We begin with the review paper of Van Geel et al. (1999), who examined what is known about the relationship 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. As they describe it, "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 the Earth's orbit about the Sun. Continuing, they add that "accepting the idea of solar forcing of Holocene and Glacial climatic shifts has major implications for our view of present and future climate," for 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, of course, call into question the conventional wisdom of attributing the global warming of the past century or so to the ongoing rise in the air's CO2 content.

In a study published the following year, Tyson et al. (2000), obtained a quasi-decadal-resolution record of oxygen and carbon-stable isotope data from a well-dated stalagmite recovered from Cold Air Cave in the Makapansgat Valley, 30 km southwest of Pietersburg, South Africa, which they augmented with temperature data reconstructed from color variations in banded growth-layer laminations of the stalagmite that were derived from a relationship calibrated against actual air temperatures obtained from a surrounding 49-station climatological network over the period 1981-1995, which had a correlation of +0.78 that was significant at the 99% confidence level.

According to the authors, both the Little Ice Age (prevailing from about AD 1300 to 1800) and the Medieval Warm Period (prevailing from before AD 1000 to around 1300) were found to be distinctive features of the climate of the last millennium. Relative to the period 1961-1990, in fact, the Little Ice Age, which "was a widespread event in South Africa specifically and southern Africa generally," was characterized by a mean annual temperature depression of about 1°C at its coolest point. The Medieval Warm Period, on the other hand, was as much as 3-4°C warmer at its warmest point. The researchers also note that the coolest point of the Little Ice Age corresponded in time with the Maunder Minimum of sunspot activity and that the Medieval Warm Period corresponded with the Medieval Maximum in solar activity. The importance of these facts, of course, resides in the demonstration that the warming of the earth since the termination of the Little Ice Age is not at all unusual or different from other climate changes of the past millennium, when atmospheric CO2 concentrations were quite stable, much lower than at present, and obviously not responsible for the observed variations in climate, which suggests that the warming of the past century or so need not be due to the contemporaneous historical increase in the air's CO2 content.

In a study demonstrating a solar-climate link on shorter decadal to centennial time scales, Domack et al. (2001) examined ocean sediment cores obtained from the Palmer Deep on the inner continental shelf of the western Antarctic Peninsula (64° 51.71' S, 64° 12.47' W) to produce a high resolution proxy temperature history of that area spanning the past 13,000 years. Results indicated the presence of five prominent palaeoenvironmental intervals over the past 14,000 years: (1) a "Neoglacial" cool period beginning 3360 years ago and continuing to the present, (2) a mid-Holocene climatic optimum from 9070 to 3360 years ago, (3) a cool period beginning 11,460 years ago and ending at 9070 years ago, (4) a warm period from 13,180 to 11,460 years ago, and (5) cold glacial conditions prior to 13,180 years ago. Spectral analyses of the data revealed that, superimposed upon these broad climatic intervals, were decadal and centennial-scale temperature cycles. Throughout the current Neoglacial period, they report finding "very significant" (above the 99% confidence level) peaks, or oscillations, that occurred at intervals of 400, 190, 122, 85 and 70 years, which they suggest are perhaps driven by solar variability.

Moving upward to the warmer ocean waters off the Cook Islands, South Pacific Ocean, Dima et al. (2005) performed Singular Spectrum Analysis on a Rarotonga coral-based sea surface temperature (SST) reconstruction in an effort to determine the dominant periods of multi-decadal variability in the series over the period 1727-1996. Results of the analysis revealed two dominant multi-decadal cycles, with periods of about 25 and 80 years. These modes of variability were determined to be similar to multi-decadal modes found in the global SST field of Kaplan et al. (1998) for the period 1856-1996. The ~25-year cycle was found to be associated with the well-known Pacific Decadal Oscillation, whereas the ~80-year cycle was determined to be "almost identical" to a pattern of solar forcing found by Lohmann et al. (2004), which, according to Dima et al., "points to a possible solar origin" of this mode of SST variability.

We conclude this brief review with the study of Bard and Frank (2006), who reviewed what is known, and unknown, about solar variability and its effects on Earth's climate, focusing on the past few decades, the past few centuries, the entire Holocene, and orbital timescales. Of greatest interest to the present discussion are Bard and Frank's conclusions about sub-orbital time scales, i.e., the first three of their four major focal points. Within this context, as they say in the concluding section of their review, "it appears that solar fluctuations were involved in causing widespread but limited climatic changes, such as the Little Ice Age (AD 1500-1800) that followed the Medieval Warm Period (AD 900-1400)." Or as they say in the concluding sentence of their abstract, "the weight of evidence suggests that solar changes have contributed to small climate oscillations occurring on time scales of a few centuries, similar in type to the fluctuations classically described for the last millennium: The so-called Medieval Warm Period (AD 900-1400) followed on by the Little Ice Age (AD 1500-1800)."

In the words of Bard and Frank, "Bond et al. (1997, 2001) followed by Hu et al. (2003) proposed that variations of solar activity are responsible for quasi-periodic climatic and oceanographic fluctuations that follow cycles of about one to two millennia." As a result, they say that "the succession from the Medieval Warm Period to the Little Ice Age would thus represent the last [such] cycle," leading to the conclusion that "our present climate is in an ascending phase on its way to attaining a new warm optimum," due to some form of solar variability. In addition, they note that "a recent modeling study suggests that an apparent 1500-year cycle could arise from the superimposed influence of the 90 and 210 year solar cycles on the climate system, which is characterized by both nonlinear dynamics and long time scale memory effects (Braun et al. 2005)."

Taken together, these several observations leave little need to invoke the historical increase in anthropogenic CO2 emissions as the primary cause of modern warming. In fact, they leave no such need at all, as solar influences appear to be sufficient to explain the bulk of the 20th-century increase in temperature.

References
Bard, E. and Frank, M. 2006. Climate change and solar variability: What's new under the sun? Earth and Planetary Science Letters 248: 1-14.

Bond, R., Hajdas, I. and Bonani, G. 2001. Persistent solar influence on North Atlantic climate during the Holocene. Science 294: 2130-2136.

Bond, G., Showers, W., Cheseby, M., Lotti, R., Almasi, P., deMenocal, P., Priore, P., Cullen, H., Hajdas, I. and Bonani, G. 1997. A pervasive millennial-scale cycle in North Atlantic Holocene and Glacial climate. Science 278: 1257-1266.

Braun, H., Christl, M., Rahmstorf, S., Ganopolski, A., Mangini, A., Kubatzki, C., Roth, K. and Kromer, B. 2005. Possible solar origin of the 1470-year glacial climate cycle demonstrated in a coupled model. Nature 438: 208-211.

Dima, M., Felis, T., Lohmann, G. and Rimbu, N. 2005. Distinct modes of bidecadal and multidecadal variability in a climate reconstruction of the last centuries from a South Pacific coral. Climate Dynamics 25: 329-336.

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.

Hu, F.S., Kaufman, D., Yoneji, S., Nelson, D., Shemesh, A., Huang, Y., Tian, J., Bond, G., Clegg, B. and Brown, T. 2003. Cyclic variation and solar forcing of Holocene climate in the Alaskan subarctic. Science 301: 1890-1893.

Kaplan, A., Cane, M.A., Kushnir, Y., Clement, A.C., Blumenthal, M.B. and Rajagopalan, B. 1998. Analyses of global sea surface temperature 1856-1991. Journal of Geophysical Research 103: 18,567-18,589.

Lohmann, G., Rimbu, N. and Dima, M. 2004. Climate signature of solar irradiance variations: analysis of long-term instrumental, historical, and proxy data. International Journal of Climatology 24: 1045-1056.

Tyson, P.D., Karlen, W., Holmgren, K. and Heiss, G.A. 2000. The Little Ice Age and medieval warming in South Africa. South African Journal of Science 96: 121-126.

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.