I would say that the climax occurs when Sam has a frightening experience when he is covered up in the wall of snow. If you have seen the film, you should be aware of some key differences in the climactic scene, as outlined here at eNotes:
"In the film, Sam is trapped by a wall of snow and is rescued by Bando. After his close encounter with death, Sam decides it is time to go home—"I've learned all I can," he says. This differs from the novel, in which Sam escapes simply by poking his head up into the soft snow and standing up, and in which Sam's solitary sojourn is interrupted by his family's decision to move to the woods."