Pleistocene glacial terminations triggered by synchronous changes in Southern and Northern Hemisphere insolation: the insolation canon hypothesis

Throughout the last ∼ 900 kyr, the Late Pleistocene, Earth has experienced periods of cold glacial climate, punctuated by seven abrupt transitions to warm interglacials, the so-called terminations. Although most of glacial ice is located in the Northern Hemisphere (NH), the Southern Hemisphere (SH) seems to play a crucial role in deglaciation. Variation in the seasonal distribution of solar insolation is one candidate for the cause of these climatic shifts. But so far, no simple mechanism has been identified. Here we present a mathematical analysis of variations in midsummer insolation in both hemispheres at 65° latitude. Applying this analysis to the entire Pleistocene, the last 2 Myr, we find that prior to each termination the insolation in both hemispheres increases in concert, with a SH lead. Introducing time and energy thresholds to these overlaps, calculated times for the onsets of the seven terminations by this insolation canon (exceptional overlaps meeting the two threshold prerequisites) are ∼ 23, 139, 253, 345, 419, 546 and 632 kyr BP, perfectly matching the geologic record. The timing originates from the interplay between the two orbital parameters obliquity and precession, explaining why terminations occur at integer multiple of the precessional cycle. There is no such constellation between 1 and 2 Myr BP, the Early Pleistocene, in agreement with Earth's climate at that time. This change in orbital forcing coincides with the Mid Pleistocene Revolution, separating the Late from the Early Pleistocene. Therefore, we hypothesize that the insolation canon is the trigger for glacial terminations.