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The Roman Warm Period and Dark Ages Cold Period in China
Yu, K.-F., Zhao, J.-X, Wei, G.-J., Cheng, X.-R., Chen, T.-G., Felis, T., Wang, P.-X. and Liu, T-.S.  2005.  δ18O, Sr/Ca and Mg/Ca records of Porites lutea corals from Leizhou Peninsula, northern South China Sea, and their applicability as paleoclimatic indicators.  Palaeogeography, Palaeoclimatology, Palaeoecology 218: 57-73.

What was done
As in the study of Wei et al. (2004), the authors derived high-resolution Sr/Ca ratios for two Porites lutea coral samples taken from the coast of the Leizhou Peninsula (2012'N, 10955'E) in the northern South China Sea, while determining their ages by means of U-Th dating.  The transfer function relating the Sr/Ca ratio to temperature was established on a modern Porites lutea coral in the same location by calibrating against sea surface temperatures (SSTs) measured from 1960 to 2000 at the nearby Haikou Ocean Observatory.

What was learned
Yu et al. first determined that the coral Sr/Ca ratio is "an ideal and reliable thermometer," after which they employed it to learn that the coral sample dated to ~541 BC "yields a mean of Sr/Ca-SST maxima of 29.3C and a mean of Sr/Ca-SST minima of 19.5C, similar to those of the 1990s (the warmest period of the last century)."  Likewise, they determined that "the ~487 AD coral yields a mean of Sr/Ca-SST maxima of 28.7C and a mean of Sr/Ca-SST minima of 16.5C, which are 0.7 and 3.8C lower than those of the 1990s."

What it means
The two dates that correspond to these relatively warm and cool temperatures occur within the Roman Warm Period and Dark Ages Cold Period, respectively.  In harmony with this conclusion, Yu et al. say "historic records show that it was relatively warm and wet in China during 800-300 BC (Eastern Zhou Dynasty), but was significantly colder and drier in east China during the period of 386-589 AD (several degrees Celsius colder than today; Chu, 1973; Ge et al., 2003; Mann et al., 2000)."  As two graphic corroborating examples of these facts, they report "it was so warm during the early Eastern Zhou Dynasty (770-256 BC) that rivers in today's Shangdong province (35-38N) never froze for the whole winter season in 698, 590, and 545 BC," but that "the period of Southern-Northern Dynasties (42-550 AD) was so dry and cold ... that Beiwei Dynasty (386-534 AD) was forced to move its capital from Pingcheng (40.10N) to Luoyang (34.67N) in 493 AD after a series of severe droughts."

These observations add to the voluminous evidence for the reality and global extent of the millennial-scale oscillation of climate that has alternately produced, not only the Roman Warm Period and Dark Ages Cold Period, but the subsequent Medieval Warm Period, Little Ice Age and Modern Warm Period, all without any help from changes in the air's CO2 content.

Wei, G., Yu, K. and Zhao, J.  2004.  Sea surface temperature variations recorded on coralline Sr/Ca ratios during Mid-Late Holocene in Leizhou Peninsula.  Chinese Science Bulletin 49: 1876-1881.

Chu, K.  1973.  A preliminary study on the climatic fluctuations during the last 5000 years in China.  Science in China, Series B, Chemistry, Life Sciences & Earth Sciences 16: 226-256.

Ge, Q.S., Zheng, J.Y., Fang, X.Q., Man, Z.M., Zhang, X.Q., Zhang, P.Y. and Wang, W.C.  2003.  Winter half-year temperature reconstruction for the middle and lower reaches of the Yellow River and Yangtze River, China, during the past 2000 years.  The Holocene 13: 933-940.

Man, Z.M., Ge, Q.S. and Zhang, P.Y.  2000.  Case studies on the impact of climatic changes on the farming-pastoral transitional zone in historical period.  Geographical Research 19: 141-147.

Reviewed 25 May 2005