In their Opinion piece “Is Climate Sensitive to Solar Variability?” (Physics Today, Physics Today 0031-9228 61

200850March 2008, page 50 ), Nicola Scafetta and Bruce West draw attention to a “phenomenological solar signature” they find in preindustrial records of global temperature. Correlations between temperature proxies and solar activity have been reported since the 18th century; see, for example, reference 1. However, their significance cannot be judged without considering volcanism, which may account for much of the apparent correlation with solar activity over the past millennium. 2 Scafetta and West assume that the only secular influence on preindustrial climate is the Sun.

Scafetta and West also state that anthropogenic influence on global warming is overestimated if climate is as sensitive to solar activity as their studies suggest. But in their referenced work, they admit that such a high sensitivity requires more powerful couplings between solar activity and climate than have been identified so far. That is hardly news; for more than a decade, the big question has been whether such powerful Sun–climate coupling mechanisms exist. For reasons we reviewed recently, 3 variation in total solar irradiance (TSI) seems to lack the required power, and the solar UV flux variation—another candidate mentioned by the authors—seems to account for less than 10% of the variance in the 20th-century global temperature. Solar modulation of cosmic rays and their possible effect on cloud cover is the most complex candidate and remains hardest to test.

Additionally, Scafetta and West use solar flare statistics as a proxy for TSI variations to derive a similarity in the power law indices of solar fluctuations and Earth temperature variations. Their claim is puzzling since solar flares make a negligible contribution to TSI variation. Why not just analyze the widely available TSI record itself?

In summary, a solar effect on global temperature may well exist, and incisive correlation studies can play a role in its investigation. However, it is increasingly clear that deployment of improved radiometric and photometric instruments will be required to discriminate between suggested Sun–climate coupling mechanisms.

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J. Climate
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