OVER THE CHUKCHI SEA, Alaska: North of the Bering Strait, a film of new ice is filling gaps between bigger ice chunks. The sea surface was only starting to freeze up in late November, even though the winter sun slips beneath the horizon at about noon.

The freeze is overdue, experts say. "A couple of weeks ago, there was no ice at all," US Coast Guard Rear Admiral Chris Colvin, head of the agency's Alaska operations, said over the roar of the C-130 aircraft lumbering over the coastline.

That's why the Coast Guard is here, far north of its usual territory where it patrols commercial fishing grounds and the daily traffic of cruise ships, cargo ships and oil tankers.