From:
http://www.co2science.org/journal/2003/v6n29c1.htm

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

Background
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."



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Reviewed 16 July 2003
Center for the Study of Carbon Dioxide and Global Change
(www.co2science.org).