I keep hearing the total solar irradiance = 0.1% statistic used to
rule out the sun's role in global warming.  One tenth of one percent
is simply not enough to account for the observed temperature
variations here on earth.  But total solar irradiance is not relevant
to the question of global warming.  What is relevant is the
variability in the sun's UV spectrum.  UV light originates in the
sun's upper photosphere, chromosphere, and corona.  Solar radiation at
wavelengths below 300 nm is almost completely absorbed by the Earth’s
atmosphere,  becoming the dominant energy source.

The ultraviolet (UV) part of the solar spectrum is important for ozone
production and middle atmosphere heating here on earth.  The variation
of the UV spectrum is shown below (Wave Length, Variation):

  200 nm, 8%
  220-260 nm, 5%
  300 nm, 0.1%

Shorter wavelengths are absorbed in the earth’s upper atmosphere and
see even larger variations:

  120 nm, 50%
  140-200 nm, 10-15%

See the following for details:  http://www.atmos-chem-phys.org/7/5391/2007/acp-7-5391-2007.pdf

The variability of solar irradiance at the UV portion of the spectrum
tracks the Solar sunspot cycle as can be seen from the "SUSIM UARS Mg
II Index" charts which can be found herehttp://wwwsolar.nrl.navy.mil/susim_uars.html

Be sure to click the "Mg II Index" hyperlink under Data.

Solar irradiance at wave lengths less than 300 nm has a place in the
global warming debate.

--Mike Jr.