As the planet cooled from peak warmth in the early Cenozoic, extensive Northern Hemisphere ice sheets
developed by 2.6 Ma ago, leading to changes in the circulation of both the atmosphere and oceans. From
w2.6 to w1.0 Ma ago, ice sheets came and went about every 41 ka, in pace with cycles in the tilt of
Earth’s axis, but for the past 700 ka, glacial cycles have been longer, lasting w100 ka, separated by brief,
warm interglaciations, when sea level and ice volumes were close to present. The cause of the shift from
41 ka to 100 ka glacial cycles is still debated. During the penultimate interglaciation, w130 to w120 ka
ago, solar energy in summer in the Arctic was greater than at any time subsequently. As a consequence,
Arctic summers werew5 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. With the
loss of land ice, sea level was about 5 m higher than present, with the extra melt coming from both
Greenland and Antarctica as well as small glaciers. The Last Glacial Maximum (LGM) peaked w21 ka ago,
when mean annual temperatures over parts of the Arctic were as much as 20 C lower than at present.
Ice recession was well underway 16 ka ago, and most of the Northern Hemisphere ice sheets had melted
by 6 ka ago. Solar energy reached a summer maximum (9% higher than at present) w11 ka ago and has
been decreasing since then, primarily in response to the precession of the equinoxes. The extra energy
elevated early Holocene summer temperatures throughout the Arctic 1e3 C above 20th century averages,
enough to completely melt many small glaciers throughout the Arctic, although the Greenland Ice
Sheet was only slightly smaller than at present. Early Holocene summer sea ice limits were substantially
smaller than their 20th century average, and the flow of Atlantic water into the Arctic Ocean was
substantially greater. As summer solar energy decreased in the second half of the Holocene, glaciers reestablished
or advanced, sea ice expanded, and the flow of warm Atlantic water into the Arctic Ocean