While satellite tracking of polar sea ice has been done only since 1979, several ice experts who have studied Russian and Alaskan records going back many 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 we observe today” in the 1930s or 1940s, said Igor Polyakov, an ice expert at the University of Alaska, Fairbanks.

Image A satellite image from last Saturday shows shrinking ice opening Canadas Northwest Passage. Scientists said on Thursday that this years ice retreat was probably unmatched in the 20th century. Credit... NASA, via Associated Press

The ice retreat has been particularly striking this year. The Alaskan side of the Arctic Ocean has stretches of thousands of square miles of open water; the fabled Northwest Passage through the islands of northern Canada was free of ice for weeks; and the sea route between the Atlantic and Pacific Oceans north of Russia was nearly clear a week ago, with one small clot of ice around a group of Siberian islands.

Mark Serreze, a senior researcher at the snow and ice center, said it was increasingly clear that climate change from the buildup of greenhouse gases was playing a role in the Arctic warming, which is seen not only in the floating ice but also in melting terrestrial ice sheets, thawing tundra and warming seawater.

“We understand the physics behind what’s going on,” Dr. Serreze said. “You can always find some aspect of natural variability that can explain some things. But now it seems patterns that used to help you don’t help as much anymore, and the ones that hurt you hurt you more.”

“You can’t dismiss this as natural variability,” he said. “We’re starting to see the system respond to global warming.”