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Millennial-Scale Climate Cycling in North-Central China
Reference
Porter, S.C. and Weijian, Z. 2006. Synchronism of Holocene East Asian monsoon variations and North Atlantic drift-ice tracers. Quaternary Research 65: 443-449.

What was done
Eighteen radiocarbon-dated aeolian and paleosol profiles (some obtained by the authors and some by others) within a 1500-km-long belt along the arid to semi-arid transition zone of north-central China were used to determine variations in the extent and strength of the East Asian summer monsoon throughout the Holocene.

What was learned
The dated paleosols and peat layers, in the words of Porter and Weijian, "represent intervals when the zone was dominated by a mild, moist summer monsoon climate that favored pedogenesis and peat accumulation," while "brief intervals of enhanced aeolian activity that resulted in the deposition of loess and aeolian sand were times when strengthened winter monsoon conditions produced a colder, drier climate." They also report that the climatic variations they discovered "correlate closely with variations in North Atlantic drift-ice tracers that represent episodic advection of drift ice and cold polar surface water southward and eastward into warmer subpolar water."

What it means
The researchers state that "the correspondence of these records over the full span of Holocene time implies a close relationship between North Atlantic climate and the monsoon climate of central China." They also state that the most recent of the episodic cold periods, which they identify as the Little Ice Age, began about AD 1370, while the preceding cold period ended somewhere in the vicinity of AD 810. Consequently, their work implies the existence of a medieval warm period that began some time after AD 810 and ended some time before AD 1370. In addition, their relating of this millennial-scale climate cycle to the similar-scale drift-ice cycle of Bond et al. (2001) implies they accept solar forcing as the most likely cause of the alternating multi-century mild/moist and cold/dry periods of North-Central China. As a result, Porter and Weijian's work helps to establish the global extent of the Medieval Warm Period, as well as its likely solar origin.

Reference
Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I. and Bonani, G. 2001. Persistent solar influence on North Atlantic climate during the Holocene. Science 294: 2130-2136.

Reviewed 6 September 2006