As science moves ever forward, ever more data are being uncovered that point to the primary role of the sun in orchestrating a wide range of climatic changes on earth. Just as small changes in solar radiation reception at the planet's surface are generally accepted to be the driving forces of the great glacial/interglacial cycles of the past two million years, so too are smaller millennial-scale changes in solar output being accepted as the causes of the smaller millennial-scale climatic changes that usher in the warm and cool climates typical of mini-epochs such as the Medieval Warm Period and Little Ice Age. Several examples of this work are described in what follows.
Karlen (1998) examined proxy climate data - including changes in the sizes of glaciers, changes in the altitude of the alpine treeline, and variations in tree-ring widths - related to changes in summer air temperatures in Scandinavia over the last 10,000 years, after which comparisons were made with solar irradiance data derived from 14C anomalies measured in tree-ring records. The author concluded from this wide array of evidence, as well as from data obtained from Greenland ice cores, that "the similarity between solar irradiation changes and climate indicate a solar influence on the Scandinavian and Greenland climates."
Perry and Hsu (2000) developed a simple solar-luminosity model by summing the amplitudes of solar radiation variances for fundamental harmonics of the eleven-year sunspot cycle and compared the results with geophysical, archaeological and historical evidence of climate variation during the Holocene. The model output was well correlated with the amount of 14C in well-dated tree rings going back to the time of the Medieval Warm Period, which finding, in the words of the authors, "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."
Perry and Hsu's results were also well correlated with the sea-level curve developed by Ters (1987); and present in all of these records over the entire expanse of the Holocene was a "little ice age"/"little warm period" cycle with a period of approximately 1300 years. In addition, these climatic mini-epochs were well correlated with major historical events. Specifically, the authors note that "great civilizations appear to have prospered when the solar-output model shows an increase in the sun's output," while they state that such civilizations "appear to have declined when the modeled solar output declined."
Chambers et al. (1999) also note that recent findings in both palaeoecology and solar science "indicate a greater role for solar forcing in Holocene climate change than has previously been recognized," and they identify a number of "multiplier effects" that can operate on solar rhythms in such a way that "minor variations in solar activity can be reflected in more significant variations within the earth's atmosphere." They also note that nonlinear responses to solar variability are inadequately represented in the global climate models used by the Intergovernmental Panel on Climate Change (IPCC) to predict future greenhouse gas-induced global warming, while at the same time other "amplifier effects" are used to model well-known glacial/interglacial cycles of temperature change, suggesting there may be a reluctance within the IPCC bureaucracy to deal even-handedly with different aspects of climate change. When multiplier effects suit their purposes - as when they enhance the warming tendency of an increase in atmospheric CO2 concentration, for example - they seem quite ready to use them; but when they don't suit their purposes - as when they amplify the climatic impact of a competing forcing factor, such as solar activity - they are not so ready to acknowledge them.
Van Geel et al. (1999) review what is known about the relationship between variations in the abundances of the cosmogenic isotopes 14C and 10Be - both of which are indicators of solar activity - and millennial-scale climate oscillations during the Holocene, finding that "there is mounting evidence suggesting that the variation in solar activity is a cause for millennial scale climate change." Indeed, they conclude 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."
Bond et al. (2001) also studied 14C and 10Be sequestered in the Greenland ice cap (10Be) and Northern Hemispheric tree rings (14C), as well as their relationship to ice-rafted debris found in three North Atlantic deep-sea sediment cores. Throughout the entire 12,000-year expanse of the Holocene, their ice-rafted debris data revealed the presence of a recurrent cycle of warm and cold periods, the latter of which, in the words of the scientists, were "broadly correlative with the so called 'Little Ice Age' and 'Medieval Warm Period'."
The signal accomplishment of the study of Bond et al. was the linking of these millennial-scale climate oscillations and their imbedded centennial-scale climate oscillations with similar-scale oscillations in cosmogenic nuclide production, which are known to be driven by oscillations in the energy output of the sun. In fact, they reported 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." In light of this observation, 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 experienced throughout the world.
Another important point made by Bond et al. is that the oscillations in drift-ice they studied "persist across the glacial termination and well into the last glaciation, suggesting that the cycle is a pervasive feature of the climate system," as would be expected for a repeatable solar-induced phenomenon. At two of their coring sites, in fact, they identified a series of such cyclical variations that extended throughout all of the previous interglacial and were "strikingly similar to those of the Holocene."
In concluding their landmark paper, Bond et al. reiterate that the results of their study "demonstrate that the earth's climate system is highly sensitive to extremely weak perturbations in the sun's energy output." They also state that their work "supports the presumption that solar variability will continue to influence climate in the future."
With the ongoing development of the Modern Warm Period, the future envisioned by Bond et al. appears already to be upon us. This opinion is shared by Karlen (1998), who concludes that "the frequency and magnitude of changes in climate during the Holocene do not support the opinion that the climatic change of the last 100 years is unique," stating bluntly that "there is no evidence of a human influence so far." Perry and Hsu (2000) join in this conclusion as well, stating that the idea of the modern temperature increase being caused solely by an increase in CO2 concentration "appears questionable."
We question it too.
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.
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.
Karlen, W. 1998. Climate variations and the enhanced greenhouse effect. Ambio 27: 270-274.
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: 12,433-12,438.
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.