ClimateWire: For the third time in four years, a dearth of sea ice has forced walruses ashore in Alaska. The lumbering marine mammals normally spend their summers resting on the ice as it floats north, making periodic dives to the ocean floor to forage for food. But this year, as in 2007 and 2009, a lack of ice in the eastern Chukchi Sea has driven thousands of walruses to congregate on land instead. Officials with the U.S. Geological Survey and the Fish and Wildlife Service estimate that …