evidence: The greenhouse warming model developed above shows how the planetary energy balance can be changed by varying atmospheric composition even when solar energy supply remains constant. However, the planetary energy balance will also change if the solar flux was altered. The degree to which this happened in more recent geological time is uncertain (Foukal et al. 2006). However, the solar energy reaching different regions of the Earth has certainly changed over time in response to cycles in the Earth's orbit around the sun (Berger 1978, Berger & Loutre 1992) (Figure 8). These cause variation in the season of Earth's closest approach to the sun (precession), the Earth's tilt (obliquity) with respect to the orbital plane, and the ellipticity of the Earth's orbit around the sun. The combination of these cycles changes energy delivered to different parts of the Earth's surface with perhaps the greatest impact at higher latitudes. Although the changes are relatively modest, the climate response has been large (again indicating positive feedbacks) as the ice ages have fluctuated to the beat with the orbital parameters (Hays et al. 1976). [...] Hays, J., Imbrie, J. & Shackleton, N. Variations in the Earth's orbit: Pacemaker of the ice ages. Science194, 1121-1132 (1976).