What was done
In an effort to determine whether climate-driven millennial-scale cycles are present in the terrestrial pollen record of North America, the authors analyzed a set of 3,076 ¹⁴C dates from the North American Pollen Database used to date sequences in more than 700 pollen diagrams across North America.

What was learned
Results of the statistical analyses indicated there were nine millennial-scale oscillations during the past 14,000 years in which continent-wide synchronous vegetation changes with a periodicity of roughly 1650 years were recorded in the pollen records.  The most recent of the vegetation transitions was centered at approximately 600 years BP (before present).  This event, in the words of the authors, "culminat[ed] in the Little Ice Age, with maximum cooling 300 years ago."  Prior to that event, a major transition that began approximately 1600 years BP represents the climatic amelioration that "culminat[ed] in the maximum warming of the Medieval Warm Period 1000 years ago."  And so it goes, on back through the Holocene and into the preceding late glacial period, with the times of all major pollen transitions being "consistent with ice and marine records."

What it means
According to the authors, "the large-scale nature of these transitions and the fact that they are found in different proxies confirms the hypothesis that Holocene and late glacial climate variations of millennial-scale were abrupt transitions between climatic regimes as the atmosphere-ocean system reorganized in response to some forcing."  They go on to say that "although several mechanisms for such natural [our italics] forcing have been advanced, recent evidence points to a potential solar forcing (Bond et al., 2001) associated with ocean-atmosphere feedbacks acting as global teleconnections agents."  Furthermore, they note that "these transitions are identifiable across North America and presumably the world."

So don't blame man for our current climatic conditions; thank the sun!

Reference
Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I. and Bonani, G.  2001.  Persistent solar influence on North Atlantic climate during the Holocene.  Science 294: 2130-2136.