What was done
The authors analyzed pollen characteristics within sediment cores retrieved from a small unnamed lake located at 71°02'18"N, 28°10'6.6"E near the coast of Nordkinnhalvoya, Finnmark, Norway, after which they used the results of this effort to construct a climatic history of the area over the course of the Holocene.

What was learned
Allen et al. found that "regional vegetation responded to Holocene climatic variability at centennial-millennial time scales." Within the timeframe that most interests us - and which is of most significance for evaluating the nature of modern global warming - they report that "the most recent widely documented cooling event, the Little Ice Age of ca 450-100 cal BP, also is reflected in our data by a minimum in Pinus:Betula [pollen] ratio beginning ca 300 cal BP and ending only in the recent past," and they add that "the Dark Ages cool interval, a period during which various other proxies indicate cooling in Fennoscandia and beyond, is evident too, corresponding to lower values of Pinus:Betula [pollen] ratio ca 1600-1100 cal BP." In addition, they state that "the Medieval Warm Period that separated the latter two cool intervals also is strongly reflected in our data, as is the warm period around two millennia ago during which the Roman Empire reached its peak."

What it means
These findings are but another common example of an important aspect of earth's climate and how it operates: it oscillates back and forth between centennial-scale intervals of relative cold and warmth with a full-period temporal mean of approximately 1500 years (see Climate Oscillations (Millennial Variability) in our Subject Index). Viewed in this light, the development of the Current Warm Period over the past century or so is readily recognized to be nothing more than the most recent - and expected - manifestation of this natural cycling of earth's climate; and this knowledge suggests that our current relative warmth is likely not a response to the historical increase in the atmosphere's CO2 concentration. It is a totally independent phenomenon.