Background
In introducing the report of their most recent study, the authors write that millennial-length temperature reconstructions have become "an important source of information to benchmark climate models, detect and attribute the role of natural and anthropogenic forcing agents, and quantify the feedback strength of the global carbon cycle." And, therefore, the four researchers have dedicated themselves and their talents to developing the most reliable long-term regional temperature reconstructions possible, focusing their attention most recently on parts of northern Sweden and Finland.

What was done
Esper et al. developed 587 high-resolution wood density profiles (Frank and Esper, 2005) from living and sub-fossil Pinus sylvestris trees of northern Sweden and Finland to form a long-term maximum latewood density (MXD) record stretching from 138 BC to AD 2006, wherein all MXD measurements were derived from high-precision X-ray radiodensitometry, as described by Schweingruber et al. (1978), and where biological age trends inherent to the MXD data were removed using regional curve standardization (RCS), as described by Esper et al. (2003), after which the new MXD record was calibrated against mean June-August temperatures obtained from the long-term (1876-2006) instrumental records of Haparanda, Karasjok and Sodankyla. And in comparing their results with the earlier temperature reconstructions of others, they say that their MXD-based summer temperature reconstruction "sets a new standard in high-resolution palaeoclimatology," as "the record explains about 60% of the variance of regional temperature data, and is based on more high-precision density series than any other previous reconstruction."

What was learned
Most importantly from our point of view, the four researchers say that the new temperature history "provides evidence for substantial warmth during Roman and Medieval times, larger in extent and longer in duration than 20th century warmth [italics added for emphasis]."

What it means
As ever more data-inclusive and carefully-analyzed studies of palaeotemperature proxies are conducted, it is becoming ever more evident that there has not been anything unusual, unnatural or unprecedented about either the rate of warming or the level of warmth achieved during the 20th century, which further suggests that there is no real-world empirical evidence for any CO2-induced global or regional warming. It has all been natural.

References
Esper, J., Cook, E.R., Krusic, P.J., Peters, K. and Schweingruber, F.H. 2003. Tests of the RCS method for preserving low-frequency variability in long tree-ring chronologies. Tree-Ring Research 59: 81-98.

Frank, D. and Esper, J. 2005. Characterization and climate response patterns of a high-elevation, multi-species tree-ring network for the European Alps. Dendrochronologia 22: 107-121.

Schweingruber, F.H., Fritts, H.C., Braker, O.U., Drew, L.G. and Schaer, E. 1978. The X-ray technique as applied to dendroclimatology. Tree-Ring Bulletin 38: 61-91.