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Decadal to Millennial Climate Cyclicity: A Response to Solar Activity or Tidal Action?
Berger, W.H. and von Rad, U. 2002. Decadal to millennial cyclicity in varves and turbidites from the Arabian Sea: hypothesis of tidal origin. Global and Planetary Change 34: 313-325.

What was done
The authors analyzed two cores of varved sediments retrieved from the upper continental slope of the Arabian Sea west of Karachi, Pakistan (2450' N, 6555' E) that covered close to the past 5000 years, searching for "cyclicity in the series of varve thickness ('varve' cycles), of unusually large excursions in varve thickness ('agitation' cycles), and of abundance of turbidites ('turbidite' cycles)."

What was learned
It was determined there were a number of high-frequency cycles with periods ranging from 10 to 100 years, as well as a number of low-frequency cycles with periods ranging from 100 to 500 years. In addition, the authors report there was evidence of "the 1470-year cycle previously reported [Grootes and Stuiver (1997), Schulz et al. (1999)] from the glacial-age Greenland ice record," which has been found to reverberate throughout the entire Holocene as well (Bond et al., 1997, 2001), producing such alternating warm and cool intervals as the Medieval Warm Period and Little Ice Age. Because a large proportion of the cycles were composed of multiples of basic tidal cycles, the authors ascribed a large proportion of the cyclicity they discovered to tidal action.

What it means
Speaking of "the presence of strong cycles in finely laminated sediments (or ice, or tree-rings)," the authors cite Wunsch (2000) as authority for their statement that such sharply defined cycles "imply sharply defined astronomic forcing," noting further that "internal oscillations of the climate system cannot produce them." Hence, although they say this fact "will make it difficult to sort solar influence from tidal forcing," it provides all the more reason to accept the development of the Modern Warm Period for what it really is: the logical next phase of the externally-forced millennial-scale oscillation of climate that has alternately given us the Roman Warm Period, the Dark Ages Cold Period, the Medieval Warm Period, the Little Ice Age and, most recently, the Modern Warm Period, all without any help or interference from the sometimes constant, sometimes changing atmospheric CO2 content.

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.

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.

Grootes, P.M. and Stuiver, M. 1997. Oxygen 18/16 variability in Greenland snow and ice with 10-3- to 105-year time resolution. Journal of Geophysical Research 102: 26,455-26,470.

Schulz, M., Berger, W.H., Sarnthein, M. and Grootes, P.M. 1999. Amplitude variations of 1470-year climate oscillations during the last 100,000 years linked to fluctuations of continental ice mass. Geophysical Research Letters 26: 3385-3388.

Wunsch, C. 2000. On sharp spectral lines in the climate record and the millennial peak. Paleoceanography 15: 417-424.

Reviewed 19 March 2003