Twigs, other plant matter, and Ice Age animal parts—bison jaws, horse femurs, mammoth bones—spilled onto a beach that sucked at Zimov’s boots. “I love Duvanny Yar,” he said as he yanked fossils from the muck. “It is like a book. Each page is a story about the history of nature.”

Across nine million square miles at the top of the planet, climate change is writing a new chapter. Arctic permafrost isn’t thawing gradually, as scientists once predicted. Geologically speaking, it’s thawing almost overnight. As soils like the ones at Duvanny Yar soften and slump, they’re releasing vestiges of ancient life—and masses of carbon—that have been locked in frozen dirt for millennia. Entering the atmosphere as methane or carbon dioxide, the