What was done
The authors evaluated different pre-industrial temperature and solar data reconstructions that started in AD 1610 as a prelude to employing a phenomenological thermodynamic model to estimate the relative contributions of solar- and anthropogenic-induced climate forcing during the 20th century.

What was learned
Based on the results of their analysis, Scafetta and West argue that "a realistic climate scenario is the one described by a large pre-industrial secular variability (as the one shown by the paleoclimate temperature reconstruction by Moberg et al. (2005)) with the total solar irradiance experiencing low secular variability (as the one shown by Wang et al. (2005))." Under this scenario, as they continue, they find that "the sun might have contributed up to approximately 50% (or more if [the] ACRIM total solar irradiance satellite composite (Wilson and Mordvinov, 2003) is implemented) of the observed global warming since 1900."

What it means
In the closing paragraph of their intriguing paper, the two researchers say they "are forced [our italics] to conclude that solar changes significantly alter climate, and that the climate system responds relatively slowly to such changes with a time constant between 6 and 12 years," which further suggests, in their words, that "the large-scale computer models of climate could be significantly improved by adding additional sun-climate coupling mechanisms."

References
Moberg, A., Sonechkin, D.M., Holmgren, K., Datsenko, N.M. and Karlen, W. 2005. Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data. Nature 433: 613-617.

Wang, Y.-M., Lean, J.L. and Sheeley Jr., N.R. 2005. Modelling the sun's magnetic field and irradiance since 1713. Astrophysics Journal 625: 522-538.

Wilson, R.C. and Mordvinov, A.V. 2003. Secular total solar irradiance trend during solar cycles 21-23. Geophysical Research Letters 30: 10.1029/2002GL016038.