What was done
The authors studied diatom assemblages in sediment cores taken from three Canadian and three United States lakes situated within the northern prairies of North America.  For five of the lakes, diatom-inferred salinity estimates were used to reconstruct relative changes in effective moisture (E/P), where E is evaporation and P is precipitation, with high salinity implying high E/P. For the sixth lake, diatom-inferred total phosphorus was used.  Chronologies were based on ²¹⁰Pb dating of recent sediments and radiocarbon dates for older sediments.  The average temporal resolution of the data ranged from five to ten years.

What was learned
The authors say their data show that "shifts in drought conditions on decadal through multicentennial scales have prevailed in this region for at least the last two millennia."  In Canada, major shifts occurred near the beginning of the Medieval Warm Period, while in the United States they occurred near its end.

In giving some context to these findings, the authors state that "distinct patterns of abrupt change in the Northern Hemisphere are common at or near the termination of the Medieval Warm Period (ca. A.D. 800-1300) and the onset of the Little Ice Age (ca. A.D. 1300-1850)."  They also note that "millennial-scale shifts over at least the past 5,500 years, between sustained periods of wetter and drier conditions, occurring approximately every 1,220 years, have been reported from western Canada (Cumming et al., 2002)," and that "the striking correspondence of these shifts to large changes in fire frequencies, inferred from two sites several hundreds of kilometers to the southwest in the mountain hemlock zone of southern British Columbia (Hallett et al., 2003), suggests that these millennial-scale dynamics are linked and operate over wide spatial scales."

What it means
There is nothing unusual about climate change.  It happens on decadal scales, centennial scales and millennial scales.  Over the past century or two, we have experienced a natural and not-unexpected millennial-scale climatic shift that may or may not have yet run its course.  That the air's CO2 content has increased concurrently is primarily due to the coincidental concurrent development of the Industrial Revolution.  There is absolutely no reason to feel that had the latter phenomenon not occurred we would still be shivering through an extended Little Ice Age.

References
Cumming, B.F., Laird, K.R., Bennett, J.R., Smol, J.P. and Salomon, A.K.  2002.  Persistent millennial-scale shifts in moisture regimes in western Canada during the past six millennia.  Proceedings of the National Academy of Sciences USA 99: 16,117-16,121.

Hallett, D.J., Lepofsky, D.S., Mathewes, R.W. and Lertzman, K.P.  2003.  11,000 years of fire history and climate in the mountain hemlock rain forests of southwestern British Columbia based on sedimentary charcoal.  Canadian Journal of Forest Research 33: 292-312.