What was done
The authors used multiple regression analysis to separate out surface and atmospheric temperature responses to solar irradiance variations over the past two and a half solar cycles (1979-2001) from temperature responses produced by variations in ENSO and volcanic activity.

What was learned
Based on the satellite-derived lower tropospheric temperature record, the authors evaluated the sensitivity (k) of temperature (T) to solar irradiance (I), where temperature sensitivity to solar irradiance is defined as k = deltaT/deltaI, obtaining the result of k = 0.11 ± 0.02°C/(W/m²).  Similar analyses based on the radiosonde temperature record of Parker et al. (1997) and the surface air temperature records of Jones et al. (2001) and Hansen and Lebedeff (1987, with updates) produced k values of 0.13, 0.09 and 0.11°C/(W/m²), respectively, with the identical standard error of ± 0.02°C/(W/m²).  They also reported that White et al. (1997) derived a decadal timescale solar sensitivity of 0.10 ± 0.02°C/(W/m²) from a study of upper ocean temperatures over the period 1955-1994 and that Lean and Rind (1998) derived a value of 0.12 ± 0.02°C/(W/m²) from a paleo-reconstructed temperature record spanning the period 1610-1800.  The authors thus concluded that "the close agreement of these various independent values with our value of 0.11 ± 0.02 [°C/(W/m²)] suggests that the sensitivity k is the same for both decadal and centennial time scales and for both ocean and lower tropospheric temperatures."

What it means
The authors suggest that if these values of k hold true for centennial time scales, which appears to be the case, their high-end value implies a surface warming of 0.2°C over the last 100 years in response to the 1.5 W/m² increase in solar irradiance inferred by Lean (2000) for this period.  This warming represents approximately one-third of the total increase in global surface air temperature estimated by Parker et al. (1997), 0.55°C, and Hansen et al. (1999), 0.65°C, for the same period.  It does not, however, include potential indirect effects of more esoteric solar climate-affecting phenomena that could also have been operative over this period.

References
Hansen, J. and Lebedeff, S.  1987.  Global trends of measured surface air temperature.  Journal of Geophysical Research 92: 13,345-13,372.

Hansen, J., Ruedy, R., Glascoe, J. and Sato, M.  1999.  GISS analysis of surface temperature change.  Journal of Geophysical Research 104: 30,997-31,022.

Jones, P.D., Parker, D.E., Osborn, T.J. and Briffa, K.R.  2001.  Global and hemispheric temperature anomalies -- land and marine instrumental records.  In: Trends: A Compendium of Data on Global Change, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, TN.

Lean, J.  2000.  Evolution of the sun's spectral irradiance since the Maunder Minimum.  Geophysical Research Letters 27: 2425-2428.

Lean, J. and Rind, D.  1998.  Climate forcing by changing solar radiation.  Journal of Climate 11: 3069-3094.

Parker, D.E., Gordon, M., Cullum, D.P.N., Sexton, D.M.H., Folland, C.K. and Rayner, N.  1997.  A new global gridded radiosonde temperature data base and recent temperature trends.  Geophysical Research Letters 24: 1499-1502.

White, W.B., Lean, J., Cayan, D.R. and Dettinger, M.D.  1997.  Response of global upper ocean temperature to changing solar irradiance.  Journal of Geophysical Research 102: 3255-3266.