In the winter, Thoreau is almost completely isolated, hemmed in by snow. So he is compelled to simplify to a degree he hasn't simplified yet. He's stuck in his cabin for a week at a time without company. Characteristically, Thoreau finds joy and beauty in all that snow. This isn't the kind of Die Hard moment we think of when we hear climax, but in a book about isolation, it certainly serves as the climactic moment.