What was done
The authors studied changes in diatom assemblages preserved in a sediment core extracted from a small lake (Middendorf Lake, 70°22' N latitude) on the western Taimyr Peninsula of northern Russia.

What was learned
The last 2500 years of the sediment record was strongly indicative of fluctuating limnological conditions, characterized, in the words of the authors, by "striking successional shifts between Fragilaria pinnata and Aulacoseira distans var. humilis."  They further report that this variability was "strongly correlated with other paleoclimatic data from northern Eurasia and Greenland."  Prior to 2000 years ago, for example, the rate of change of the diatom assemblage was at a several-century minimum, but it rose rapidly as the Roman Warm Period established itself.  This parameter then dropped to lower values during the Dark Ages Cold Period, but rose to new heights (the greatest of the entire 4700-year record) during the Medieval Warm Period, only to experience the greatest decline of the entire record with the appearance of the Little Ice Age, after which the development of the Modern Warm Period once again saw a rapid rise in the rate of change of the diatom assemblage.

What it means
These results are another ringing testament to the reality of the non-CO2-induced millennial-scale oscillation of climate that pervades earth's history and periodically ushers in global warming such as that of the past century and a half [see Climate Oscillations (Millennial Variability) in our Subject Index].  Furthermore, as the air's CO2 content was essentially constant over the bulk of the 4700-year sediment record, its rising in tandem with the planet's temperature over the past century and a half is readily recognized as being but a coincidental side effect of the concomitant development of the Industrial Revolution, which social development is unique to the most recent cycle of the climatic oscillation.