http://serc.carleton.edu/eslabs/cryosphere/4a.html shows this:
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http://ossfoundation.us/projects/environment/global-warming/natural-cycle shows this:
The 100,000 year cycles shown at far right (just above) are glaciation back to warm back to glaciation cycles; toward middle of graph the cycles are 41,000 years, warm to cold again. what we have are about 9 glaciations across last million years, very rhythmic, and before that very rhythmic again across another 2 million years. What could be the cause of these very rapid warming, then cooling cycles? Here below are 3 possible clues–
1-1-2015 https://www.iceagenow.info/greenland-ice-melt-caused-heat-flow-deep-earth/
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Earth’s outer core iron jet stream 12-19-16 http://www.bbc.com/news/science-environment-38372342
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12-9-15 http://www.ibtimes.co.uk/antarctic-glacial-underbelly-scanned-first-time-hinting-volcanoes-magma-blowtorch-1532531
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Additionally, a Serbian mathematician of the 1920s offers another possible explanation: MILANKOVITCH CYCLES