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Prior Warm Periods at Austria's Lake Oberer Landschitzsee
Reference
Schmidt, R., Roth, M., Tessadri, R. and Weckstrom, K. 2008. Disentangling late-Holocene climate and land use impacts on an Austrian alpine lake using seasonal temperature anomalies, ice-cover, sedimentology, and pollen tracers. Journal of Paleolimnology 40: 453-469.

What was done
The authors analyzed sediment grain size, as well as the concentrations of major and trace elements and minerals found in a sediment core recovered from an Austrian alpine lake, Oberer Landschitzsee (4714'52" N, 1351'40" E), which covered the past 4,000 years, together with autumn and spring temperature anomalies and ice-cover estimated from selected pollen markers and a diatom and chrysophyte cyst thermistor-based regional calibration dataset, in order to recreate the surrounding region's late-Holocene climate and land-use history.

What was learned
The findings of most interest to us were Schmidt et al.'s detection of the Roman Warm Period (300 BC to AD 400) and the Medieval Warm Period (AD 1000 to AD 1600), as well as their determination that "spring temperature anomalies during Roman and Medieval times equaled or slightly exceeded [our italics] the modern values." Also of significance was their detection of two other warm periods -- 1800 to 1300 BC and 1000 to 500 BC -- as well as the cooler periods that were sandwiched between them, including the Little Ice Age that occurred between the Medieval Warm Period and the Current Warm Period. In addition, they were able to ascertain that "four waves of alpine land use were coupled mainly with warm periods."

What it means
There is a well-established millennial-scale oscillation of climate that has reverberated throughout glacial and interglacial times alike, which has alternately brought the planet relatively warmer and colder climatic conditions, independent of any changes in the air's CO2 content; and the results of this study bear further witness to this fact. They also indicate that in the vicinity of Oberer Landschitzsee, the two warm periods that preceded the Current Warm Period (when the atmosphere's CO2 concentration was fully 100 ppm less than it is today) were at least as warm as -- or even slightly warmer than -- it is at present. Hence, it is likely that earth's current run-of-the-mill warmth is totally unrelated to its much higher atmospheric CO2 concentration and is instead but a manifestation of this natural climatic cycle.

Reviewed 27 August 2008