What was done
The authors studied a sediment core retrieved from Big Lake (51°40'N, 121°27'W) on the Cariboo Plateau of British Columbia, Canada, carefully dating it and deriving estimates of changes in climatically sensitive limnological variables (salinity and lake depth) from transfer functions based on modern distributions of diatom assemblages in 219 lakes from western Canada.

What was learned
On the basis of observed changes in patterns of the floristic composition of diatoms over the past 5,500 years, the authors report that "alternating millennial-scale periods of high and low moisture availability were inferred, with abrupt [our italics] transitions in diatom communities occurring 4960, 3770, 2300 and 1140 cal. yrs. BP."  They also indicate that "periods of inferred lower lake depth correspond closely to the timing of worldwide Holocene glacier expansions," and that the mean length of "the relatively stable intervals between the abrupt transitions ... is similar to the mean Holocene pacing of IRD [ice rafted debris] events ... in the North Atlantic," which have been described by Bond et al. (1997) and attributed to "solar variability amplified through oceanic and atmospheric dynamics," as detailed by Bond et al. (2001).

What it means
This study provides more evidence for the reality of the millennial-scale climatic oscillation that has alternately brought the planet relatively warmer and cooler conditions, such as the Medieval Warm Period and Little Ice Age.  It also provides more evidence for the likelihood that this oscillation (which has recently ushered in the Modern Warm Period) is totally natural and only coincidentally related to the historic increase in atmospheric CO2 concentration that has accompanied the progression of the Industrial Revolution.

References
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.

Bond, G., Showers, W., Chezebiet, 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.