Background
The authors write that "one challenge in reconstructing climate from tree rings at some northern sites is that tree growth, in some instances, has become decoupled from temperature trends in recent decades, a phenomenon known as 'divergence' (D'Arrigo et al., 2008; Driscoll et al., 2005; Wilson et al., 2007)." And they indicate that "this divergence has been attributed to warming-induced drought stress that has forced a shift in tree growth response to climate (D'Arrigo et al., 2008)."

What was done
Using "tree-ring records from living hemlock at mid-elevation GOA [Gulf of Alaska] sites where such trees appear to still be responding positively to temperature as in the past," Wiles et al. went on to "minimize divergence in the recent period that might bias [their] results and thus provide a more accurate assessment of contemporary warming relative to previous centuries," after which "these modern forest records [were] combined with records from sub-fossil wood killed by late Holocene glacial advances to generate a 1200-year-long ring-width chronology."

What was learned
The work of the seven scientists revealed a number of centennial trends that included "a warm interval centered on AD 950 for coastal Alaska that occurred around the time of the Medieval Warm Period," and which "for the first time," as they describe it, "includes a divergence-free view of contemporary warming that is ongoing and is comparable to the MWP." But from the graph of their results, it can be seen that the peak warmth of the MWP was actually slightly greater than the peak warmth of the Current Warm Period (CWP).

In addition to this important finding, Wiles et al. determined that their tree-ring reconstruction "supports centennial modes of solar forcing as a driver of surface air temperatures in the Gulf of Alaska, with lunar tidal, solar variability, internal variability, and volcanism impacting climate on annual to decadal timescales." More specifically, they write that "the observed 220-year and 170-year modes are consistent with both the glacial records (Wiles et al., 2004) and the De Vries cycle, indicating that solar variability is a major climate driver at the century timescale," while they add that "Little Ice Age coolings are fairly coherent across southern Alaska and the Yukon with the coolest intervals corresponding to the Sporer and Maunder [solar] minima."

What it means
Nothing in this study suggests there is anything unusual, unnatural or unprecedented about the Current Warm Period, in that temperatures slightly warmer than those of recent times occurred a full millennium ago, when there was way less CO2 in the atmosphere than there is currently, which in turn suggests that the much greater CO2 concentrations of today need not be the cause of earth's current - and not unusual - level of warmth.

References
D'Arrigo, R.D., Wilson, R., Liepert, B. and Cherubini, P. 2008. On the 'Divergence Problem' in northern forests: A review of the tree-ring evidence and possible causes. Global and Planetary Change 60: 289-305.

Driscoll, W., Wiles, G.C., D'Arrigo, R.D. and Wilmking, M. 2005. Divergent tree growth response to recent climatic warming, Lake Clark National Park and Preserve, Alaska. Geophysical Research Letters 32: 10.1029/2005GL024258.

Wiles, G.C., D'Arrigo, R.D., Villalba, R., Calkin, P.E. and Barclay, D.J. 2004. Century-scale solar variability and Alaskan temperature change over the past millennium. Geophysical Research Letters 31: 10.1029/2004GL020050.

Wilson, R., D'Arrigo, R., Buckley, B., Buntgen, U., Esper, J., Frank, D., Luckman, B., Payette, S., Vose, R. and Youngblut, D. 2007. A matter of divergence - Tracing recent warming at hemispheric scales using tree-ring data. Journal of Geophysical Research: Atmospheres 112: 1.1029/2006JD008318.