FIGURE 1. Ice-age climate and solar variability.

A 3-million-year record of ¹⁸O (ref. 8) (a); orbital obliquity (blue) compared with integrated summer insolation (red)³ (b); and summer insolation for the Northern Hemisphere (on 21 June at 65° N; red) and the Southern Hemisphere (on 21 December at 65° S; blue)⁶ (c). ¹⁸O is considered a proxy of global ice-volume change, which is assumed to occur mostly in the Northern Hemisphere over this interval. From 3 to 1 million years ago, ¹⁸O varies primarily at the 41,000-year period characteristic of obliquity and integrated insolation. From 1 million years ago to the present, longer cycles of climate change, with a roughly 100,000-year period, are more obvious. The double-headed arrow indicates a transition more gradual than abrupt over the time indicated. GJ, gigajoule; W, watts.