Ice ages have thick glaciers and ice caps over many lands that today are ice-free. The amount of water on earth doesn’t change, so vast amounts of land ice means there is less water in the oceans and the sea-level is lower.

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When snow doesn’t melt during the summer, it builds up on the land, compressing the lower layers to turn it into ice. That snow has to come from somewhere. Water evaporates from the oceans, the wind blows it over the cold land turning it into snow. Eventually, so much water is evaporated from the oceans it causes their levels to fall.

A: Basically because that ocean water gets locked up in the ice caps.

Within the ice ages (or at least within the last one), more temperate and more severe periods occur. The colder periods are called glacial periods, the warmer periods interglacials, such as the Eemian Stage.

Glacials are characterized by cooler and drier climates over most of the Earth and large land and sea ice masses extending outward from the poles. Mountain glaciers in otherwise unglaciated areas extend to lower elevations due to a lower snow line. Sea levels drop due to the removal of large volumes of water above sea level in the icecaps. There is evidence that ocean circulation patterns are disrupted by glaciations. Since the Earth has significant continental glaciation in the Arctic and Antarctic, we are currently in a glacial minimum of a glaciation. Such a period between glacial maxima is known as an interglacial.

The Earth has been in an interglacial period known as the Holocene for more than 11,000 years. It was conventional wisdom that “the typical interglacial period lasts about 12,000 years,” but this has been called into question recently. For example, an article in Nature argues that the current interglacial might be most analogous to a previous interglacial that lasted 28,000 years. Predicted changes in orbital forcing suggest that the next glacial period would begin at least 50,000 years from now, even in absence of human-madeglobal warming (see Milankovitch cycles). Moreover, anthropogenic forcing from increasedgreenhouse gases might outweigh orbital forcing for as long as intensive use of fossil fuels continues. At a meeting of the American Geophysical Union (December 17, 2008), scientists detailed evidence in support of the controversial idea that the introduction of large-scale rice agriculture in Asia, coupled with extensive deforestation in Europe began to alter world climate by pumping significant amounts of greenhouse gases into the atmosphere over the last 1,000 years. In turn, a warmer atmosphere heated the oceans making them much less efficient storehouses of carbon dioxide and reinforcing global warming, possibly forestalling the onset of a new glacial age.

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