{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"3d7aab671528b44d81dd972a04b71f70a8804c9bbccfe342083019af8414b6d6","created":"2026-10-03T07:18:03Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"08df2dee66362018d934185a8589b28e6b75db289c2be52ecd9426cf73729428","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"mechanism","assisted_by":[],"external_id":"earth.paleoclimate-puzzles.mid-pleistocene-transition","kind":"phenomenon","literature_status":"contested","n":"1","parents":[],"plain":"Before about a million years ago ice ages came every 41 thousand years, matching a wobble in Earth's tilt, then shifted to bigger cycles every 100 thousand years. Earth's orbit did not change at that time, so the cause must be inside the climate system.","posed_since":"","precise":"Between about 1.25 and 0.7 Myr ago the dominant period of benthic delta-18O changed from 41 kyr (obliquity) to roughly 100 kyr with larger amplitude, with no corresponding change in orbital forcing. Identify the internal change (removal of deformable regolith under ice sheets, long-term CO2 decline, Southern Ocean stratification, or a threshold in ice-sheet dynamics) and show it reproduces the period change and the sawtooth shape in a model forced only by insolation.","problem_ref":null,"references":"","settled_by":"A physically based ice-sheet-climate-carbon model forced only by orbital insolation and a measured slow boundary-condition change that reproduces the timing and spectral shift of the $\\delta-18\\mathrm{O}$ record.","status_note":"","title":"Why did glacial cycles switch from 41-kyr to 100-kyr 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