Background
The 20th century is said by climate alarmists to have experienced a temperature increase that was unprecedented over the past two millennia, due primarily to CO2-induced global warming. For the moment, we will forget about the "unprecedented" part of this claim and focus on the attribution portion, in a review of a paper that looks at the role played by solar radiation variability over the 20th century.

What was done
Scafetta and West developed "two distinct TSI [total solar irradiance] reconstructions made by merging in 1980 the annual mean TSI proxy reconstruction of Lean et al. (1995) for the period 1900-1980 and two alternative TSI satellite composites, ACRIM (Wilson and Mordvinov, 2003), and PMOD (Frolich and Lean, 1998), for the period 1980-2000," after which they used what they deemed to be appropriate climate sensitivity transfer functions to transform the TSI histories they developed into 20th-century temperature histories.

What was learned
The two researchers determined that the sun contributed some 46-49% of the 1900-2000 global warming of the earth; and considering that there may have been uncertainties of 20-30% in their sensitivity parameters, they suggest that the sun may possibly have been responsible for as much as 60% of the 20th-century temperature increase.

What it means
The role of the sun in 20th-century global warming, according to Scafetta and West, has been vastly underestimated by the climate modeling community, with various energy balance models producing estimates of solar-induced warming over this period that are "two to ten times lower" than what they found.. Why is this so? The two researchers say "the models might be inadequate because of the difficulty of modeling climate in general and a lack of knowledge of climate sensitivity to solar variations in particular." They also note that "theoretical models usually acknowledge as solar forcing only the direct TSI forcing," thereby ignoring "possible additional climate effects linked to solar magnetic field, UV radiation, solar flares and cosmic ray intensity modulations." In this regard, we additionally note that some of these phenomena may to some degree be independent of, and thereby add to, the simple TSI forcing Scafetta and West employed, which suggests that the totality of solar activity effects on climate may be even greater than what they calculated.

References
Frohlich, C. and Lean, J. 1998. The Sun's total irradiance: Cycles, trends and related climate change uncertainties since 1976. Geophysical Research Letters 25: 4377-4380.

Lean, J., Beer, J. and Bradley, R. 1995. Reconstruction of solar irradiance since 1610: Implications for climate change. Geophysical Research Letters 22: 3195-3198.

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.