What was done
Using a variety of temperature proxies that had all "previously been shown to relate to local or regional temperature," together with "a reconstruction method, LOCal (LOC), that recently has been shown to confidently reproduce low-frequency variability," the authors developed a new multi-proxy reconstruction of the mean temperature of the extra-tropical Northern Hemisphere (30-90°N) stretching all the way back to the year 1 AD.

What was learned
Christiansen and Ljungqvist report that the two-millennia-long reconstructed temperature history shows a well-defined Medieval Warm Period, with "a well-defined peak in the period 950-1050 AD with a maximum temperature anomaly of 0.6°C" - relative to the reference period of 1880-1960 AD - the timing of which they say "is in agreement with the reconstructions of Esper et al. (2002) and Ljungqvist (2010)," with "the level of warmth during the peak of the MWP in the second half of the 10th century, equaling or slightly exceeding the mid-20th century warming," which result they say is "in agreement with the results from other more recent large-scale multi-proxy temperature reconstructions," citing among others, in this regard, the studies of Moberg et al. (2005), Ljungqvist (2010) and Ljungqvist et al. (2012).

As for the timing of the MWP in different parts of the globe-girdling extra-tropical Northern Hemisphere, they report that Ljungqvist et al. (2012) have shown that, "on centennial time-scales, the MWP is no less homogeneous than the Little Ice Age if all available proxy evidence, including low-resolution records are taken into consideration in order to give a better spatial data coverage."

What it means
This newest work of Christiansen and Ljungqvist adds significantly to the growing body of real-world evidence that demonstrates both the global nature and temporal consistency of the MWP (see our Medieval Warm Period Project), as well as the fact that the MWP's peak temperatures throughout the extra-tropical Northern Hemisphere rivaled those of the Current Warm Period, even though the atmospheric CO2 concentrations of today are some 40% greater than they were during the MWP, which is highly suggestive of the very real possibility that the warmth of the Current Warm Period may well be due to something other than the current high level of the air's CO2 content.

References
Esper, J., Cook, E.R. and Schweingruber, F.H. 2002. Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability. Science 295: 2250-2253.

Ljungqvist, F.C. 2010. A new reconstruction of temperature variability in the extra-tropical Northern Hemisphere during the last two millennia. Geografiska Annaler 92A: 339-351.

Ljungqvist, F.C., Krusic, P.J., Brattstrom, G. and Sundqvist, H.S. 2012. Northern Hemisphere temperature patterns in the last 12 centuries. Climate of the Past 8: 227-249.

Moberg, A., Sonechkin, D.M., Holmgren, K., Datsenko, N.M. and Karlen, W. 2005. Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data. Nature 433: 613-617.