times during which large portions of the earth's surface were covered with thick glacial ice sheets. In the Pleistocene epoch, in the Carboniferous and Permian periods of the Paleozoic era era, and in Huronian time of the Precambrian era, the earth experienced an overall cooling of the climate, resulting in great ice sheets covering great portions of the oceans and continents. More or less extensive continental glaciations, or glacial advances, may have occurred at other times. The study of glacial periods owed its first impetus to the Swiss-American naturalist Louis Agassiz, whose conception of Pleistocene glaciation was presented in his address before the Helvetic Society (1837) and in his Études sur les glaciers (1840). No satisfactory theory on the cause of glacial periods has yet been accepted. The earliest conception was that the earth's history has been one of progressive cooling, resulting in a major glaciation during the Pleistocene epoch. This concept lost its validity when the existence of earlier glacial periods, after which the earth again became warm, was established. In the mid-1800s, the Scot James Croll, an amateur scientist who later became a Fellow of the Royal Society, realized that the earth's heat balance could change as its orbit alters, thus accounting for the ice ages over time. Milutin Milankovitch, a Serbian mathematician, between World War I and II believed that ice ages occurred because of solar radiation fluctuations due to periodic changes in the earth's axis tilt and in its orbit. Other possible explanations for glacial advances include the changes in the direction of ocean currents, shifting of the continents over the earth's surface, fluctuation in the sun's energy and size, and loss of heat from the earth's surface through reduction of the carbon dioxide content of the atmosphere. One more recent theory postulates that large volcanic eruptions contribute to the short-term cooling of the globe and could increase their effects if many erupted over longer periods of time. One link between volcanoes and climate may be the Little Ice Age (1645–1715), when volcanic activity increased and worldwide temperatures decreased dramatically. Another theory states that dust thrown into the atmosphere by large asteroidal strikes would decrease solar radiation enough to cause global cooling; such a scenario has been invoked to explain the mass extinction that occurred at the end of the Cretaceous period.

See R. F. Flint, Glacial and Quaternary Geology (1971); R. P. Sharp, Living Ice: Understanding Glaciers and Glaciation (1991).

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