What was done
Pollen in sediment cores obtained from a recently in-filled lake (40 years ago) in the Patacancha Valley at Marcacocha, Peru, was analyzed and used to develop a vegetation history and, hence, proxy climate record, for the past 4000 years.

What was learned
An overall decline in pollen content was witnessed over the period AD 100-1050, which the authors indicate is reflective of increasingly colder conditions relative to the period before that time.  Indeed, they state that the "low proportion of Chenopodiaceae and Ambrosia is a clear indication that temperatures were still suppressed" toward the end of this 1000-year interval.  Also, a relative rise in sedges was noted in the record around AD 100, which the authors say may reflect "a shift to wetter conditions."

A "more optimum climate," as they describe it, with warmer temperatures and drier conditions, prevailed for several centuries after about AD 900, as indicated by the establishment and rapid growth of the tree Alnus acuminata (Aliso) during this period.  Between AD 1700 and 1800, however, during what the authors call the "most intense episode of the Little Ice Age," a major decline of Alnus occurred, indicating once again a return to colder conditions.

What it means
The vegetation history analyzed in this record clearly demonstrates the existence of four important global climate epochs that climate revisionists would have us ignore: the Roman Warm Period, which was in decline over the first portion of the record studied, the subsequent Dark Ages Cold Period, the well-known Medieval Warm Period, and the following Little Ice Age. The existence of the millennial-scale climatic oscillation that alternately brings us these warm and cold periods argues strongly against the Modern Warm Period being a consequence of CO2-induced global warming.  It is but the logical continuation of the normal oscillatory behavior of earth's climate.