Volume 11, Number 53: 31 December 2008
Working with a stalagmite found in China's Wanxiang Cave (33°19'N, 105°00'E) -- which Zhang et al. (2008) say is located on the fringes of the area currently affected by the Asian Monsoon and is thus sensitive to (and integrates broad changes in) that annually-recurring phenomenon -- the seventeen researchers developed a δ18O record with an average resolution of 2.5 years that "largely anti-correlates with precipitation" and runs continuously from AD 190 to 2003. Even more important than its close ties with precipitation, in our opinion, Zhang et al. demonstrate that the record "exhibits a series of centennial to multi-centennial fluctuations broadly similar to those documented in Northern Hemisphere temperature reconstructions, including the Current Warm Period, Little Ice Age, Medieval Warm Period and Dark Age Cold Period." And when one compares the peak warmth thus implied by their data for the Current and Medieval Warm Periods, it is readily seen that the Medieval Warm Period comes out on top as having been the warmer of the two.
In another important set of comparisons, Zhang et al. superimpose their δ18O record upon individual plots of Northern Hemispheric temperature as derived by Esper et al. (2002), Moberg et al. (2003) and Mann and Jones (2003). In the first of these comparisons, the two records closely mimic each other, with both of them indicating greater peak warmth during the Medieval Warm Period than during the Current Warm Period. The same is likewise true of the second comparison; and in the third comparison the records also closely mimic each other over the vast majority of their expanse. Over the last decades of the 20th century, however, the temperatures of the Mann and Jones record rise far above the temperatures implied by the Zhang et al. record (and, therefore, those of the Esper et al. and Moberg et al. records as well), which suggests to us that this anomalous behavior of the Mann and Jones record is indicative of its possessing a major defect that is not found in the other three datasets. And that defect, in our estimation, is Mann and Jones' use of directly-measured as opposed to reconstructed temperatures over their record's last few decades, which leads to their anomalous end-point "oranges" not telling the same story as that told by everyone else's "apples."
Another point of great interest about the Zhang et al. record is that it "correlates with solar variability, Northern Hemisphere and Chinese temperature, Alpine glacial retreat, and Chinese cultural changes." And since none of the last four phenomena can influence the first one, it stands to reason that solar variability is what has driven the variations in every other factor mentioned. In fact, in a commentary that accompanies Zhang et al.'s article, Kerr (2008) states that the Zhang et al. record is described by other researchers as "amazing," "fabulous," and "phenomenal," and that it "provides the strongest evidence yet for a link among sun, climate, and culture." In addition, we note that it provides equally strong evidence for at least the Northern-Hemispheric-extent of the Medieval Warm Period and its greater and more persistent warmth than that of the Current Warm Period.
Sherwood, Keith and Craig Idso
Esper, J., Cook, E.R. and Schweingruber, F.H. 2002. Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability. Science 295: 2250-2253.
Kerr, R.A. 2008. Chinese cave speaks of a fickle sun bringing down ancient dynasties. Science 322: 837-838.
Mann, M.E. and Jones, P.D. 2003. Global surface temperatures over the past two millennia. Geophysical Research Letters 30: 10.1029/2003GL017814.
Moberg, A., Sonechkin, D.M., Holmgren, K., Datsenko, N.M. and Karlen, W. 2005. Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data. Nature 433: 613-617.
Zhang, P., Cheng, H., Edwards, R.L., Chen, F., Wang, Y., Yang, X., Liu, J., Tan, M., Wang, X., Liu, J., An, C., Dai, Z., Zhou, J., Zhang, D., Jia, J., Jin, L. and Johnson, K.R. 2008. A test of climate, sun, and culture relationships from an 1810-year Chinese cave record. Science 322: 940-942.