Background
The authors write that "studying climate change over the past 2000 years … is essential to better understanding climate variability and provides background knowledge necessary for improving predictions of future changes," citing the IGBP (2009). More specifically, they ask: "Has rapid warming such as that in the 20th century occurred previously, especially during the past 2000 years?" Noting that "this is a key issue in understanding the forces of climate warming in the 20th century," they say that "few studies so far have addressed this question," which they thus proceed to do.

What was done
Using 24 preexistent proxy temperature series, the four researchers at the Institute of Geographic Sciences and Natural Resources Research of the Chinese Academy of Sciences analyzed rates of temperature change in China at 30- to 100-year time scales for the past 2000 years and at a 10-year time scale for the past 500 years.

What was learned
Ge et al. report that "the warming rates at centennial and decadal scales in the 20th century were not exceptional for the past 2000 years." At the 30-year time scale, for example, they found that the peak rate was "less than rates for previous periods, such as the rapid warming from the Little Ice Age to the 20th century and from the 270s-290s to 300s-320s." Likewise, they say that "the peak rates of the 100-year scale warming in the AD 180s-350s in northeastern China as well as those in the 260s-410s and 500s-660s in Tibet were all greater than those from the mid-19th to 20th century." And they additionally note that although "the rates of the most rapid cooling at scales of 30 to 100 years in the Little Ice Age were prominent," they state that they "were also not unprecedented in the last 2000 years."

What it means
"In conclusion," in the words of Ge et al., "it is demonstrated that although human-induced greenhouse effects may have contributed to rapid global warming in the 20th century, in the case of China such rapid decadal to centennial warming has occurred in preindustrial times [bold and italics added]," which significantly weakens the worn-out climate-alarmist claim of late 20th-century warming being unprecedented over the past one to two millennia.

Reference
IGBP (International Geosphere-Biosphere Programme). 2009. PAGES Science Plan and Implementation Strategy. Report 57: 3-9.