Figure 7.

Solar irradiance changes from Hoyt and Schatten (33). Their data span 1700–1992; estimated values are shown to 1994. To obtain top-of-the-atmosphere forcing values (to give changes that are then directly comparable with the anthropogenic forcings shown in Fig. 4), the values shown should be multiplied by 0.175. The main climate model calculations begin in 1765 and assume constant irradiance at the 1765 level before this date, as shown. Similar calculations using 1700 as a starting date with constant irradiance at the 1700 level for the time before 1700 lead to only negligible differences after the mid-19th century.