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Solar Activity and Climate Change Over the 20th Century
Foukal, P.  2002.  A comparison of variable solar total and ultraviolet irradiance outputs in the 20th century.  Geophysical Research Letters 29: 10.1029/2002GL015474.

Variations in both total solar irradiance and ultraviolet (UV) irradiance have long been suspected of driving climate change on earth, and the author rightly notes that the ability to discriminate between the roles played by these two factors would significantly advance the state of sun-climate studies.  Hence, it was his objective to devise a means to provide that discrimination.

What was done
Foukal based his work on the finding from analysis of space-borne radiometry that "variations in total solar irradiance, S, measured over the past 22 years, are found to be closely proportional to the difference in projected areas of dark sunspots, AS, and of bright magnetic plage elements, APN, in active regions and in enhanced network," plus the finding that "this difference varies from cycle to cycle and is not simply related to cycle amplitude itself," which facts suggest there is "little reason to expect that S will track any of the familiar indices of solar activity."  On the other hand, he notes that "empirical modeling of spectro-radiometric observations indicates that the variability of solar ultraviolet flux, FUV, at wavelengths shorter than approximately 250 nm, is determined mainly by APN alone."

Building upon this conceptual foundation, and based upon daily data from the Mt. Wilson Observatory that covered the period 1905-1984 and partially-overlapping data from the Sacramento Peak Observatory that extended through 1999, Foukal derived time series of both total solar and UV irradiances between 1915 and 1999, which he then compared with global temperature data for the same time period.

What was learned
It was determined, in the words of Foukal, that "correlation of our time series of UV irradiance with global temperature, T, accounts for only 20% of the global temperature variance during the 20th century," but that "correlation of our total irradiance time series with T accounts statistically for 80% of the variance in global temperature over that period."

What it means
The UV findings of Foukal were not incredibly impressive, but the results of his total solar irradiance analysis were, leading him to emphatically state that "the possibility of significant driving of 20th century climate by total irradiance variation cannot be dismissed."  Although the magnitude of the total solar effect was determined to be "a factor 3-5 lower than expected to produce a significant global warming contribution based on present-day climate model sensitivities [our italics]," what Foukal calls the "high correlation between S and T" strongly suggests that changes in S largely determine changes in T, the confirmation of which suggestion likely merely awaits what he refers to as an "improved understanding of possible climate sensitivity to relatively small total irradiance variation."

Reviewed 16 July 2003