Glacial terminations or glacial interruptions?

In the early 20th century, after contributing major advances in calculating radiation forcing on planetary bodies, Milutin Milankovitch the Serbian mathematician took up the challenge of explaining why Earth has experienced recurrent episodes of glaciation. Influenced by the ideas of his predecessors, Milankovitch developed a theory that centered on the notion that summertime temperature at high northern latitudes is the most important influence on the advance and retreat of glaciations. The calculations revealed a periodicity in summer insolation that had a reasonable correspondence with what was then known about the occurrence of ice ages. From that was born the elemental foundation of the orbital theory of the ice ages. That theory evolved over the next three decades while retaining the fundamental tenant that summer season insolation at the higher northern latitudes determines Earth's climate variability. Scientists of the day were skeptical, and it was not until the 1960s that new techniques became available to test the temporal predictions of Milankovitch's theory. The orbital theory gained support in the 1950s and 60s when methods for paleoclimate reconstructions documented an orbital-like recurrence pattern of cold and warm climate conditions spanning the past 2.5 million years. Accompanying the documentation of Earth's climate rhythmicity from marine archives have been advances in other areas, including ice core records of atmospheric CO2 that pose challenges to the original orbital theory, namely what role have variations in atmospheric CO2 played in dictating the transitions from warm to cold and, what caused orbital scale variations in greenhouse gas concentrations? In this contribution we review the current state of knowledge about the Earth's carbon cycle on glacial/interglacial timescales and explore how new information has begun to shed light on the long-standing goal to understand Earth's natural climate rhythmicity. The findings presented here highlight the need to expand research on Earth's geologic processes that influence the carbon budget on glacial timescales. And with this comes a new hypothesis that incorporates geologic processes in orbital scale climate cycles.