Wiles, G.C., Barclay, D.J., Calkin, P.E. and Lowell, T.V. 2008. Century to millennial-scale temperature variations for the last two thousand years indicated from glacial geologic records of Southern Alaska. Global and Planetary Change 60: 115-125.
What was done
The authors used "comparisons of temperature sensitive climate proxy records with tree-ring, lichen and radiocarbon dated histories from land-terminating, non-surging glaciers for the last two millennia from southern Alaska" to "identify summer temperature as a primary driver of glacial expansions," based on "field and laboratory work over the past decade" that yielded "five new or updated glacier histories," one each for Bear Glacier (Kenai Mountains), Marathon Mountain Cirque (Kenai Mountains), Amherst Glacier (Chugach Mountains), Crescent Glacier (Chugach Mountains) and Yakutat Glacier (St. Elias Mountains), all located just above the Gulf of Alaska (about 60°N) between approximately 140 to 150°W.
What was learned
Wiles et al.'s findings suggest the presence of the Roman Warm Period near the beginning of their 2000-year record, because of detected "general glacier expansions during the First Millennium AD" that experienced their "strongest advance" at AD 600, which latter cold interval -- with ice extent "as extensive as [the] subsequent Little Ice Age" -- is typically known as the Dark Ages Cold Period. This latter cold interval was followed by the Medieval Warm Period (MWP), the evidence for which "consists of soil formation and forest growth on many forefields in areas that today are only just emerging from beneath retreating termini," which suggests that the MWP was likely both warmer and longer-lived than what we have so far experienced of the Current Warm Period. They also report, in this regard, that at the Sheridan, Tebenkof and Princeton glaciers, "tree-ring chronologies show that forest growth on these forefields was continuous between the 900s and 1200s."
What it means
Noting that the alternating warm-cold-warm-cold-warm sequence of the past 2000 years "is consistent with millennial-scale records of ice-rafted debris flux in the North Atlantic and Northern Hemisphere temperature reconstructions," and that "variable Holocene solar irradiance has been proposed as a potential forcing mechanism for millennial-scale climate change," the four researchers conclude that "this is supported by the Southern Alaskan glacial record." And this conclusion implies that the past century's lead-in to the Current Warm Period may well have been similarly orchestrated and have had essentially nothing to do with the concomitant increase in the air's CO2 content.