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Slow Variations in Solar Luminosity
Foukal, P.  2003.  Can slow variations in solar luminosity provide missing link between the sun and climate?  EOS, Transactions, American Geophysical Union 84: 205, 208.

The author unequivocally states that it is an acknowledged fact that "recent evidence from ocean and ice cores suggests that a significant fraction of the variability in northern hemisphere climate since the last Ice Age correlates with solar activity (Bond et al., 2001)."  He also notes that "a recent reconstruction of S [total solar irradiance] from archival images of spots and faculae obtained daily from the Mt. Wilson Observatory in California since 1915 shows remarkable agreement with smoothed global temperature in the 20th century [Foukal, 2002]."  Nevertheless, he is forced to acknowledge that the observed variations in S between 1978 and 2002 are not large enough to explain the observed temperature changes on earth within the context of normal radiative forcing.

What was done
Foukal reviews the status of research into various subjects that might possibly be able to explain this dichotomy.  Specifically, he presents an overview of current knowledge relative to the idea that "the solar impact on climate might be driven by other variable solar outputs of ultraviolet radiation or plasmas and fields via more complex mechanisms than direct forcing of tropospheric temperature."

What was learned
As could be expected, the article contains no grand revelations.  However, when all is said and done, Foukal returns pretty much to where he started, concluding that "we cannot rule out multi-decadal variations in S sufficiently large to influence climate, yet overlooked so far through limited sensitivity and time span of our present observational techniques."

What it means
There is just too much evidence for a solar-climate connection to dismiss it out of hand.  Hence, the possibility remains that the sun alone may well be responsible for the lion's share of all climate change on earth.

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.

Foukal, P.  2002.  A comparison of variable solar total and ultraviolet irradiances in the 20th century.  Geophysical Research Letters 29: 10.1029/2002GL015474.

Reviewed 17 September 2003