The Fate of Greenland: Lessons from Abrupt Climate Change

1. Mystery of the Ice Ages

1 MYSTERY OF THE ICE AGES The Theory of Ice Ages Tourists love to get near glaciers, but most people prefer to live elsewhere, and for good reasons. Strong, cold winds often drain down-valley from the icy glaciers . The soils just beyond the ice are usually rocky and poor, and often there is no soil at all, just bare rock or gravel. A hungry farmer looks elsewhere for a new row to hoe. Centuries ago, the people who lived near glaciers, whether high in European valleys or elsewhere, knew that the glaciers grew and shrank with the changing climate. At least some of those people realized that the stony soils, bare rocks, and other features being formed by the glaciers were nearly identical to features down the valley, and those people correctly inferred that the glaciers had once been bigger. But in eighteenth-century Europe, many university scientists did not seriously consider the possibility that they lived on the deposits of glaciers. The evidence was everywhere, but the idea seemed monstrously strange to many, and only a few were engaged in asking why the landscape looked as it did. A glacier moving down a valley may bulldoze stones and soil in front of it, scrapingsurficialdepositsdowntobedrock.Meltingontheglaciersurfacecreates Figure 1.1 This glacier was larger during the Little Ice Age, and is melting back. The stream fans out towards the lower right of the picture, just below the dark-colored pile of glaciertransported rocks, called a moraine, marking the maximum size of the glacier during the Little Ice Age. (Photo by Philip Conkling) 29 MYSTERY OF THE ICE AGES streams that drain through great holes in the ice to feed steep, turbulent rivers beneath the ice that further clean the bedrock. A cautious person standing by such a river where it emerges from under the glacier can hear boulders knocking together in the current, and an incautious wader risks a crushed ankle, or worse. The glacier-fed rivers often pile gravel benches or terraces along their paths down the valley. Beneath a glacier, the moving ice picks up boulders and smaller rocks and drags them over the bedrock beneath, scratching and polishing the rock in many places. In other places, the glacier breaks chunks loose and shoves them into the bedrock to make beautiful crescent-shaped cracks. The glacier usually outlines itself with a pile of mixed-up big and little rocks, and the sides of some of those rocks have been worn flat and scratched or polished against bedrock before emerging from beneath the ice. Rocks left behind by a glacier may have been carried long distances in the ice, and many different rock types often are mixed together in deposits of the glacier. A wealth of other features—layers of mixed-up rock and soil, ponds left by the slow melting of ice blocks buried in the glacially transported material, sinuous ridges marking the former courses of subglacial streams, valley bottoms eroded by the moving ice into broad “U” shapes with waterfalls cascading down as in Yosemite or the fjords of Norway—all these features and more testify to the former presence of glaciers. But to people who had never seen or studied a glacier, and who may have been raised to believe in a young Earth that had changed little over time, the idea of a vast sea of ice overrunning much of the modern world must have seemed too fantastic for serious consideration. Scotland and England, Scandinavia, the lowlands far beyond the modern glaciers of the Alps, and other parts of Europe bore features from glaciers, but these features were misinterpreted for centuries by peoplewithtoolittleexperienceinthemountainsandtoolittletrustintheirown science. The odd boulders carried far from their sources came to be called “drift,” based on the mistaken idea that they drifted into place in icebergs unleashed by Noah’s flood, perhaps scratching the bedrock on their tumbling paths. For scientists who had not yet visited the polar regions to observe that drifting icebergs produce very different features remarkably unlike those observed across the European landscape, this story sufficed for a time. We now know that the rounded hills of Scotland, the Matterhorn, the waterfalls of Yosemite, the Great Lakes of the United States and Canada, the Ten Figure 1.2 Except in especially cold places where ice is frozen to the rocks beneath, glaciers usually erode the landscape more rapidly than rivers or wind. Valleys that are “Ushaped ” in...