We find evidence for multi-centennial climate cycles within the ages commonly described as the Medieval Warming Period and the Little Ice Age. We compare six proxy temperature records and find evidence for synchronicity of ~200-year cycles in the northern hemisphere. The first two data sets are (1) a reconstruction from an ice core in the Colle Gnifetti (CG) glacier on the Swiss-Italian border (Bohleber et al 2018), and (2) length records of the Great Aletsch glacier (GAG), Switzerland (Holzhauser, 2009). A third is a proxy (3) by Cabedo-Sanz et al (2016) using the biomarker IP25 in sea sediments north of Iceland, which serves as a proxy for drift ice and hence arguably for Arctic ice areal coverage. Further temperature proxies for (4) all-China (Ge et al, 2017), (5) north China and (6) central China (Wang et al, 2018) provide a distribution of coverage over the non-tropical northern hemisphere. These six regional temperature proxy data sets are also compared with the G7 global temperature reconstruction (Ludecke and Weiss, 2017). Prominent minima in temperature proxy data occur circa 1350CE (all 6 proxies), 1480-1520CE (4 proxies), 1650-1700CE (all 6 proxies) and 1800-1860CE (all 6 proxies). The last three of these are also visible in the G7 global temperature reconstruction. Over the period 1550-1949 CE the Pearson correlation coefficients for the IP25 (Arctic ice) data with the GAG and the all-China temperature proxy are 0.69 and 0.70 respectively; these high correlations in data sets from opposite sides of the globe suggest a global cause rather than regional internal variability. Since 1600CE we note that IP25 (Arctic ice) and the CG temperature proxy lag the glacier record by 17 and 10 years respectively; that lag is counterintuitive and may reflect precipitation variations in the Alps preceding the temperature drop, or it may be attributable to uncertainties in age dating between the data sets. Power spectral analysis shows the dominant centennial periods in the data sets are centered at 180, 240 and 500 which (within dating uncertainties) may relate to the ~160 year Jose cycle, the 208-year de Vries cycle and possible 350 and 500 year cycles previously recognized in solar activity via study of terrestrial cosmogenic isotopes. The consistency between the spectral maxima of temperature proxies studied here and spectral maxima of cosmogenic isotopes, supports the hypothesis of some association of these cycles with an “astronomical clock”, although the mechanism of possible forcing remains a subject for further study.

Two sites in adjacent catchments located in eastern NSW provide hydrological data over 200 years since European settlement: (a) height of the Hawkesbury River at Windsor, within the Sydney Basin (HR); (b) level of the ephemeral Lake George, sited 100 km inland (LG). HR has experienced 43 moderate to major floods since 1799 with the timing of floods grouping into approximate 40-year segments of greater or lesser flood frequency. LG has a reconstructed history of annual levels (Short et al, 2020) which shows obvious spacings with range 50 to 80 years. Three features are clear. The close correlation in time between HR floods, and the deep LG records, in separate hydrological catchments, suggests that these were not random occurrences. The sunspot record shows clear correlation in timing of occurrence (but not of amplitudes) of anomalously weak sunspot maxima with high rainfall/flood-prone segments. High sunspot maxima are associated with dryer 40 yr segments. Both datasets yield meaningful spectra via Lomb-Scargle spectral analysis (the data lengths being too short for reliable Fourier spectra). These power spectra show maxima at periods 82-88 yr (HR) and 80 yr (LG). Subsidiary peaks at periods 50, 30, 20and 11 yr appear on both. These peaks align with the first three of the six named periodicities in solar activity; the Schwabe(11yr); Hale (22yr) and the Gleissberg (87 yr) periodicities. These three periodicities are present in the sunspot record from1609 CE, and in the cosmogenic (Be and C ) record of the past 50+ kyr where high sunspot activity correlates with low cosmic radiation and high total solar insolation (TSI). In particular HR for 1810-20 CE and LG for 1819-20 CE coincide with the last portion of the sunspot Dalton Minimum, the last of the “Little Ice Ages” experienced worldwide. The timing of HR floods correlates closely with La Nina events, and a subset correlates with solar cycle terminators described by Leamon et al (2021). We believe it is significant that a subset of terminators associated with dryer segments also approximate a pattern consistent with the 87 yr Gleissberg cycle. We conclude that just as solar cycle terminators appear to have predictive value for La Nina events, recognition of the Gleissberg cycle may have predictive value for ~80 yr cycles of flood-prone and drought-prone times.