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Solar Effects on Earth's Climate
Reference
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.

What was done
The authors review 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.

What was learned
Of greatest interest to us 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)."

What it means
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, as they continue, "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 increase in temperature. Nevertheless, much more work is needed to clarify the specific mechanisms by which the solar-induced warming is accomplished.

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.

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.

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.

Reviewed 21 February 2007