Reference
Yafeng, S., Tandong, Y. and Bao, Y. 1999. Decadal climatic variations recorded in Guliya ice core and comparison with the historical documentary data from East China during the last 2000 years. Science in China Series D-Earth Sciences 42 Supp.: 91-100.
What was done
The authors analyzed high-resolution records of delta18O obtained from the Guliya ice cap (35.2°N, 81.5°E, 6200 m a.s.l.) located in the Qinghai-Tibet Plateau of China as a proxy for temperature in that region of the world over the past 2000 years. A precipitation record was also constructed for this time period based upon a model analysis of annual snowfall accumulation rates.
What was learned
Prior to AD 270, climatic conditions were characterized as relatively warm and wet, followed by a cold and dry period that lasted until around 970 AD. A moderately warm and dry period dominated the climate between 970 and 1510 AD, after which conditions deteriorated into a "well-defined 'Little Ice Age'" that lasted until around 1930.
Perhaps the most striking discovery reported in this paper is the authors' finding that there have been 33 abrupt climatic shifts on the order of 3°C that took place over the course of two or three decades over the past 2000 years. Furthermore, among these 33 abrupt transitions, there have been "several large ones," including a 7°C decrease between 250 and 280 AD and a 7°C increase between 550 and 580 AD. Another 7°C increase was seen over the longer time interval between 1120 and 1260 AD, corresponding to the Medieval Warm Period.
What it means
The results of this paper clearly demonstrate a dynamic feature of earth's climate system that is totally independent of human activities, while at the same time revealing the reality of both the Medieval Warm Period and the Little Ice Age, in yet another rebuff of the climate alarmist claim that both of these climatic excursions are but figments of people's imaginations. Thank goodness for real-world data!