SERGEY ZIMOV, AN ECOLOGIST BY training, tossed a woolly mammoth bone on the pile. He was squatting in mud along the cool, wide Kolyma River, below a towering cliff of crumbling earth. It was summer in eastern Siberia, far above the Arctic Circle, in that part of Russia that’s closer to Alaska than to Moscow. There wasn’t a speck of frost or snow in sight. Yet at this cliff, called Duvanny Yar, the Kolyma had chewed through and exposed what lies beneath: a layer of frozen ground, or permafrost, that is hundreds of feet deep—and warming fast. ¶ 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…