The great ice fields of the last glacial advance loomed across the entire North American continent and at their maximum around 20,000 years ago constituted an impenetrable barrier to human migration. The glaciers themselves served as barometers to ice-age peoples and when they began to melt about 15,000 years ago they signaled the onset of approaching climate change.

      Even today, I note that the question of where the ice is located still resonates in my personal life. In my small world of mountains and wilderness, recognition of the hard face of modern global warming comes in large measure not from reading the pile of scientific research which I receive most every day, but from old friends and some new ones who climb mountains. I am blessed to call among my friends a handful of the world’s great mountain climbers. They climb in British Columbia, Alaska, the Andes, Africa and, most of all, in the Himalayas. My small forays and treks along the Continental Divide of the Northern Rockies and coastal British Columbia are meek in comparison to their expeditions to the high white wilderness of glaciers that mark the roof of the world. But the topic is the same: the disappearance of the world’s glaciers, especially the finite ones we know well and explore like the body of a secret lover.

      My own fragile and diminutive mistresses lie along a great traverse of grizzly bear country along the Continental Divide in Montana’s Glacier National Park. The trek starts from the paved road the locals call “Going to the Six-Pack Highway” and departs from all trails as it leads you north towards Canada. It’s not an easy bushwhack and the weather can shut you down. I’ve made the traverse over a half-dozen times beginning forty years ago, often alone or with a trusted friend and, the last, just a few years ago, an abbreviated trip with my own daughter.

      As you climb up to the divide and wind around the peaks, Glacier’s famous ice fields come into view. Most are tiny hanging glaciers clinging to the northeast-facing cliffs. A few of the larger ones have names: Vulture, Gyrfalcon or Two Ocean. You walk on snowfields and the tundra of the high country, where you can scarcely put a boot down without stepping on a grizzly dig. You finally scramble your way out of a perfect glacial cirque punctuated with a circular turquoise tarn, using your ice axe here and there for balance and backsliding, and step onto the Continental Divide. It opens up into a narrow expanse of subalpine splendor with stunted fir trees framing chains of shallow mountain lakes. Grizzly sign is everywhere; some of the high meadows look plowed by the long-clawed bears who dig corms and tubers from the thin soil.

      Once I followed a well-used animal trail around the shore of small lake up there and almost stumbled into a garbage can-sized dish-shaped depression next to a tree; the grass at the edge of the grizzly daybed sprang upright from the weight of the bear who had just arose from his nap. Considerably more cautious, I crept down the trail. That’s way too close.

      In the 1970s, the small glaciers at the head of Valentine Creek still contained blue ice. By the late 1980s, these glaciers had become mere snowfields. Twenty years later, the snow was gone. Now, the ribbon of ice above lonely Gyrfalcon Lake is but a shadow of its hefty parent only thirty-five years ago. On my last traverse of the basin, I climbed up to the hanging glacier and broke off an icicle, sucking the water out of it like a last good kiss.

      All this is but a brief moment in a single life watching small, beloved ice-fields shrink and die. A similar tragedy is playing out in the Wind River Range of Wyoming, where large glaciers, though diminished, still flow. My friends tell similar stories of the giant ice fields of the Himalayas. It is a tale of loss, of the end of something beautiful in a melting century.

      One can imagine the minds of ice-age explorers dancing with images of glaciers. The first Americans of the Late Pleistocene would not have walked or boated down the icy defiles without a distinct foreboding of approaching change and an appreciation of the coming need for humans to pioneer new habitats, to confront these never-before-seen animals and endure shifting climates.

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      The Pleistocene or Great Ice Age lasted about 2 million years (published estimates run from 1.6 to 2.6 million years), during which the northern polar caps surged and ebbed on a cycle of every 100,000 years or so. The last advance, the Wisconsin, started almost a hundred thousand years back in North America and, within that cycle, oscillations in the ice produced smaller advances and retreats. The Late Wisconsin, 12,000 to 38,000 years ago, was generally a time of expanding glaciers that reached their maximums about 20,000 years ago. This period is when humans first show up in North America.

      Non-polar glaciers often have their origins in the high country when a cooling climate dumps more snow in the mountains in winter than melts during summer—accumulation exceeds ablation. As the snow builds up over decades and centuries, the pressure causes snow to granulate. The weight of ice compacts the glacier until, like a plastic, it begins to flow downhill. The rivers of mountain glaciers may coalesce and become ice sheets, like the Cordilleran of western North America.

      The history of ice in North America is important because it outlines the great mysteries of continental colonization: Who were the first Americans, when and how did they get here and what routes did they take to get south of the ice? Since the archaeological record from the American Late Pleistocene is not robust, reconstruction of ancient environmental habitats contributes substantially to understanding when people could live there. And these habitats depended on where the glaciers were. The ice altered environments and climate. Constantly changing vegetative communities, chewed into a patchwork by the huge Pleistocene grazers and browsers, were the norm. The presence of the great ice sheets carved these plant communities into arrangements for which there are no modern equivalents.

      Northern North America probably lay covered by ice during the Last Glacial Maximum (about 18,000 to 20,000 years ago). The Laurentide ice sheet buried the land from the North Atlantic coast to the Rocky Mountain trench, south into Ohio. The Cordilleran ice sheet came out of the Coastal Range and Rocky Mountains, huge valley glaciers coalescing into a composite sheet, smaller than the Laurentide, covering northwestern North America. Temperatures in Greenland during the last glacial maximum were 41 degrees Fahrenheit cooler than today. A key issue for archaeologists is whether there was a corridor between the two sheets sufficiently benign to permit human migrations coming south out of Beringia. Textbook maps often show the two great ice-sheets conjoined. Glacier erratic trains (boulders carried by the glacial ice) on the foothills of the Northern Rockies indicate this was the case 18,000 years ago. But there is conflicting evidence. The Ice Free Corridor (IFC), and other pre-last glacial maximum routes between the ice sheets, could have been open much of the time during the past 30,000 years. The debate continues.

      People moved around the edges of these glaciers, looking for food and shelter from the cold. The Greatest Adventure began whenever those bands of Siberian hunters moved east across Beringia into present day Alaska. That crossing could have taken place anytime beginning about 30,000 years ago. The Bering Strait land bridge was almost certainly open 10,000-27,000 years back. Dating for the last stages of the Ice Age is imprecise; the geography of ice in North America, which areas might have been ice-free, is not clear. Many archaeologists believe the far north of Late Pleistocene Siberia was too damn cold for people to live there. Since no sites dating between 30,000 and about 14,000 years ago have been found in northeastern Siberia, some scientists think these children of the ice retreated south to sub-Arctic central Siberia during this time. Similar claims about the inhospitable climate—bleak, frigid, uninhabitable—are made for the same period in Alaska.