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The Case for a Global Medieval Warm Period Grows Ever Stronger
Hong, B., Liu, C.-Q., Lin, Q.-H., Yasuyuki, S., Leng, X.-T., Wang, Y., Zhu, Y.-X. and Hong, Y.-T. 2009. Temperature evolution from the δ18O record of Hani peat, Northeast China, in the last 14000 years. Science in China Series D: Earth Sciences 52: 952-964.

Of the Medieval Warm Period (MWP), the authors write that "because it is a distinct warm period nearest to the modern warming period and happened before the Industrial Revolution, it naturally becomes a [source of] comparison with modern warming." And in this regard, they add that "a universal concern in academic circles is [1] whether it also existed outside the European region and [2] whether it is a common phenomenon."

What was done
In a study designed to broach both questions, the authors extracted cores of peat from a location close to Hani Village, Liuhe County, Jilin Province, China (42°13'N, 126°31'E); and they used them to develop, as they describe it, "a peat cellulose δ18O temperature proxy record proximately existing for 14,000 years."

What was learned
Their efforts revealed, first of all, that the MWP had indeed held sway on the Chinese mainland over the period AD 700-1400, peaking at about AD 900. And the eight researchers report that phenological data from east China (Ge et al., 2006) and tree-ring records from west China (Yang et al., 2000) also indicate that "the temperature on the Chinese mainland was distinctly warmer during the MWP." In fact, they say MWP temperatures were as much as "0.9-1.0°C higher than modern temperatures (Zhang, 1994)."

Secondly, with respect to the last 14,000 years, Hong et al. write that "sudden cooling events, such as the Older Dryas, Inter-Allerod, Younger Dryas, and nine ice-rafted debris events of the North Atlantic" -- which are described by Stuiver et al. (1995) and Bond et al. (1997, 2001) -- "are almost entirely reiterated in the temperature signals of Hani peat cellulose δ18O." And they state that "these cooling events show that the repeatedly occurring temperature cooling [and warming] pattern not only appeared in the North Atlantic Region in the high latitudes, but also in the Northwest Pacific Region in the middle latitudes," indicating that the recurring warming and cooling did indeed occur "outside the European region" and that it truly was "a common phenomenon."

Last of all, we note that the earlier paper of Hong et al. (2000) -- which describes a 6,000-year peat cellulose δ18O record derived from nearby Jinchuan Town, Huinan County, Jilin Province, China (42°20'N, 126°22'E) -- identified δ18O periodicities of 86, 93, 101, 110, 127, 132, 140, 155, 207, 245, 311, 590, 820 and 1046 years, which they say "are similar to those detected in solar excursions," and which they consider "further evidence for a close relationship between solar activity and climate variations on timescales of decades to centuries." These findings were highly praised by Fairbridge (2001), who noted that "almost identical equivalents are seen in solar emission periodicities and their harmonics, e.g., 86.884 years = 40 x 2.172 year Quasi Biennial Oscillation (QBO) as well as in the lunar tidal/apsides beat frequency (17.3769 years) which also matches closely with most of the longer spectral peaks, e.g., 140 (139) years, 207 (208.5), 311 (312.8), 590 (590.8) and 1046 (1042.6) years." And for these spectacular spectral findings, Fairbridge wrote that "Hong et al. deserve the appreciation of the entire Holocene community."

What it means
As ever more pertinent work is conducted, the case for a global and solar-induced Medieval Warm Period grows ever stronger, as it also does for all of the similar warm periods that preceded it over the prior 13,000 years, which makes the case for a similar origin for the Current Warm Period ever more likely as well.

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.

Bond, G., Showers, W., Cheseby, M., Lotti, R., Almasi, P., deMenocal, P., Priore, P., Cullen, H., Hajdas, I. and Bonani, G. 1997. A pervasive millennial-scale cycle in North Atlantic Holocene and Glacial climate. Science 278: 1257-1266.

Fairbridge, R.W. 2001. Six millennia in Chinese peats, relating to planetary-solar-luniterrestrial periodicities: a comment on Hong, Jiang, Liu, Zhou, Beer, Li, Leng, Hong and Qin. The Holocene 11: 121-122.

Ge, Q.S., Zheng, J.Y. and Liu, J. 2006. Amplitude and rhythm of winter half-year temperature change in eastern China for the past 2000 years. Advances in Climate Change Research 2: 108-112.

Hong, Y.T., Jiang, H.B., Liu, T.S., Zhou, L.P., Beer, J., Li, H.E., Leng, X.T., Hong, B. and Qin, X.G. 2000. Response of climate to solar forcing recorded in a 6000-year δ18O time-series of Chinese peat cellulose. The Holocene 10: 1-7.

Stuiver, M., Grootes, P.M. and Braziunas, T.F. 1995. The GISP2 δ18O climate record of the past 16,500 years and the role of the sun, ocean, and volcanoes. Quaternary Research 44: 341-354.

Yang, B., Kang, X.C. and Shi, Y.F. 2000. Decadal climatic variations indicated by Dulan tree-ring and comparison with other proxy data in China of the last 2000 years. Chinese Geographical Science 10: 193-201.

Zhang, D.E. 1994. Evidence for the existence of the Medieval Warm Period in China. Climatic Change 26: 293-297.

Reviewed 15 September 2010