SCOTT BASE, Antarctica — A group of hikers in red parkas approached a half-dozen seals resting on floating sea ice. The leader of the entourage — Secretary of State John Kerry — raised his arms and ordered everyone to halt.

As an ethereal silence descended, Mr. Kerry cocked his head in the stillness of one of the world’s last truly wild places.

In that moment, the frozen landscape seemed timeless, but it is actually in grave peril, as Mr. Kerry had been told by scientists only minutes before. The ice across large parts of West Antarctica may be starting to disintegrate because of global warming, and if it goes, the world’s coastal cities face destruction, too.

The presence of Mr. Kerry, the highest-ranking United States government official ever to visit Antarctica, lifted the morale of scientists working to understand the icebound continent. Yet the visit, at the end of last week, was shadowed by anxiety.