In a review of the relationship between variations in solar activity and millennial-scale climate oscillations during the Holocene and portions of the last great ice age, Van Geel et al. (1999) state there is "mounting evidence 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 the earth's orbit about the sun. In this summary, we highlight some of the recent scientific literature that supports Van Geel et al.'s statement.
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," Perry and Hsu (2000) developed a simple solar-luminosity model and used it to estimate total solar-output variations over the past 40,000 years, as well as 10,000 years into the future, after which they compared the model output with historical evidence of climate variation during the Holocene. The results of their analysis showed that 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 about A.D. 1100, 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." The model output was also well correlated with the sea-level curve developed by Ters (1987); and present in both of these records over the entire expanse of the Holocene was a millennial-scale climate oscillation with a period of approximately 1,300 years.
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. (2001) 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, however, 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." 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.
Other authors have also implicated the sun in forcing millennial-scale climate. Recent modeling work by Ganopolski and Rahmstorf (2001, 2002) and Alley and Rahmstorf (2002), for example, suggests that the North Atlantic branch of the global thermohaline circulation possesses two potential modes of operation during glacial times: a cold stable mode and a warm marginally unstable mode, the latter of which typically lasts for but a few hundred years. The cold stable mode is characterized by deep-water formation south of Iceland; while the warm unstable mode is characterized by deep-water formation in the Nordic Seas and shares many characteristics with the circulatory mode of the current interglacial, although it is not quite as strong.
All else being equal, the cold stable mode of the ocean's thermohaline circulation would be expected to persist throughout an entire glacial period. However, as Ganopolski, Rahmstorf and Alley (GRA) note, a weak real-world forcing with a periodicity on the order of 1500 years produces small cyclical variations in freshwater input to high northern latitudes at approximately the same periodicity; and these perturbations, when in the declining phase, often, but not always, initiate a transition to the warm unstable mode of thermohaline circulation, which includes a shift in the location of deep-water formation from south of Iceland to the Nordic Seas. This new mode of circulation (warm unstable, which is accompanied by rapidly warming air temperatures) then persists for a few hundred years before reverting back (because of its inherent instability) to the cold stable mode of circulation (and its accompanying colder air temperatures). In the end, Ganopolski and Rahmstorf (2001) state that the low-amplitude cycle in freshwater forcing responsible for the large-amplitude cyclical changes in glacial climate could be "ultimately due to solar variability," while Alley and Rahmstorf (2002) say that "a possible cause could be a weak periodic variation in the output of the sun."
Lastly, in an effort to determine whether climate-driven millennial-scale cycles are present in the terrestrial pollen record of North America, Viau et al. (2002) analyzed a set of 3,076 14C dates from the North American Pollen Database used to date sequences in more than 700 pollen diagrams across North America. Results of their analyses indicated there were nine millennial-scale oscillations during the past 14,000 years in which continent-wide synchronous vegetation changes with a periodicity of roughly 1650 years were recorded in the pollen records. In contemplating the reason for the transition the authors state that "although several mechanisms for such natural forcing have been advanced, recent evidence points to a potential solar forcing (Bond et al., 2001) associated with ocean-atmosphere feedbacks acting as global teleconnections agents."
What is the significance of knowing about these millennial-scale solar forcings of earth's climate? For starters, this important knowledge base implies that "the 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 millennial-scale solar forcings of climate are identified, understood and incorporated in climate models, all climate model output should be considered suspect at best and unreliable at worst.
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, 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."
So don't get caught up in all the hype about anthropogenic CO2 emissions being responsible for 20th century global warming. It's not CO2 that's been causing the earth to warm. It's the sun.
References
Alley, R.B.S. and Rahmstorf, S. 2002. Stochastic resonance in glacial climate. EOS, Transactions, American Geophysical Union 83: 129, 135.
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.
Ganopolski A. and Rahmstorf, S. 2001. Rapid changes of glacial climate simulated in a coupled climate model. Nature 409: 153-158.
Ganopolski, A. and Rahmstorf, S. 2002. Abrupt glacial climate changes due to stochastic resonance. Physical Review Letters 88: 038501.
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.
Viau, A.E., Gajewski, K., Fines, P., Atkinson, D.E. and Sawada, M.C. 2002. Widespread evidence of 1500 yr climate variability in North America during the past 14,000 yr. Geology 30: 455-458.