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There is a growing understanding as well that ice ages are not uniformly icy, nor interglacial periods unchangingly warm. About 40,000 years ago, for example, right in the middle of the last Ice Age, the world warmed briefly, forcing glaciers to retreat. And while the current interglacial period has been stably temperate, the previous one, according to at least one study, was evidently interrupted by frigid spells lasting hundreds of years. If that period was more typical than the present one, humanity's invention of agriculture, and thus civilization, may have been possible only because of a highly unusual period of stable temperature -- a fluke.
Just 150 years ago, the notion that much of the Northern Hemisphere had once been covered by thick sheets of ice was both new and highly controversial. Within a few decades, though, most scientists were convinced and began looking for explanations. Several suggested that astronomical cycles were involved, and by the 1930s the Yugoslav astronomer Milutin Milankovitch had constructed a coherent theory. The ice ages, he argued, were triggered by changes in the shape of the earth's slightly oval orbit around the sun and in the planet's axis of rotation. Studies of the chemical composition of ocean-floor sediments, which depend on climate conditions when the material was laid down, more or less supported Milankovitch's predicted schedule of global glaciation.
According to Milankovitch cycles, an ice age could start sometime within the next 1,000 or 2,000 years. But geophysicists have realized for years that while the cycles are real, and influence climate, they alone cannot explain ice ages. For one thing, Milankovitch's timing of glaciation may be broadly correct, but major glacial episodes happen when his cycles call for minor ones, and vice versa.
Besides, a simple astronomical model would predict smooth and gradual climate transitions -- the opposite of what really happens. The last Ice Age was in full retreat about 13,000 years ago when temperatures suddenly reversed and began heading lower again. They stayed low for 1,000 years, an episode known as the Younger Dryas period. The periodic "spikes" of warmer weather that have interrupted ice ages and the cold weather that often came on suddenly in the last interglacial period are also impossible to explain with astronomy. And so is the astonishingly rapid changeover from warm to cold.
A number of theories have been floated to explain these irregular, rapid variations. The leading one, advanced by Lamont-Doherty's Wallace Broecker and George Denton of the University of Maine, involves a kind of cyclic ocean current that has been likened to a conveyer belt. Broecker and Denton note that a stream of unusually salty (and thus especially dense) water flows underneath the Gulf Stream as it moves from the tropics to the North Atlantic. When this salty stream reaches the far north, it is forced to the surface as water above it is blown aside by the winds; it then discharges its tropical heat into the arctic air, cools off and sinks to the bottom, where it returns to the tropics to be heated again.