What was done
Working with winter half-year temperatures of the "Big Chunk of China" studied by Ge et al. (2003b), which extend back in time a full 2000 years and are described in our Editorial of 19 Nov 2003, together with additional data that extend back in time a full 3000 years, Ge et al. (2003a) were able to identify a millennial-scale climatic oscillation that has a periodicity of approximately 1350 years.  Hence, to see if there was anything unusual or unnatural about the warming of the recent past, and especially the past three decades, they compared similar portions of the two most recent cycles of this natural oscillation by plotting 30-year anomalies of reconstructed temperatures from the AD 1500s-1990s against similar anomalies from the AD 150s-650s and fitting a linear regression to the resulting data.

What was learned
The authors report that "the temperature anomaly in the 1980s-1990s is much higher than the regression value," stating that "if the regression value is regarded as the basic value of temperature changes in nature, the unusual high temperature in the 1980s-1990s could likely be regarded as the forcing of the greenhouse effect induced by human activities on climatic changes," which they say "coincides with the conclusion that the greenhouse effect induced by human activities has been increasing remarkably since the 1950s [as] concluded by the IPCC based on results of modeling."

What it means
Actually, the authors' analysis provides no support whatsoever for this conclusion, for there are other data points that are even further removed from the regression line than is the 1980s-1990s data point; and these more aberrant data points all occurred during the 1600s, well before any major changes in the air's CO2 concentration.  The data point labeled 1635, for example, extends further above the regression line than does the most recent data point, while the very next data point labeled 1665 deviates the most of all, but below the regression line.  What is more, the next data point labeled 1695 reverses course to once again extend further above the line than does the most recent data point.  Hence, the relative warmth of the 1980s-1990s is seen by this analysis to not be anomalous at all and, therefore, not a manifestation of CO2-induced global warming.  It is merely a consequence of natural climatic variability of a well-defined cyclical nature.

Reference
Ge, Q., Zheng, J., Fang, X., Man, Z., Zhang, X., Zhang, P. and Wang, W.-C.  2003b.  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.