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As the planet cooled from peak warmth in the early Cenozoic, extensive Northern Hemisphere ice sheets developed by 2.6Ma ago, leading to changes in the circulation of both the atmosphere and oceans. From ∼2.6 to ∼1.0Ma ago, ice sheets came and went about every 41ka, in pace with cycles in the tilt of Earth’s axis, but for the past 700ka, glacial cycles have been longer, lasting ∼100ka, separated by brief, warm interglaciations, when sea level and ice volumes were close to present. The cause of the shift from 41ka to 100ka glacial cycles is still debated. During the penultimate interglaciation, ∼130 to ∼120ka ago, solar energy in summer in the Arctic was greater than at any time subsequently. As a consequence, Arctic summers were ∼5°C warmer than at present, and almost all glaciers melted completely except for the Greenland Ice Sheet, and even it was reduced in size substantially from its present extent …