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A photo provided by Steffen Graupner and MOSAiC shows a German icebreaker taking part in a large multidisciplinary polar research expedition in the Arctic Ocean, in August.

STEFFEN GRAUPNER/The New York Times News Service

The effects of global warming in the Arctic are so severe that the region is shifting to a different climate, one characterized less by ice and snow and more by open water and rain, scientists said Monday.

Already, they said, sea ice in the Arctic has declined so much that even an extremely cold year would not result in as much ice as was typical decades ago. Two other characteristics of the region’s climate, seasonal air temperatures and the number of days of rain instead of snow, are shifting in the same way, the researchers said.

The Arctic is among the parts of the world most influenced by climate change, with sharply rising temperatures, thawing permafrost and other effects in addition to shrinking sea ice. The study, by Laura Landrum and Marika M. Holland of the National Center for Atmospheric Research in Boulder, Colo., is an effort to put what is occurring in the region in context.

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“Everybody knows the Arctic is changing,” said Dr. Landrum, a climate scientist and the lead author of the study, published in the journal Nature Climate Change. “We really wanted to quantify if this is a new climate.”

Scientists said the vast and ancient ice sheet sitting atop Greenland had shed a 113-square-kilometre chunk of ice last month. Reuters

In other words, she said, “has the Arctic changed so much and so fast that the new climate cannot be predicted from the recent past?”

Using years of observational data from the region and computer models, the researchers found that sea ice is already in a new climate, in effect: The extent of ice in recent years is consistently less than what would be expected in even the worst year for ice in the mid-20th century.

Arctic sea ice has declined by about 12 per cent per decade since satellite measurements began in the late 1970s, and the 13 lowest sea-ice years have all occurred since 2007. This year is expected to be a record or near-record low for ice extent, which will be determined by the end of this month as the summer melt period ends.

For fall and winter air temperatures and rain versus snow days, the simulations found that the transition to a new climate is occurring more slowly, with the shift expected to be complete by the middle of the century.

Overall, Dr. Landrum said, “We are beginning to get to the point where we can no longer know what to expect.”

Jennifer Kay, a climate scientist at the University of Colorado who was not involved in the research, said the new study builds on previous ones that had looked at fewer climate elements.

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“It’s nice to see all those variables discussed,” Dr. Kay said. And determining the timing of the various shifts is an interesting contribution.

But scientists have known for a long time that fundamental changes were occurring in the region. “We know what used to be,” Dr. Kay said. “We call it the ‘new Arctic’ because it’s not the same.”

Dr. Landrum said that Arctic communities are already suffering from the changes. Eroding coastlines are forcing some Alaska Native villages to consider relocating. Other changes are affecting the food supply. Warmer storms that bring rain on existing snow, for example, can lead to starvation of the animals Indigenous groups rely on.

“Arctic climate change is not in the future for them,” she said. “It’s now.”

Dr. Landrum said the climate models used in the study simulated the future in a world where planet-warming emissions of greenhouse gases remained high. That provides some fodder for optimism, she said.

“We still have an opportunity to change how rapidly the Arctic evolves,” she said, “if we end up changing our emissions.”

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“You can’t just give up. If you work hard and make some changes there’s a possibility you’d have some dramatic effects.”

Another study released Monday suggested that two Antarctic glaciers that have long been of concern to scientists over their potential to contribute to sea-level rise may be in worse shape than previously thought.

The Thwaites and Pine Island glaciers are rivers of ice, slowly moving from the West Antarctic Ice Sheet in the continent’s interior to the ocean, where it melts and adds to sea-level rise. In recent decades the two glaciers' movement has accelerated, leading to more ice loss from the interior, largely because of melting by warm water underneath the glaciers.

Even with the acceleration, however, complete melting of this part of the West Antarctic sheet could take centuries.

The new study, published in Proceedings of the National Academy of Sciences, analyzed satellite imagery and found cracks and other signs of stress damage to the glaciers' ice shelves, the leading edges that float on the water.

This evidence of damage, the paper’s authors wrote, is the first sign of structural weakening of the ice shelves, a process that can end in the shelves' disintegration and even faster glacial flow of ice to the ocean. The authors said that incorporating these damage processes into models of ice-sheet dynamics is critical for more accurate assessments of potential sea-level rise.

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