In 1840, Louis Agassiz made the outrageous suggestion that ice once blanketed large parts of Europe and North America. Thus was born the glacial theory. Over a century and a half later, scientists are still asking the question: What causes the Earth to freeze? It remains a weighty problem with no completely satisfactory answer. It involves several related questions: What triggered the last Ice Age and why was it so action-packed, with alternating cold and warm stages? Why did several glacial episodes in the past freeze the entire planet? Why does the Earth's climate system alternate between hothouse and icehouse states?
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