Arctic sea ice behaves a bit like a human waistline, packing on weight in the winter and slimming down in the heat of summer. But while many of us struggle to lose weight, the Arctic has been struggling to gain it.

The maximum extent of Arctic sea ice cover this winter was the second-lowest since satellite record-keeping began, researchers said Friday.

The loss of sea ice is a bellwether of global warming, suggesting that climate change is not just something to worry about far off in the future: It is here.

“We’ve probably known for 100 years that as the climate warms up in response to loading the atmosphere with greenhouse gases, we would see the changes first in the Arctic,” said Mark Serreze, director of the National Snow and Ice Data Center in Boulder, Colo., which issued the new data. “This is what we expected and this is exactly what has happened. It’s a case where we hate to say we told you so, but we told you so.”

With each passing decade, the ice grows a bit less in winter, and melts a bit more in summer. The record for the least amount of sea ice gained in the winter was set last year, when the ice covered 5.57 million square miles (14.42 million square kilometers) at its peak. This winter’s maximum extent was slightly greater, at 5.59 million square miles (14.48 million square kilometers), according to the data center.

Despite the small increase this year, the downward trend in winter ice coverage is unmistakable, and the past four years have been the four lowest on record.

Maximum Winter Sea Ice Extent

The disappearing sea ice is a key indicator of a warming Arctic. And the consequences of a warming Arctic can be felt further south. A growing number of researchers are linking the changes up north to unusual winter weather in North America and Europe.

In recent weeks, the Northeastern United States faced four nor'easters in as many weeks, and Western Europe encountered subzero temperatures that were far lower than at the North Pole.

These weather patterns are influenced by the jet stream, the river of wind that encircles the Northern Hemisphere, said Jennifer A. Francis, an Arctic researcher at Rutgers University. Temperature differences between the Arctic and the lower latitudes help create the jet stream. Because the Arctic is warming twice as fast as the rest of the Earth, that temperature difference is getting smaller. As a result, the jet stream is getting weaker and shifting its behavior, sending cold air south from the Arctic and pumping warm air north.

The weakening jet stream also helps keep weather patterns locked in place, Dr. Francis said. “We’ve been in this pattern along the East Coast that is very conducive to the formation of nor’easter-type storms,” she said. “Part of the reason for that is because we’ve had this pattern in the jet stream that’s been so persistent.”

Dr. Francis’s theory is not settled science, Dr. Serreze said. Other researchers have posited that changes in ocean currents in the tropical Pacific are the source of the recent weather events in the midlatitudes.

“But I would say that the weight of evidence is resting on the side of a significant Arctic influence,” Dr. Serreze said.