In their review of the relationship between solar activity and climate during the Holocene and portions of the prior ice age, Chambers et al. (1999) conclude there is "increasing evidence for solar-driven variations in earth-atmospheric processes, over a range of timescales." In this summary, we highlight some of the recent scientific literature in support of a solar-influence on climate at centennial time scales.
Perhaps the most significant study of recent times to link centennial-scale climate oscillations to variations in the energy output of the sun was the study of Bond et al. (2001). In an effort to determine what is responsible for the approximate 1300-year cycle of global climate change that has been intensely studied in the region of the North Atlantic Ocean and demonstrated to prevail throughout glacial and interglacial periods alike, Bond et al. examined the history 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). Based on arduous analyses of the deep-sea sediment cores that yielded the variable-with-depth amounts of three proven proxies for the prior presence of overlying drift-ice, the scientists were able to discern and, with the help of an accelerator mass spectrometer, date a number of recurring alternate periods of relative cold and warmth that wended their way throughout the entire 12,000-year expanse of the Holocene. The mean duration of the several complete climatic cycles thus delineated was 1340 years.
The signal accomplishment of the Bond et al. study was the linking of these millennial-scale climate oscillations -- and their imbedded centennial-scale oscillations -- with similar-scale oscillations in cosmogenic nuclide production that are known to be driven by contemporaneous oscillations in the energy output of the sun. In fact, they were able to report that "over the last 12,000 years virtually every centennial time-scale increase in drift ice documented in our North Atlantic records was tied to a solar minimum." They also 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 experienced throughout the world.
With an average periodicity of approximately 211 years (ranging from about 180 to 220 years), the Suess Cycle is one of the more publicized centennial-scale solar cycles thought to exert a significant influence on earth's climate. In an analysis of a sediment core from a closed-basin lake in the northern Great Plains of North America, Yu and Ito (1999) postulated the Suess Cycle to be a "major cause of century-scale drought frequency" in that region. Across the globe, Neff et al. (2001) provided additional evidence for a solar-induced influence on the hydrologic cycle. For the period 9,600-6,100 years before present, they investigated the relationship between a 14C tree-ring record and a proxy record of monsoon rainfall intensity inferred from calcite ð18O data obtained from a stalagmite in northern Oman. Their investigation revealed an "extremely strong" correlation between the two data sets; and spectral analyses revealed both of them to possess a statistically significant centennial periodicity of about 205 years.
Not far from Oman, Castagnoli et al. (2002) studied a 1400-year ð13C record derived from the remains of the foraminifera Globigerinoides rubber, which were extracted from a sediment core located in the Gallipoli terrace of the Gulf of Taranto (39°45'53"N, 17°53'33"E). Variations in the ð13C of the symbiontic foraminifera reflect the effects of productivity varying with the ambient light level; and because the ð13C time series can thus provide information on sea surface illumination at the time of planktonic foraminifera growth, it can be utilized as a proxy for solar radiation variability. Similar to the studies referenced above, Castagnoli et al. found an approximate 200-year cycle in the record. Furthermore, comparison of their data with historical aurora and sunspot time series revealed that the three records are "associable in phase" and "disclose a statistically significant imprint of the solar activity in a climate record."
Evidence of the Suess Cycle's solar influence on climate has also been found off the coast of Antarctica, where Domak et al. (2001) subjected ocean sediment cores to radiocarbon and spectral analyses to provide a high resolution proxy temperature history spanning the past 13,000 years. As in the studies listed above, a significant oscillation was identified in the record with a period of about 200 years. In addition, Domak et al. report two other significant centennial-scale periodicities of around 125 and 400 years, all of which periodicities, the authors suggest, may be driven by solar variability.
As noted in the study above, the approximate 200-year Suess Cycle is not the only centennial-scale solar cycle linked to climate change. Dean and Schwalb (2000), for example, report a recurring 400-year cycle of major drought in the Northern Great Plains of the United States during the past 2000 years, which they and Black et al. (1999) postulate to be of solar origin. Likewise, Dean et al. (2002) implicate the sun as a major factor responsible for producing the strong 400-year periodicity observed in a sediment core taken from Elk Lake, MN, covering the last 1500 years. In addition, still other solar-climate links have been discerned at centennial-scale periodicities on the order of 100 years (Castagnoli et al., 2002; Dean et al., 2002), 125 years (Yu and Ito, 1999; Domak et al., 2001; Neff et al., 2001), 150 years (Neff et al., 2001), and 800 years (Neff et al., 2001).
What is the significance of these observations? For one thing, they imply earth's climate system "is far more sensitive to small variations in solar activity than generally believed" (Van Geel et al., 1999). Furthermore, until the mechanisms by which these centennial-scale solar forcings of climate are identified, understood and incorporated in climate models, all output from such models should be considered suspect at best and unreliable at worst (Chambers et al., 1999).
Second, these several studies provide ample ammunition for defending the premise that the global warming of the past century or so may well have been nothing more than the solar-mediated recovery of the earth from the chilly conditions of the Little Ice Age. In addition, they suggest that any further warming of the planet that might occur will be but a continuation of the natural trend that seems destined to make the Modern Warm Period of comparable warmth to the prior Medieval Warm Period and even earlier Roman Warm Period.
In light of these observations, we tend to agree with the conclusions of Van Geel et al. and Perry and Hsu (2000), respectively, that "global temperature fluctuations during the last decades are partly or completely explained by small changes in solar radiation," and that "the modern temperature increase being caused solely by an increase in CO2 concentrations appears questionable."
References
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.
Black, D.E., Peterson, L.C., Overpeck, J.T., Kaplan, A., Evans, M.N. and Kashgarian, M. 1999. Eight centuries of North Atlantic Ocean atmosphere variability. Science 286: 1709-1713.
Castagnoli, G.C., Bonino, G., Taricco, C. and Bernasconi, S.M. 2002. Solar radiation variability in the last 1400 years recorded in the carbon isotope ratio of a Mediterranean sea core. Advances in Space Research 29: 1989-1994.
Chambers, F.M., Ogle, M.I. and Blackford, J.J. 1999. Palaeoenvironmental evidence for solar forcing of Holocene climate: linkages to solar science. Progress in Physical Geography 23: 181-204.
Dean, W.E. and Schwalb, A. 2000. Holocene environmental and climatic change in the Northern Great Plains as recorded in the geochemistry of sediments in Pickerel Lake, South Dakota. Quaternary International 67: 5-20.
Dean, W., Anderson, R., Bradbury, J.P. and Anderson, D. 2002. A 1500-year record of climatic and environmental change in Elk Lake, Minnesota I: Varve thickness and gray-scale density. Journal of Paleolimnology 27: 287-299.
Domack, E., Leventer, A., Dunbar, R., Taylor, F., Brachfeld, S., Sjunneskog, C. and ODP Leg 178 Scientific Party. 2001. Chronology of the Palmer Deep site, Antarctic Peninsula: A Holocene palaeoenvironmental reference for the circum-Antarctic. The Holocene 11: 1-9.
Neff, U., Burns, S.J., Mangini, A., Mudelsee, M., Fleitmann, D and Matter, A. 2001. Strong coherence between solar variability and the monsoon in Oman between 9 and 6 kyr ago. Nature 411: 290-293.
Perry, C.A. and Hsu, K.J. 2000. Geophysical, archaeological, and historical evidence support a solar-output model for climate change. Proceedings of the National Academy of Sciences USA 97: 12433-12438.
Van Geel, B., Raspopov, O.M., Renssen, H., van der Plicht, J., Dergachev, V.A. and Meijer, H.A.J. 1999. The role of solar forcing upon climate change. Quaternary Science Reviews 18: 331-338.
Yu, Z. and Ito, E. 1999. Possible solar forcing of century-scale drought frequency in the northern Great Plains. Geology 27: 263-266.