The cap of floating sea ice on the Arctic Ocean, which retreats during summer, shrank more than one million square miles this year, below the average minimum ice area reached in recent decades.
The scientists noted that the ice retreated to 1.59 million square miles, the minimum ice area for this year, till Sunday. The ice is now spreading again under the influence of the deep Arctic chill that settles in as the sun drops below the horizon at the North Pole for six months.
The findings were reported by the National Snow and Ice Data Center in Boulder, Colo and posted online at www.nsidc.org.
Ice experts, who have studied Russian and Alaskan records for decades, said the ice retreat this year was probably unmatched in the 20th century, including during a warm period in the 1930s.
''I do not think that there was anything like what we observe today in the 1930s or 1940s,'' said Igor Polyakov, an ice expert at the University of Alaska, Fairbanks.
The ice retreat has been striking this year with the Alaskan side of the Arctic Ocean having stretches of thousands of square miles of open water, the Northwest Passage through islands of northern Canada remaining free of ice for weeks and the sea route between the Atlantic and Pacific Oceans north of Russia having only a small clot of ice around a group of Siberian islands.
Mark Serreze, a senior researcher at the snow and ice center, said it was clear that climate change due to increasing greenhouse gases was playing a role in the Arctic warming. This resulted in melting terrestrial ice sheets, thawing tundra and warming seawater.
''You can't dismiss this as natural variability,'' Dr Serreze said, adding that, "the world is starting to see the system respond to global warming.'' Sea ice around Antarctica has seen unusual winter expansions recently, and this week was near a record high.