What was done
Several periods of expansion and decline of human cultures in Northwest Europe were identified and compared with a history of reconstructed climate "based on insolation, glacier activity, lake and sea levels, bog growth, tree line, and tree growth."

What was learned
The author determined there was "a positive correlation between human impact/land-use and climate change."  Specifically, in the latter part of the record, where both cultural and climate changes were best defined, there was, in his words, a great "retreat of agriculture" centered on about AD 500, which led to "reforestation in large areas of central Europe and Scandinavia."  He additionally notes that "this period was one of rapid cooling indicated from tree-ring data (Eronen et al., 1999) as well as sea surface temperatures based on diatom stratigraphy in [the] Norwegian Sea (Jansen and Koc, 2000), which can be correlated with Bond's event 1 in the North Atlantic sediments (Bond et al., 1997)."

Next came what the author calls a "boom period" that covered "several centuries from AD 700 to 1100."  This interval of time proved to be "a favourable period for agriculture in marginal areas of Northwest Europe, leading into the so-called Medieval Warm Epoch," when "the climate was warm and dry, with high treelines, glacier retreat, and reduced lake catchment erosion."  This period "lasted until around AD 1200, when there was a gradual change to cool/moist climate, the beginning of the Little Ice Age ... with severe consequences for the agrarian society."

What it means
Even human enterprise bears witness to the reality and great significance of the natural non-CO2-induced millennial-scale oscillation of climate that, within the bounds of the historical record just described, has been responsible for producing the Dark Ages Cold Period, Medieval Warm Period and Little Ice Age, and which has most recently blessed the earth with the Modern Warm Period.

References
Bond, G., Showers, W., Cheseby, M., Lotti, R., Almasi, P., deMenocal, P., Priore, P., Cullen, H., Hajdas, I. and Bonani, G.  1997.  A pervasive millennial-scale cycle in North Atlantic Holocene and glacial climates.  Science 278: 1257-1266.

Eronen, M., Hyvarinen, H. and Zetterberg, P.  1999.  Holocene humidity changes in northern Finnish Lapland inferred from lake sediments and submerged Scots pines dated by tree-rings.  The Holocene 9: 569-580.

Jansen, E. and Koc, N.  2000.  Century to decadal scale records of Norwegian sea surface temperature variations of the past 2 millennia.  PAGES Newsletter 8(1): 13-14.