In 2007, Ross McKitrick wrote a paper on the Fourth Assessment Report which included a short section on the IPCC's use of Judith Lean's paper:
The IPCC acknowledges that solar activity is high, and possibly exceptionally high, compared to the last 8,000 years. The two most prominent proxy-based reconstructions (from teams led by Solanki and Muescheler, respectively), differ on whether an interval in the 1700s included a spike comparable to today’s but both agree that today’s solar output is very high compared to most of the current interglacial era.
There have been many reconstructions of total solar activity based on sunspot counts, which
began in the early 1600s. Up until recently, most reconstructions showed a strong upward trend in total solar irradiance since the 17th century, with low-frequency trends that track 19th and 20th century average temperatures reasonably well. Very recently, however, a different-looking reconstruction by Wang et al.7 suggested that climatic forcing due to total irradiance had risen very little since the 1700s, implying an increase in solar output as little as one-tenth the size reported by other solar reconstructions. This would imply that solar changes could have little to do with climate change since 1600.Rather than treating the Wang et al. results as one of a range of recent findings, it was presented as the sole and sufficient basis for a conclusive dismissal of the larger solar forcing estimates used in earlier IPCC reports, a dismissal that carried over into the Summary for Policy Makers, where it is concluded that solar influence on climate is much smaller than has been earlier proposed. Here the bias amounts to cherry-picking. Readers are told that one new result, standing at odds with a host of earlier studies, is the definitive word on the matter. Had the one new result found evidence for much stronger solar forcing than earlier thought, it is likely it would hardly have been mentioned, or it would have been presented in a disputatious and grudging aside.
There is also a lengthy discussion of the issue in this Fraser Institute paper by D'Aleo et al. (the et al including Richard Willson, the principal investigator on the ACRIM results that forms the basis of Lean's work.
Lean and Fröhlich added degradation corrections to the results of Nimbus7/ERB and ACRIM1 results, which had the effect of lowering their TSI results during the solar cycle 21 maximum and conforming the TSI time series to the predictions of Lean’s solar proxy model. Their method is not consistent with the degradation analyses published by the ACRIM1 science team. Fröhlich and Lean chose overlapping ERBS/ERBE results to relate ACRIM1 and ACRIM2 results across the crucial “ACRIM Gap.” Willson has argued that the ERBS/ERBE results are inferior to those of the Nimbus7/ERB in general, and specifically during the “ACRIM Gap,” when uncorrected sensor degradation of the ERBS/ERBE results causes lower results after the “Gap” and the absence of a trend in the Lean-Fröhlich composite TSI time series (Willson and Mordvinov, 2003).