The two researchers began by noting that in nearly all attribution studies, where attempts have been made to determine what was responsible for orchestrating the course of earth's past temperature history, the approach used by the Intergovernmental Panel on Climate Change and most climate modelers has been to use pre-determined forcing and feedback mechanisms in the models they employ. "One difficulty with this approach," according to Scafetta and West, "is that the feedback mechanisms and alternative solar effects on climate, since they are only partially known, might be poorly or not modeled at all." Consequently, "to circumvent the lack of knowledge in climate physics," as they describe it, they adopted "an alternative approach that attempts to evaluate the total direct plus indirect effect of solar changes on climate by comparing patterns [our italics] in the secular temperature and TSI reconstructions," where "a TSI reconstruction is not used as a radiative forcing, but as a proxy [for] the entire solar dynamics," parts of which, we again emphasize, may be only partially understood or even totally unknown.

Based on this underlying philosophy, Scafetta and West proceeded on the assumption "that the secular climate sensitivity to solar change can be phenomenologically estimated by comparing ... solar and temperature records during the pre-industrial era, when, reasonably, only a negligible amount of anthropogenic-added climate forcing was present," and when "the sun was the only realistic force affecting climate on a secular scale."

In pursuing this course of action, the two scientists found what they call a "good correspondence between global temperature and solar induced temperature curves during the pre-industrial period, such as the cooling periods occurring during the Maunder Minimum (1645-1715) and the Dalton Minimum (1795-1825)." In addition, they note that since the time of the 17th century solar minimum, "the sun has induced a warming of ΔT ~ 0.7 K," and that "this warming is of the same magnitude [as] the cooling of ΔT ~ 0.7 K from the medieval maximum to the 17th century minimum," which finding, in their words, "suggests the presence of a millenarian solar cycle, with ... medieval and contemporary maxima, driving the climate of the last millennium," as was first suggested fully three decades ago by Eddy (1976) in his seminal study of the Maunder Minimum.

In discussing their findings, Scafetta and West assert that their work provides substantive evidence for the likelihood that "solar change effects are greater than what can be explained by several climate models," citing, in this regard, the models of Stevens and North (1996), the Intergovernmental Panel on Climate Change (2001), Hansen et al. (2002) and Foukal et al. (2004); and in further explaining how this may be, they note that a solar change "might trigger several climate feedbacks and alter the greenhouse gas (H2O, CO2, CH4, etc.) concentrations, as 420,000 years of Antarctic ice core data would also suggest (Petit et al., 1999)," once again reiterating that "most of the sun-climate coupling mechanisms are probably still unknown," and that "they might strongly amplify the effects of small solar activity increase," as Scafetta and West's findings clearly indicate they do, although by what singular or multiple means remains to be clarified.

We agree with this assessment of the subject; for it is the story told by real-world data, as revealed by the logical way in which Scafetta and West conducted their insightful but straightforward analysis. The sun's fingerprints on earth's climatic history have been laid bare by them for all to see. Without even a need to squint, there is simply no mistaking them.

That being said, however, the researchers note that in the 20th century there was "a clear surplus warming" above and beyond what is suggested by their solar-based temperature reconstruction, such that something in addition to the sun may have been responsible for approximately 50% of the total global warming since 1900; and this anomalous increase in temperature could be argued to be due to anthropogenic greenhouse gas emissions.

On the other hand, Scafetta and West say the temperature difference since 1975, where the most noticeable part of the discrepancy occurred, may have been due to "spurious non-climatic contamination of the surface observations such as heat-island and land-use effects (Pielke et al., 2002; Kalnay and Cai, 2003)," which they say is also suggested by "an anomalous warming behavior of the global average land temperature vs. the marine temperature since 1975 (Brohan et al., 2006)."

The take-home message of the Scafetta and West paper would thus appear to be that the sun alone was responsible for most of the temperature variability of earth's Northern Hemisphere over all but perhaps the final 25 years of the past four centuries, as well as over much of the prior 600 years (which includes a good portion the Medieval Warm Period), while it is yet to be conclusively determined if the non-solar-induced portion of the warming of the last quarter of the 20th century was due to (1) anthropogenic greenhouse gas emissions, (2) spurious non-climatic contamination of the temperature record or (3) some mix of these two factors.

Sherwood, Keith and Craig Idso

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