Many scientific papers have presented evidence of centennial-scale fluctuations in proxy climate data. In the central United States, for example, Woodhouse and Overpeck (1998) describe many such studies that deal with drought within this context, while in an original research paper, Willard et al. (2003) report that several dry periods ranging in length from decades to centuries are evident in proxy climate data derived from sediment cores retrieved from Chesapeake Bay on the eastern seaboard of the United States.
In Canada, similar discoveries have been made. Campbell (2002), for example, detected decadal-, centennial- and millennial-scale oscillations of relative moisture availability in the sediments of Pine Lake in Alberta, while Laird et al. (2003) report that shifts in drought conditions on decadal through multi-centennial scales have prevailed for at least the last two millennia on the U.S-Canadian border within the northern prairies of North America.
Working in northeast Nigeria, Holmes et al. (1997) determined that drought there has occurred on an interdecadal to centennial timescale over the last 1500 years, while in analyzing data from northern Lake Malawi, Africa (near 10°S, 34°E), Johnson et al. (2001) found evidence for century-scale oscillations of temperature over the past 700 years. In addition, Grud et al. (2002) found century-timescale climatic variations over the last two millennia in Swedish Lapland.
Many studies have also attempted to discover the cause(s) of these thermal and/or hydrological oscillations. Working off the west coast of the United States in sediments of the Santa Barbara Basin, for example, Schimmelmann et al. (2003) discovered evidence of flood events with a quasi-periodicity of approximately 200 years that matched "the ~200-year periodicities found in a variety of high-resolution palaeoclimate archives and, more importantly, a c.208-year cycle of solar activity and inferred changes in atmospheric circulation." Based on these observations, they hypothesized that "solar-modulated climatic background conditions are opening a ~40-year window of opportunity for flooding every ~200 years."
These findings buttress those of Loehle (2004), who found of a 228-year cycle in the multi-millennial temperature records of Keigwin (1996) and Holmgren et al. (1999, 2001) that he says "approximates the 210-year cycle found by Damon and Jirikowic (1992)" in their study of solar forcing of global climate change. Likewise, working with a sediment core they obtained from the Mediterranean Sea, Castagnoli et al. (2002) identified cyclical components of the proxy climate record with periods of approximately 11, 100 and 200 years. Comparing the raw ð13C and component data with aurorae and sunspot time series, respectively, they found that the records were "associable in phase" and "disclose a statistically significant imprint of the solar activity in a climate record."
One of the most impressive studies of this subject to be conducted in recent years was that of Bond et al. (2001), who set out to determine the cause of the approximate 1500-year millennial-scale oscillation of climate that has been detected in regions surrounding the North Atlantic Ocean and has been demonstrated to prevail throughout glacial and interglacial periods alike (Oppo et al., 1998; Raymo et al., 1998). This they did via an analysis of ice-rafted debris found in three North Atlantic deep-sea sediment cores and cosmogenic nuclides sequestered in the Greenland ice cap (10Be) and Northern Hemispheric tree rings (14C), which analysis revealed variable solar activity to be the factor responsible for the millennial-scale climatic oscillation. In addition, they state that "over the last 12,000 years virtually every centennial timescale increase in drift ice documented in our North Atlantic records was tied to a solar minimum." In consequence of these observations, they concluded that "a solar influence on climate of the magnitude and consistency implied by our evidence could not have been confined to the North Atlantic," suggesting that the cyclical climatic effects of the variable solar inferno are likely experienced throughout the entire world; and they cited a wealth of independent evidence to support this conclusion.
In a subsequent related study that also spanned the past 12,000 years, Hong et al. (2003) developed a high-resolution proxy record of the Indian Ocean summer monsoon based on δ13C time series of plant cellulose in the Hongyuan peat bog of the Tibetan Plateau, finding that "on centennial to millennial time scales there is a close teleconnection between the Indian Ocean summer monsoon variations and the abrupt climate change events characterized by the ice-rafted debris events in the North Atlantic over the last 12,000 years." In fact, they report that "corresponding to each of the eight ice-rafted debris events in the North Atlantic the monsoon strength decreased clearly, which shows that the close correlation between the Indian Ocean summer monsoon and the North Atlantic climate is present not only in the last glacial, but also in the Holocene." In addition, they discuss evidence which suggests that "the ocean thermohaline circulation may play a linking role for the teleconnective variations between the Indian Ocean summer monsoon and the North Atlantic climate," echoing Bond et al. who had suggested that solar signals may be "transmitted through the deep ocean as well as through the atmosphere, further contributing to their amplification and global imprint."
Many other scientists have also produced evidence of centennial-scale oscillations of climate that likely have a solar origin. In a high-resolution study of sediments in the southern Caribbean that were deposited over the past 825 years, Black et al. (1999) uncovered evidence of substantial variability of both a decadal and centennial nature that is indicative of natural climate regime shifts. Comparing these features to still other records of climate variability, they concluded that "these shifts may play a role in triggering changes in the frequency and persistence of drought over North America." They also found a strong correspondence between changes in North Atlantic climate and changes in 14C production rate (a measure of solar activity), which led them to conclude that "small changes in solar output may influence Atlantic variability on centennial time scales."
Yu and Ito (1999) came to a similar conclusion about solar activity and drought in a study of sediments extracted from a closed-basin lake in the northern Great Plains of North America that produced a 2100-year record of drought. Spectral analysis of the data revealed four main periodicities of 400, 200, 130 and 100 years. Comparing these results with various solar indices, they found they matched "in surprising detail," leading them "to consider solar variability as the major cause of century-scale drought frequency in the northern Great Plains."
More compelling evidence for solar-induced centennial-scale climatic variability was provided by Poore et al. (2003), who developed a 14,000-year record of Holocene climate based primarily on the relative abundance of the planktic foraminifer Globigerinoides sacculifer found in two sediment cores extracted from the Gulf of Mexico (GOM). In reference to the North Atlantic millennial-scale cool events 1-7 identified by Bond et al. as belonging to a pervasive climatic oscillation with a period of approximately 1500 years, they say of their own study that distinct excursions to lower abundances of G. sacculifer "match within 200 years the ages of Bond events 1-6," noting that "major cooling events detected in the subpolar North Atlantic can be recognized in the GOM record." In addition, they report that "the GOM record includes more cycles than can be explained by a quasiperiodic 1500-year cycle," and these centennial-scale cycles with periods ranging from 200 to 500 years are also observed in the study of Bond et al.
Poore et al. additionaly remark that their results "are in agreement with a number of studies indicating the presence of substantial century-scale variability in Holocene climate records from different areas," specifically citing the reports of Campbell et al. (1998), Peterson et al. (1991) and Hodell et al. (2001), whose analysis of sediment cores taken from Lake Chichancanab on the Yucatan Peninsula of Mexico, reconstructing the climatic history of that region over the past 2600 years, revealed a recurrent drought periodicity of 208 years that matched well with a cosmic-ray-produced 14C record preserved in tree rings that is believed to reflect variations in solar activity. Because of the good correspondence between the two data sets, Poore et al. concluded that "a significant component of century-scale variability in Yucatan droughts is explained by solar forcing." Last of all, they discuss how this and other evidence leads them to conclude that "some of the high-frequency variation (century scale) in G. sacculifer abundance in our GOM records is forced by solar variability."
Another study indicative of the linkages that exist among these various phenomena is that of Tan et al. (2004), who derived an annual layer thickness chronology (LTC) for a stalagmite found in Beijing Shihua Cave and reconstructed a 2650-year warm season temperature record (WTR) for Beijing by calibrating the LTC with the observed WTR of Tan et al. (2003). They report that the resulting warm season temperature reconstruction "is consistent with oscillations in total solar irradiance inferred from cosmogenic 10Be and 14C," and that it "is remarkably consistent with Northern Atlantic drift ice cycles that were identified [by Bond et al.] to be controlled by the sun through the entire Holocene." As a result, they conclude that "the synchronism between the two independent sun-linked climate records therefore suggests that the sun may directly couple hemispherical climate changes on centennial to millennial scales."
In light of these many observations, it would appear that centennial-scale climatic oscillations (together with millennial-scale oscillations) are common to nearly all parts of the world; and it would appear that wherever a cause for the oscillations is suggested, solar variability seems to top the list of possibilities. In fact, it is often the only cause suggested.
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