done
The Jämtland multi-millennial tree-ring width chronology is comprised of living and subfossil Scots pines (Pinus sylvestris L.) sampled close to the present tree-line in the central Scandinavian Mountains.  It spans the time period from 2893 BC to AD 2002, with minor gaps at 1633-1650 BC and AD 887-907.  In the present study, the authors focus their analysis on the well replicated period of 1632 BC to AD 2000, utilizing it as a proxy for summer temperatures.

What was learned
Several periods of anomalously warm and cold summers were noted throughout the record: (1) 550 to 450 BC (Roman Warm Period), when summer temperatures were the warmest of the entire record, exceeding the 1961-1990 mean by more than 6°C, (2) AD 300 to 400 (Dark Ages Cold Period), which was "the longest period of consecutive cold summers," averaging 1.5°C less than the 1961-1990 mean, (3) AD 900 to 1000, a warm era corresponding to the Medieval Warm Period, and (4) AD 1550 to 1900, a cold period known as the Little Ice Age.

With respect to the latter portion of the tree-ring record, which encompasses the period of modern global warming, Linderholm and Gunnarson declare that this phenomenon "does not stand out as an anomalous feature in the 3600-year record, in fact other periods show more rapid warming and also higher summer temperatures."  What should be even more embarrassing to the world's climate alarmists is the fact that the last half of the 20th century actually experienced cooling.

What it means
The results of this analysis suggest there is nothing unusual or unprecedented about the Modern Warm Period in the Scandinavian Mountains; it has not been anywhere near as warm there in modern times as it was some 2500 years ago during the Roman Warm Period, nor has it been as warm there recently as it was during the less extreme Medieval Warm Period of a thousand years ago.  In fact, the unprecedented warmth that climate alarmists claim to have been present during the latter two decades of the 20th century is nowhere to be seen in the Scandinavian Mountains, nor is it detectable at a whole host of other places on earth.

References
Mann, M.E., Bradley, R.S. and Hughes, M.K.  1998.  Global-scale temperature patterns and climate forcing over the past six centuries.  Nature 392: 779-787.

Mann, M.E., Bradley, R.S. and Hughes, M.K.  1999.  Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations.  Geophysical Research Letters 26: 759-762.

Mann, M.E. and Jones, P.D.  2003.  Global surface temperatures over the past two millennia.  Geophysical Research Letters 30: 10.1029/2003GL017814.