On geological time scales the two polar regions have shown generally synchronous changes, as well as on the 10,000-100,000year scale. The major exception was from 33 Ma to 14 Ma when the Antarctic had a major ice sheet and the Arctic did not. The last glacial cycle saw major Northern Hemisphere ice sheets in North America and Fennoscandia that only disappeared in the Holocene. The post-glacial warming was interrupted by the 1300-year-long Younger Dryas cold event, particularly around the North Atlantic sector. In the last millennium the same sector experienced the LIA. In the early twentieth century the northern North Atlantic warmed strongly. From the 1980s, there is a marked global warming signature, strongly amplified in the Arctic. Arctic glaciers, the Greenland ice sheet, and particularly Arctic sea ice all are showing a marked response to this warming. In contrast, in southern high latitudes only the Antarctic Peninsula has displayed substantial warming over the last five decades and this is reflected in the retreat or collapse of ice shelves. Antarctic sea ice has shown minor changes.
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