The floating cap of sea ice on the Arctic Ocean shrank this summer to what is probably its smallest size in at least a century of record keeping, continuing a trend toward less summer ice, a team of climate experts reported yesterday.

That shift is hard to explain without attributing it in part to human-caused global warming, the team’s members and other experts on the region said.

The change also appears to be headed toward becoming self-sustaining: the increased open water absorbs solar energy that would otherwise be reflected back into space by bright white ice, said Ted A. Scambos, a scientist at the National Snow and Ice Data Center in Boulder, Colorado, which compiled the data along with the National Aeronautics and Space Administration.

“Feedbacks in the system are starting to take hold,” Dr. Scambos said.