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Climate Oscillations (Millennial Variability: Forcing Factors) -- Summary
What is the source of the pervasive millennial-scale oscillation of climate that has brought the earth, in receding order, the Modern Warm Period, Little Ice Age, Medieval Warm Period, Dark Ages Cold Period, Roman Warm Period and so on, backwards in time ad infinitum?  Most analyses of this question suggest a millennial-scale variability of some aspect of solar activity; but there are other contenders as well.

Van Geel et al. (1999) review what is known about relationships between variations in the abundances of the cosmogenic isotopes 14C and 10Be and millennial-scale climate oscillations during the Holocene and portions of the last great ice age.  In doing so, they demonstrate "there is mounting evidence suggesting that the variation in solar activity is a cause for millennial scale climate change," which is known to operate independently of the glacial-interglacial cycles that are forced by variations in earth's orbit about the sun.  They also review the evidence for two potential mechanisms by which the postulated solar-climate connection may be driven, making a strong case for the validity of one or both of them.

In discussing the significance of their work, van Geel et al. say that "accepting the idea of solar forcing of Holocene and Glacial climatic shifts has major implications for our view of present and future climate."  It implies, as they note, that "the climate system is far more sensitive to small variations in solar activity than generally believed" and that "it could mean that the global temperature fluctuations during the last decades are partly, or completely explained by small changes in solar radiation."  These observations clearly call into question the climate-alarmist ruse of attributing the global warming of the past century or so to the ongoing rise in the air's CO2 content.  It is much more likely that we have the sun to thank for rescuing us from the cold and inclement weather conditions of the recently-departed Little Ice Age.

Perry and Hsu (2000) also opt for a solar-climate connection as the source of the well-documented millennial-scale oscillation of climate.  Noting that "the most direct mechanism for climate change would be a decrease or increase in the total amount of radiant energy reaching the earth," they developed a simple solar-luminosity model and used it to estimate total solar-output variations over the past 40,000 years and 10,000 years into the future.  This model was derived by summing the amplitude of solar radiation variance for fundamental harmonics of the eleven-year sunspot cycle throughout an entire 90,000-year glacial cycle.  The results of this exercise were then compared with geophysical, archaeological and historical evidence of climate variation during the Holocene.

The model output was well correlated with the amount of carbon 14 (which is produced in the atmosphere by cosmic rays that are less abundant when the sun is active and more abundant when it is less active) in well-dated tree rings gong back to the time of the Medieval Warm Period (about A.D. 1100), which finding, in the words of Perry and Hsu, "supports the hypothesis that the sun is varying its energy production in a manner that is consistent with the superposition of harmonic cycles of solar activity."  The model output was also well correlated with the sea-level curve developed by Ters (1987).  Present in both of these records over the entire expanse of the Holocene was a "little ice age"/"little warm period" cycle with a period of approximately 1,300 years.  In addition, the climate changes implied by these records were well correlated with major historical events, such that Perry and Hsu say that "great civilizations appear to have prospered when the solar-output model shows an increase in the sun's output," while they "appear to have declined when the modeled solar output declined."

Continuing, Perry and Hsu say that "current global warming commonly is attributed to increased CO2 concentrations in the atmosphere."  However, they note that "geophysical, archaeological, and historical evidence is consistent with warming and cooling periods during the Holocene as indicated by the solar-output model."  They therefore conclude that the idea of "the modern temperature increase being caused solely by an increase in CO2 concentrations appears questionable."  Their findings also clearly suggest that, as far as humanity is concerned, warmer is generally always better than colder.

Providing a different perspective on the issue, Keeling and Whorf (2002) developed the hypothesis that abrupt millennial-scale climate changes are produced in part by periodic variations in the strength of the global oceanic tide-raising forces caused by resonances in the periodic motions of the earth and moon.  Such variations, according to them, cause "periodic cooling of surface ocean water by modulating the intensity of vertical mixing that brings to the surface colder water from below."  To test this idea, they compared calculated tidal cycles with proxy climate records over the past 34,000 years and found their hypothesis to be supported, as cooler periods of climate were found to correspond to periods of greatest tidal forces.  Of particular interest, in this regard, is the fact that the most recent maximum of the prominent 1,800-year tidal cycle coincided with the cold temperatures of the Little Ice Age in the 16th through 19th centuries.

If the tidal cycle hypothesis is correct, temperatures will continue to warm for several more centuries, greatly compromising our ability to determine any time soon whether the warming of the recent past and possible near future may be attributed to solar forcing, tidal forcing, anthropogenic CO2 forcing, or some combination of some or all of these factors, or something different still.  Hence, it is clearly premature to conclude, as the IPCC has concluded, that the balance of evidence suggests a discernable human influence on earth's climate that is primarily attributable to the historical rise in the air's CO2 content.

Keeling, C.D. and Whorf, T.P.  2000.  The 1,800-year oceanic tidal cycle: A possible cause of rapid climate change.  Proceedings of the National Academy of Sciences, USA 97: 3814-3819.

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.

Ters, M.  1987.  Variations in Holocene sea level on the French Atlantic coast and their climatic significance.  In: Rampino, M.R., Sanders, J.E., Newman, W.S. and Konigsson, L.K. (Eds.)  Climate: History, Periodicity, and Predictability.  Van Nostrand Reinhold, New York, NY, pp. 204-236.

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.