ENORMOUS ICE SHEETS hav, at times, spread across the globe, bringing with them a cold climate. One definition holds that the advance of glaciers to middle latitudes marks an ice age. Another, and perhaps complementary, definition holds that the spread of glaciers to encompass roughly one-third of the planet is the signature of an ice age. Glaciers have advanced and retreated from Earth s surface and from the ocean several times. The most exuberant estimate tallies 17 ice ages, A more conservative estimate is that Earth has endured the tumult of six ice ages. Scholars have debated not only the number of ice ages, but also their causes. The causes fall into three categories: astronomical, atmospheric, and terrestrial. The recognition of ice ages is recent. ...

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