IPCC Overestimate of TSI

From Judith Curry:overconfidence-in-ipccs-detection-and-attribution-part-iv

The level of scientific understanding of radiative forcing is ranked by the AR4 (Table 2.11) as high only for the long-lived greenhouse gases, but is ranked as low for solar irradiance, aerosol effects, stratospheric water vapor from CH4, and jet contrails. Radiative forcing time series for the natural forcings (solar, volcanic aerosol) are reasonably well known for the past 25 years (although these forcings continue to be debated), with estimates further back in time having increasingly large uncertainties.
Based upon new and more reliable solar reconstructions, the AR4 (Section 2.7.1.2) concluded that the increase in solar forcing during the period 1900-1980 used in the AR3 reconstructions is questionable and the direct radiative forcing due to increase in solar irradiance is reduced substantially from the AR3.  However, consideration of Table S9.1 in the AR4 shows that each climate model used outdated solar forcing (from the AR3) that was assessed to substantially overestimate the magnitude of the trend in solar forcing prior to 1980.  The IPCC AR4 states: “While the 11-year solar forcing cycle is well documented, lower-frequency variations in solar forcing are highly uncertain.”  “Large uncertainties associated with estimates of past solar forcing (Section 2.7.1) and omission of some chemical and dynamical response mechanisms make it difficult to reliably estimate the contribution of solar forcing to warming over the 20th century.”