What was done
The authors inferred the thermal conditions of Smreczynski Staw Lake (49°12'N, 19°51'E) in the Tatra Mountains of southern Poland via analyses of the distributions of various cladocera, chironomid and diatom species they identified and quantified in a sediment core they had extracted from the center of the lake in the spring of 2003, which contained sediments that had accumulated there over the prior 1500 years.

What was learned
Gasiorowski and Sienkiewicz report the presence of "a diverse ecosystem at the beginning of [the] record, ca. AD 360-570," which period of time has classically been assigned to the Dark Ages Cold Period. Thereafter, however, they found that from AD 570 to 1220 "environmental conditions were better," and that various cold-water taxa were "totally absent." In addition, they write that the younger section of this zone -- approximately its upper third (AD 850-1150), which contained the highest concentration of warm-water Chironomus species -- "can be correlated with the Medieval Warm Period (Moberg et al., 2005)."

Next came the Little Ice Age, which was the focal point of their study, extending all the way to the start of the 20th century, after which relative warmth once again returned, persisting to the present. And based on the peak Chironomus concentrations of this portion of their record, which we call the Current Warm Period or CWP, their data suggest that the peak warmth of the CWP and the earlier MWP were about the same, which is also what the temperature reconstruction of Moberg et al. suggests.

What it means
Once again, we have another paleoclimate record that displays the millennial-scale oscillation of climate that reverberates throughout the Holocene and about as far back in time as researchers have looked for it. And once again we have another demonstration of the fact that the peak warmth of the late 20th-century and the early 21st-century has not been as unprecedented as the world's climate alarmists have typically claimed it to be.

Reference
Moberg, A., Sonechkin, D.M., Holmgren, K., Datsenko, N.M. and Karlen, W. 2005. Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data. Nature 433: 613-617.