What was done
The authors compiled and analyzed new and previously published glacial geological data obtained from 17 glacier fields in coastal and near-coastal British Columbia (Canada) and Alaska (USA), including evidence pertaining to "overridden forests in glacier forefields, buried paleosols and forest vegetation in lateral moraines, lichen- and ¹⁴C-dated moraines, and glacier-dammed lake sediments."

What was learned
Reyes et al.'s analysis indicated there had been "a widespread glacier advance during the first millennium AD ... along an ~2000 km transect of the Pacific North American cordillera" that was "centered on AD 400-700."

What it means
The eleven researchers concluded that "the synchroneity of this glacier advance and inferred cooling over a large area suggest a regional climate forcing and, together with other proxy evidence for ... regional climate amelioration ca. AD 850-1200 (Hu et al., 2001) during the Medieval Warm Period (Cook et al., 2004; Moberg et al., 2005), and subsequent Little ice Age glacier expansion (Larocque and Smith, 2003; Wiles et al., 2004), are consistent with a millennial-scale climate cycle in the North Pacific region."

We additionally add that since all of the major ups and downs in temperature that produced the multi-century warm and cold periods that preceded the Current Warm Period occurred during a time of rather steady atmospheric CO2 concentration, there is no reason to believe that the final "up" that produced the Current Warm Period was caused by anything other than the most recent natural warming phase of the well-established millennial-scale climatic oscillation that produced the earlier warmings that led to the development of the Roman and Medieval Warm Periods. Also of significance is the fact that this important millennial-scale climatic oscillation operates globally, as is suggested by Reyes et al.'s comment that "glacier advances during the first millennium AD have also been documented in the Canadian Rocky Mountains (Luckman, 1996), Iceland (Gudmundsson, 1997), the European Alps (Holzhauser et al., 2005) and New Zealand (Gellatly et al., 1988)."

References
Cook, E.R., Esper, J. and D'Arrigo, R.D. 2004. Extra-tropical Northern Hemisphere land temperature variability over the past 1000 years. Quaternary Science Reviews 23: 2063-2074.

Gellatly, A.F., Chinn, T.J.H. and Rothlisberger, F. 1988. Holocene glacier variations in New Zealand: A review. Quaternary Science Reviews 7: 227-242.

Gudmundsson, H.J. 1997. A review of the Holocene environmental history of Iceland. Quaternary Science Reviews 16: 81-92.

Holzhauser, M., Magny, M. and Zumbuhl, H.J. 2005. Glacier and lake-level variations in west-central Europe over the last 3500 years. The Holocene 15: 789-801.

Hu, F.S., Ito, E., Brown, T.A., Curry, B.B. and Engstrom, D.R. 2001. Pronounced climatic variations in Alaska during the last two millennia. National Academy of Sciences Proceedings 98: 10,552-10.556.

Larocque, S.J. and Smith, D.J. 2003. Little Ice Age glacial activity in the Mt. Waddington area, British Columbia Coast Mountains, Canada. Canadian Journal of Earth Sciences 40: 1413-1436.

Luckman, B.H. 1996. Dendroglaciology at Peyto Glacier, Alberta. In: Dean, J.S. et al., Eds. Tree rings, environment and humanity. Radiocarbon 38: 679-688.

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.

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/.