Global and Arctic climates have varied widely in the past. On time scales of tens of millions to hundreds of millions of years, these changes were at least partly a result of the shifting configurations of land and ocean and mountain-building events associated with continental drift. While it appears that the Earth has experienced ice ages in many periods of its history, including the Proterozoic, 800 and 600 million years before present (hereafter abbreviated as Ma), the Ordovician and Silurian (460 and 430 Ma) and the Carboniferous and Permian (350 and 250 Ma), we know little about the details of these events. Much more is known of global and Arctic climates during the Quaternary period. The Quaternary, comprising of the Pleistocene and Holocene epochs, extends from about 1.8 million years ago to the present.
The ice ages of the Pleistocene captivate the imagination. The Pleistocene was characterized by many glacial advances and retreats. While the climate of the Arctic varied with these glacial cycles, which seem to be ultimately driven by periodicities in the Earth's orbit that influence the geometry and seasonality of solar radiation, high-latitude processes, such as ice-albedo feedbacks and deepwater formation in the northern North Atlantic, likely contributed to change. Our most complete information on the Pleistocene is for the last glacial cycle, extending from the Holocene back to the peak of the Eemian Interglacial, about 124 000 years ago (124 ka). Climates during the last glacial cycle were extremely variable, characterized by millennial-scale oscillations between cold and relatively warm conditions known as Dansgaard-Oeschger (D-O) cycles. Transitions into and out of D-O cycles, as seen in oxygen isotope records preserved in Greenland ice cores, may have occurred on time scales of decades. In ocean cores, D-O cycles can be stratigraphically related to layers of ice-rafted detritus (IRD) released from armadas of melting icebergs calved from the ice sheets, the most pronounced of which are termed Heinrich events. Causes of the D-O cycles are still incompletely understood.
The Last Glacial Maximum (LGM) occurred from 18 to 25 ka (depending on the region and interpretation). Terrestrial ice extent during the LGM is fairly well known. There were immense ice sheets in North America (the Laurentide, second in size only to the Antarctic ice sheet) and parts of Eurasia. Along with the major Fennoscandian Ice Sheet, ice sheets covered Svalbard, Franz Josef Land, the Barents Sea, Novaya Zemlya, the Kara Sea, Iceland and part of the British Isles. The process of deglaciation after the LGM was very irregular. Of note is the Younger Dryas (YD) event from about 13 to 11.7 ka, which represented a temporary return to near glacial conditions. The YD is considered to be the last D-O event. Regional climate expressions of the YD were quite varied.
The Holocene epoch (formally defined as the last 11.5 ka) has been generally warm, and although characterized by a more stable climate than the Pleistocene, has still been quite variable. Especially mild conditions during the first part of the Holocene were followed by general cooling, during which the sea ice cover expanded and glacier extent increased. The Little Ice Age (LIA) represents the most recent major cooling and in many regions was the coldest part of the Holocene. Ice core records from Greenland indicate that temperatures there reached their lowest point in the past millennium between AD 1579 and 1730. Causes of the LIA are highly debated. Volcanism probably played some part, as did solar variability. The LIA has been followed by general warming extending through the present. As outlined in Chapter 11, warming evident in the period of instrumental records is not completely understood, but is likely in part a response to anthropogenic greenhouse gas loading of the atmosphere.
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