Over the past several ~100 ka glacial-interglacial cycles, the concentration of atmospheric CO2 was closely coupled to global temperature, which indicates the importance of CO2 as a greenhouse gas. The reasons for changes in atmospheric CO2 have mainly been sought in the ocean, but remain elusive. Moreover, the mid-Pleistocene transition from the ‘41 ka world’ during the early Pleistocene before ~0.7 Ma to the ~100 ka ice age cycles is poorly understood. The classical Milankovitch theory of summer insolation forcing at high northern latitudes can not fully explain the Pleistocene ice age rhythm. Based on the recent findings that the amount of soil organic carbon stored in high-latitude permafrost regions has been greatly underestimated and the simple logic that permafrost regions and respective carbon pools were likely much larger during glacials than during interglacials, a ‘permafrost glacial hypothesis’ is proposed: (i) Gradual sequestration of CO2 in permafrost soils during coolings and rapid release of CO2 and methane during terminations, respectively, provide important positive feedbacks for the climate. (ii) Integrated annual insolation at the southern and thus most sensitive permafrost boundary may act as a trigger for global climate changes. (iii) The mid-Pleistocene transition might be readily explained with permafrost extents reaching ~45ÅãN during the long-term Pleistocene cooling, resulting in a transition from high-latitude obliquity (~41 ka) to mid-latitude eccentricity (~100 ka) forcing.
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