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The Millennial-Scale Oscillation of Climate in the Qinghai-Tibetan Region of China
Reference
Liu, Z., Henderson, A.C.G. and Huang, Y. 2006. Alkenone-based reconstruction of late-Holocene surface temperature and salinity changes in Lake Qinghai, China. Geophysical Research Letters 33: 10.1029/2006GL026151.

What was done
The authors developed a quantitative reconstruction of temperature changes over the past 3500 years based on alkenone distribution patterns in a sediment core retrieved from China's Lake Qinghai (37N, 100E), based on the alkenone unsaturation index (Uk37) and its simplified form (Uk'37), which "have been calibrated to growth temperature of marine alkenone producers (Prahl et al., 1988)" and "to temperature changes in lacustrine settings on a regional scale (Chu et al., 2005; Zink et al., 2001)."

What was learned
In the words of Liu et al., "our temperature record based on Uk'37 clearly shows oscillating warm/cold periods." They note, for example, that "periods at 0-200 yr BP, 500-1100 yr BP and 1500-2000 yr BP were relatively warm, which could be related to the 20th-century warm period, the Medieval Warm Period, and the Roman Warm Period." Also, they say that "cold periods at 200-500 yr BP and 1100-1500 yr BP corresponded to the Little Ice Age and the Dark Ages Cold Period." What is more, their plotted data indicate that the peak warmth of the Roman Warm Period exceeded the temperature of the latter part of the 20th century by about 0.4C, while the peak warmth of the Medieval Warm Period exceeded the temperature of the latter part of the 20th century by nearly 1C.

What it means
The existence of this millennial-scale oscillation of climate, with its prior periods of higher-than-current temperatures, clearly demonstrates there is nothing unusual about earth's present climatic state, except that it is surprisingly cool, considering how much more CO2 there is in the air nowadays than there was during the warmer Medieval and Roman Warm Periods.

References
Chu, G., Sun, Q., Li, S., Zheng, M., Jia, X., Lu, C., Liu, J. and Liu, T. 2005. Long-chain alkenone distributions and temperature dependence in lacustrine surface sediments from China. Geochimica et Cosmochimica Acta 69: 4985-5003.

Prahl, F.G., Muehlhausen, L.A. and Zahnle, D.L. 1988. Further evaluation of long-chain alkenones as indicators of paleoceanographic conditions. Geochimica et Cosmochimica Acta 52: 2303-2310.

Zink, K.G., Leythaeuser, D., Melkonian, M. and Schwark, L. 2001. Temperature dependency of long-chain alkenone distributions in recent to fossil limnic sediments and in lake waters. Geochimica et Cosmochimica Acta 65: 253-265.

Reviewed 27 September 2006