What do the Everglades and Wrangell-St. Elias–two iconic national parks at the far ends of North America–have in common? According to scientists, both face a grave future as temperatures rise. To see the impact up close, our reporter pushes deep into the backcountry, discovering landscapes both beautiful and threatened beyond his imagination.

In August 1999, British artist Hamish Fulton set out on a long hike through the mountains of Wrangell-St. Elias National Park, in southeast Alaska. Fulton's destination was Iceberg Lake, a crystalline body of glacier-dammed water in a remote valley above the Bagley Icefield.

Fulton had a week of rough backpacking behind him when he crested a low pass on a sunny morning and paused for his first look at the lake. On the near side of the valley, a creek emerged from an alpine glacier. But instead of flowing into a blue lake, the stream meandered across a plain of gray mud, then vanished into a field of dripping icebergs. Iceberg Lake had disappeared.

Although many of Alaska's glacier-dammed lakes drain occasionally, Iceberg Lake was so stable that local pilots, climbers, and rangers had considered it permanent. The news of Iceberg Lake's demise soon made its way to Mike Loso, a geologist at Alaska Pacific University. Loso traveled to the scene to examine the lakebed's sediment, which is set down in layers that coincide with annual cycles, similar to rings in a tree. What Loso found surprised him. "There was no evidence of the lake draining at any time during the last 1,500 years," Loso says. "It appears that 20th-century warming is more intense, and accompanied by more extensive glacier retreat, than at any other time in the last 1,500 years....