Background
In an initial explanation about why they did what they did, the authors write that "lakes are excellent sensors of environmental change," and that "lake sediments can provide well-resolved records of change on different time scales," while noting that "crater and maar lakes are especially sensitive to climate change because typically they have a small catchment area and limited inflow/outflow." Moreover, they say such lakes "often provide high-resolution records due to limnological processes favorable to the development and preservation of seasonally laminated sediments," citing Zolitschka et al. (2000) in this regard. And they add that "diatoms are excellent indicators of environmental conditions and have been widely used to reconstruct Holocene climate variability," citing Smol and Cumming (2000), Battarbee et al. (2001) and Mackay et al. (2003).

What was done
Focusing on Lake Erlongwan, one of eight maar lakes in the Long Gang Volcanic Field of Jilin Province, NE China (42°18'N, 126°21'E) - which they describe as a closed dimictic lake that occupies an area of 0.3 km² and has a small catchment (0.4 km²) with no natural inflows or outflow - Wang et al. retrieved a 66.5-cm-long sediment core from its central, deepest region in 2001, which they dated with the help of radiometric ²¹⁰Pb, ¹³⁷Cs and ¹⁴C analyses, and which they analyzed for diatom species and quantities. Although they note, in this regard, that diatoms "are generally not known to be very sensitive to water temperature," they indicate that "climate affects the physical properties of the lake water column, especially as it controls the seasonal durations of ice cover, water column mixing and stratification, which all have profound effects on the availability of nutrients and light necessary for algal photosynthesis and growth," so that "climate has an indirect influence on the composition and productivity of phytoplankton, especially non-motile organisms such as diatoms," which thus allows them to undertake "a detailed qualitative paleolimnological interpretation of the Lake Erlongwan sediment sequence based mainly on the growing body of literature that focuses on the ecology of planktonic diatoms, especially their responses to climate-driven changes in limnology."

What was learned
The ten researchers report that "three intervals were identified by their diatom assemblages and correspond within dating uncertainties to the Medieval Warm Period, the Little Ice Age and the 20th century warming trend." During the MWP, they further indicate that "the duration of the summer was longer while the spring and autumn were shorter than the 20th century." And they unequivocally declare that "the period between ca. AD 1150 and 1200 was the warmest interval of the past 1000 years."

What it means
In view of the fact that, prior to the time of their study, there was no record of mean annual temperatures from NE China covering the past 1000 years with the same resolution as their diatom record, Wang et al.'s work demonstrates - for yet another part of the planet (see our Medieval Warm Period Project) - that late-20th-century warmth, even with the help of an extra 100 ppm of CO2, was less than that of the MWP, which makes it extremely difficult to believe that earth's current level of warmth largely owes its existence to anthropogenic CO2 emissions, as the world's climate alarmists continue to claim it does.

References
Battarbee, R.W., Jones, V.J., Flower, B.P., Cameron, N.G., Bennion, H., Carvalho, L. and Juggins, S. 2001. Diatoms. In: Smol, J.P., Birks, H.J.B. and Last, W.M. (Eds.). Tracking Environmental Change Using Lake Sediments. Kluwer Academic Publishers, Dordrecht, The Netherlands, pp. 155-201.

Mackay, A.W., Jones, V.J. and Battarbee, R.W. 2003. Approaches to Holocene climate reconstruction using diatoms. In: Mackay, A.W., Battarbee, R.W., Birks, H.J.B. and Oldfield, F. (Eds.). Global Change in the Holocene. Arnold, London, United Kingdom, pp. 294-309.

Smol, J.P. and Cumming, B.F. 2000. Tracking long-term changes in climate using algal indicators in lake sediments. Journal of Phycology 36: 986-1011.

Zolitschka, B., Brauer, A., Negendank, J.F.W., Stockhausen, H. and Lang, A. 2000. Annually dated late Weichselian continental paleoclimate record from the Eifel, Germany. Geology 28: 783-786.