The Arctic ice cap shrank so much this summer that waves briefly lapped along two long-imagined Arctic shipping routes, the Northwest Passage over Canada and the Northern Sea Route over Russia.
Over all, the floating ice dwindled to an extent unparalleled in a century or more, by several estimates.
Now the six-month dark season has returned to the North Pole. In the deepening chill, new ice is already spreading over vast stretches of the Arctic Ocean. Astonished by the summer's changes, scientists are studying the forces that exposed one million square miles of open water, or 2.6 million square kilometers, beyond the average since satellites started measurements in 1979.
At a recent gathering of sea-ice experts in Fairbanks, Alaska, a geophysicist summarized it this way: "Our stock in trade seems to be going away." Scientists are also unnerved by the summer's implications for the future, and their ability to predict it.
Complicating the picture, the dramatic Arctic change was as much a result of ice moving as melting, many say. A study led by Son Nghiem at NASA, and appearing this week in Geophysical Research Letters, used satellites and buoys to show that winds since 2000 had pushed huge amounts of thick old ice out of the Arctic basin past Greenland. The thin floes that formed on the resulting open water melted quicker or could be shuffled together by winds and similarly expelled, the authors said.
The pace of change has far exceeded what had been estimated by almost all of the supercomputer simulations used to envision how the Arctic will respond to rising concentrations of greenhouse gases linked to global warming. But that disconnect between observations and projections can cut two ways: Are the models overly conservative? Or are they missing natural influences that can cause wide swings in ice and temperature, thereby dwarfing the slow background warming?
The world is paying more attention than ever. Russia, Canada and Denmark, prompted in part by years of warming and the ice retreat this year, ratcheted up rhetoric and actions aimed at securing sea routes and seabed resources.
Proponents of cuts in greenhouse gases cited the dramatic meltdown as proof that human activities are propelling a slide toward climate calamity.
Arctic analysts say things are not that simple. More than a dozen specialists on the Arctic said in interviews that the extreme summer ice retreat has 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, analysts 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.
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."
Analysts say that next summer is quite likely to see an even bigger ice retreat because this winter's freeze is starting from such a huge ice deficit. At least one researcher, of the Naval Postgraduate School in Monterey, California, 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 disappears entirely in the summer.
While open Arctic waters could be a boon for shipping, fishing and oil exploration, an annual seesawing between ice and no ice could be a particularly harsh jolt to polar bears.
Many researchers warned that it was still far too soon to start sending container ships over the top of the world.
"Natural variations could turn around and counteract the greenhouse-gas-forced change, perhaps stabilizing the ice for a bit," said Marika Holland of the National Center for Atmospheric Research in Boulder, Colorado.
But, she added, that will not last. "Eventually the natural variations would again reinforce the human-driven change, perhaps leading to even more rapid retreat," Holland said. "So I wouldn't sign any shipping contracts for the next five to 10 years, but maybe the next 20 to 30."
While analysts debate details, many agree that the vanishing act of the sea ice this year was probably caused by superimposed forces including heat-trapping clouds and water vapor in the air and the ocean-heating influence of unusually sunny skies in June and July.
Other factors were warm winds flowing from Siberia around a high-pressure system parked over the ocean. The winds not only would have melted thin ice but also pushed floes offshore where currents and winds could push them out of the Arctic Ocean.
But another factor was probably involved, one with roots going back to about 1989.
At that time, a periodic flip in winds and pressure patterns over the Arctic Ocean, called the Arctic Oscillation, settled into a phase that tended to stop ice from drifting in a gyre for years, so it could thicken, and instead carried it out to the North Atlantic.
The new NASA study of expelled old ice builds on previous measurements showing the proportion of thick, durable floes that were at least 10 years old dropped to 2 percent this spring from 80 percent in the spring of 1987, said Ignatius Rigor, an ice expert at the University of Washington and an author of the new NASA-led study.
Without the thick ice, which can endure months of nonstop summer sunshine, more dark open water and thin ice absorbed solar energy, adding to melting and delaying the winter freeze.