lands that now support only polar desert and tundra. Global oceanic and atmospheric circulation was substantially different between 3 and 2.5 Ma than subsequently. The development of the first continental ice sheets over North America and Eurasia led to changes in the circulation of both the atmosphere and oceans. The onset of continental glaciation is most clearly defined by the first appearance of rock fragments in sediment cores from the central Atlantic Ocean about 2.6 Ma. These rock fragments, often referred to as ice-rafted debris (IRD), are too heavy to have blown or been washed into the central Atlantic; they must have been delivered by large icebergs emanating from continental ice sheets. The first appearance of IRD marks the onset of the Quaternary Period (2.6–0 Ma), generally equated with “ice-age” time, even though for a small fraction (about 10%) of the time the ice sheets were very likely to have been as small as or smaller than their present size. From about 2.7 to about 0.8 Ma, the ice sheets came and went about every 41 k.y., the same timing as cycles in the tilt of Earth’s axis. Ice sheets grew when Earth’s tilt was at a minimum, resulting in less seasonality (cooler summers, warmer winters), and they melted when tilt was at a maximum and seasonality was at its greatest (warmer summers and cooler winters). For the past 600 k.y., ice sheets have grown larger and ice-age times have been longer, lasting about 100 k.y.; those icy intervals have been separated by brief warm periods (interglaciations), when sea level and ice volumes were close to those at present. The duration of interglaciations ranges from about 10 k.y. to perhaps 40 k.y. The cause of the shift from 41 k.y. to 100 k.y. glacial cycles is still being debated. Most explanations center on the continued gradual planetary cooling that may have produced larger ice sheets that were more resistant to melting, or with removal of soft sedimentary cover over bedrock in glaciated regions that, once removed, increased the frictional coupling of the ice sheet to its bed, resulting in steeper ice-sheet profiles and thicker ice sheets, again more resistant to melting. The relatively warm planetary state during which human civilization developed is the most recent of the warm interglaciations, the Holocene (about 11.5–0 kiloannum (thousands of years ago (ka)). During the penultimate warm interval, about 130–120 ka, solar energy in summer in the northern high latitudes was greater than at any time in the current warm interval. As a consequence, the Arctic summer was about 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. With the increased ice melt, sea level was about 5 meters (m) higher than at present; the extra melt came from both Greenland and Antarctica as well as from small glaciers (Overpeck et al., 2006; Meier et al., 2007). Although sea ice is difficult to reconstruct, the evidence suggests that the central Arctic Ocean retained some permanent ice cover or was periodically ice free, even though the flow of warm Atlantic water into the Arctic Ocean was very likely to have been greater than during the present warm interval. The Last Glacial Maximum (LGM) peaked about 21 ka when mean annual temperatures in parts of the Arctic were as much as 20°C lower than at present. Ice recession was well underway by 16 ka, and most of the Northern Hemisphere ice sheets had melted by 7 ka. Summertime solar energy rose steadily in the Arctic from 21 ka to a maximum (10% higher than at present) about 11 ka and has been decreasing since then, primarily in response to the precession of the equinoxes causing Earth’s distance from the Sun during Northern Hemisphere summer to decrease from 21 to 11 ka and then to increase to the present. The extra energy received in early Holocene summers warmed summers throughout the Arctic about 1°–3°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 present). 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 re-established or advanced, sea ice extended, and the flow of warm Atlantic water into the Arctic Ocean diminished. Late Holocene cooling reached its nadir during the Little Ice Age (about 1250–1850 AD), when sun-blocking volcanic eruptions and perhaps other causes added to the orbital cooling, allowing most Arctic glaciers to reach their maximum Holocene extent. During the warming of the past century and a half, glaciers have receded throughout the Arctic, terrestrial ecosystems have advanced northward, and perennial Arctic Ocean sea ice has diminished. Paleoclimate reconstructions indicate that Arctic temperature changes typically have been larger than corresponding hemispheric or globally averaged changes. This behavior is observed with conditions both warmer and colder than recently, indicating that Arctic amplification is a pervasive feature of the climate system.