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Until the advent of computers, tides were predicted by complex “brass brain” machines. This periodicity is interesting in that it could be related to the pentadecadal variability described in sea level pressure and temperatures in the North Pacific (Minobe 2000). Of the spectral predictions, the one published (Clilverd et al., 2006) used a low-frequency modulation model that has some clear inadequacies, like including the Gleissberg 88-year cycle that is no longer observable, assigning an extremely low amplitude to the 208-year de Vries cycle, and not including the modulation by the ~ 2400-year Bray cycle that we have discussed previously.

The Eddy lows that correspond to this periodicity (orange bars) are numbered from most recent. Notice that the Bray periodicity is continuous over the entire Holocene, while the Eddy periodicity is very strong in the early Holocene and very weak in the mid-to-late Holocene. The Eddy lows that correspond to this periodicity (orange bars) are numbered from most recent. Yet the biggest group of researchers just call any periodicity between 50 and 150 years the Gleissberg cycle, often giving the name simultaneously to two different bands. Of interest to us here is only the ~ 88-year periodicity present in cosmogenic records that we can also call the Gleissberg cycle, if only to avoid further confusion.

Grand solar minima that correspond to these lows are indicated with boxes with their names. Joan Feynman, sister of the famous physicist, has studied the centennial solar cycle under the Gleissberg flag of convenience (Feynman & Ruzmaikin, 2014). The ~ 88-year Gleissberg cycle during the Holocene. The problem is that wavelet analysis shows that this periodicity was only apparent between 6,500 and 3,500 BP (figure 86).

These last two might be simply harmonics of the de Vries cycle, but as they are currently observable, they may be useful to interpret the past, as well as project future solar activity.

The ~ 1000-year Eddy solar cycle seems to have dominated Holocene climate variability between 11,500-4,000 years BP, and in the last two millennia, where it defines the Roman, Medieval, and Modern warm periods. (2010), and its lows have been numbered here, from more recent, as E1, E2, … The climatic effect of the Eddy cycle should manifest in the two periods when solar activity was most affected by this millennial periodicity. b) Sunspot based solar activity reconstruction from the radiocarbon record showing the disposition of the GSM associated with the Bray (blue) and Eddy (orange) cycle lows. Outside these windows centered in the Bray cycle lows, the de Vries periodicity has very low power in wavelet analysis indicating it has little effect on solar activity (figure 58). The result continued being significant after the removal of the volcanic signal, and was most prominent in records from Asia and Europe (figure 85; Breitenmoser et al., 2012).

Or are some of them simply artifacts and not solar variability cycles?

Instead of assuming every peak in a frequency analysis constitutes sufficient evidence for the existence of a cycle, I only consider those where abundant evidence exists in the scientific literature that solar cycles match the climate evidence precisely. Of interest are also the periodicities recognizable in the sunspot record, the Schwabe (11-year), Pentadecadal, and Centennial (Feynman) cycles.

In 1862 Rudolf Wolf, after completing the first continuous record of sunspot numbers, “concluded from the sunspot observations available at that time that high and low maxima did not follow one another at random: a succession of two or three strong maxima seemed to alternate with a succession of two or three weak maxima”. Despite this precedent, most solar physicists were expecting SC24 to have a slightly lower level of activity than SC23 and were surprised by the depth and duration of the 2008 minimum and the subsequent low activity of SC24. Despite a low bias, the model predicted the current centennial minimum for cycles 24 and 25. Importantly, the model also predicted in 2006 that SC25 will again be a below average cycle of similar amplitude to SC24.