FIGURE 2-6 July-August average net daytime (12Z-23Z) radiation received at the surface for the pre-1900 landscape (top), 1989 large-scale meteorology with the 1993 landscape (middle), and the difference between the 1989 simulation and the pre-1900 case (bottom) in units of W m⁻². The daytime land average for the pre-1900 case is 530 W m⁻² and for the 1989 simulation is 503 W m⁻². SOURCE: Adapted from Marshall et al. (2004b).

spacecraft have measured the total solar irradiance since the late 1970s. There is an 11-year cycle in total solar irradiance of peak-to-peak amplitude ~1 W m⁻² (0.1 percent) in the past three cycles. Allowing for reflection of 30 percent of this incident energy (Earth’s albedo) and averaging over the globe, the corresponding climate forcing is of order 0.2 W m⁻².

Multiple TSI datasets have been combined into a composite time series of daily total solar irradiance from 1979 to the present. This requires the cross-calibration of measurements made by overlapping datasets to adjust