The Arctic ( or )〔(American Heritage Dictionary )〕 is a polar region located at the northernmost part of the Earth. The Arctic consists of the Arctic Ocean and parts of Alaska (United States), Canada, Finland, Greenland (Denmark), Iceland, Norway, Russia, and Sweden. The Arctic region consists of a vast ocean with a seasonally varying ice cover, surrounded by treeless permafrost. The area can be defined as north of the Arctic Circle (66° 33'N), the approximate limit of the midnight sun and the polar night. Alternatively, it can be defined as the region where the average temperature for the warmest month (July) is below ; the northernmost tree line roughly follows the isotherm at the boundary of this region.〔("arctic." ) ''Dictionary.com Unabridged (v 1.1).'' Random House, Inc. Retrieved 2 May 2009.〕
Socially and politically, the Arctic region includes the northern territories of the eight Arctic states, although by natural science definitions, much of this territory is considered subarctic. The Arctic region is a unique area among Earth's ecosystems. For example, the cultures in the region and the Arctic indigenous peoples have adapted to its cold and extreme conditions. In recent years, the extent of the sea ice has declined.〔〔("Global Sea Ice Extent and Concentration: What sensors on satellites are telling us about sea ice." ) National Snow and Ice Data Center. Retrieved 1 May 2009.〕 Life in the Arctic includes organisms living in the ice, zooplankton and phytoplankton, fish and marine mammals, birds, land animals, plants and human societies.〔Christopher Krembs and Jody Deming. ("Organisms that thrive in Arctic sea ice." ) National Oceanic and Atmospheric Administration. 18 November 2006.〕
== Etymology ==
The word Arctic comes from the Greek word ἀρκτικός (''arktikos''), "near the Bear, northern"〔Henry George Liddell and Robert Scott. ("Arktikos." ) ''A Greek-English Lexicon''. Perseus Digital Library.〕 and that from the word ἄρκτος (''arktos''), meaning bear.〔Henry George Liddell and Robert Scott. ("Arktos." ) ''A Greek-English Lexicon''. Perseus Digital Library.〕 The name refers either to the constellation Ursa Major, the "Great Bear", which is prominent in the northern portion of the celestial sphere, or to the constellation Ursa Minor, the "Little Bear", which contains Polaris, the Pole star, also known as the North Star.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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