Arctic experts say things are not that simple. More than a dozen experts said in interviews that the extreme summer ice retreat had revealed at least as much about what remains unknown in the Arctic as what is clear. Still, many of those scientists said they were becoming convinced that the system is heading toward a new, more watery state, and that human-caused global warming is playing a significant role.

For one thing, experts are having trouble finding any records from Russia, Alaska or elsewhere pointing to such a widespread Arctic ice retreat in recent times, adding credence to the idea that humans may have tipped the balance. Many scientists say the last substantial warming in the region, peaking in the 1930s, mainly affected areas near Greenland and Scandinavia.

Some scientists who have long doubted that a human influence could be clearly discerned in the Arctic’s changing climate now agree that the trend is hard to ascribe to anything else.

“We used to argue that a lot of the variability up to the late 1990s was induced by changes in the winds, natural changes not obviously related to global warming,” said John Michael Wallace, a scientist at the University of Washington. “But changes in the last few years make you have to question that. I’m much more open to the idea that we might have passed a point where it’s becoming essentially irreversible.”

Image Arctic Study Researchers haul a buoy across the Arctic sea ice in August, led by two Coast Guard crew whose job was to ward off polar bears or rescue anyone who slipped into the sea. Credit... McKenzie Funk

Experts say the ice retreat is likely to be even bigger next summer because this winter’s freeze is starting from such a huge ice deficit. At least one researcher, Wieslaw Maslowski of the Naval Postgraduate School in Monterey, Calif., projects a blue Arctic Ocean in summers by 2013.

In essence, Arctic waters may be behaving more like those around Antarctica, where a broad fringe of sea ice builds each austral winter and nearly disappears in the summer. (Reflecting the different geography and dynamics at the two poles, there has been a slight increase in sea-ice area around Antarctica in recent decades.)