<P> An ice age is a period of long - term reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers . Within a long - term ice age, individual pulses of cold climate are termed "glacial periods" (or alternatively "glacials" or "glaciations" or colloquially as "ice age"), and intermittent warm periods are called "interglacials". In the terminology of glaciology, ice age implies the presence of extensive ice sheets in both northern and southern hemispheres . By this definition, we are in an interglacial period--the Holocene--of the ice age . The ice age began 2.6 million years ago at the start of the Pleistocene epoch, because the Greenland, Arctic, and Antarctic ice sheets still exist . </P> <P> In 1742 Pierre Martel (1706--1767), an engineer and geographer living in Geneva, visited the valley of Chamonix in the Alps of Savoy . Two years later he published an account of his journey . He reported that the inhabitants of that valley attributed the dispersal of erratic boulders to the glaciers, saying that they had once extended much farther . Later similar explanations were reported from other regions of the Alps . In 1815 the carpenter and chamois hunter Jean - Pierre Perraudin (1767--1858) explained erratic boulders in the Val de Bagnes in the Swiss canton of Valais as being due to glaciers previously extending further . An unknown woodcutter from Meiringen in the Bernese Oberland advocated a similar idea in a discussion with the Swiss - German geologist Jean de Charpentier (1786--1855) in 1834 . Comparable explanations are also known from the Val de Ferret in the Valais and the Seeland in western Switzerland and in Goethe's scientific work . Such explanations could also be found in other parts of the world . When the Bavarian naturalist Ernst von Bibra (1806--1878) visited the Chilean Andes in 1849--1850, the natives attributed fossil moraines to the former action of glaciers . </P> <P> Meanwhile, European scholars had begun to wonder what had caused the dispersal of erratic material . From the middle of the 18th century, some discussed ice as a means of transport . The Swedish mining expert Daniel Tilas (1712--1772) was, in 1742, the first person to suggest drifting sea ice in order to explain the presence of erratic boulders in the Scandinavian and Baltic regions . In 1795, the Scottish philosopher and gentleman naturalist, James Hutton (1726--1797), explained erratic boulders in the Alps by the action of glaciers . Two decades later, in 1818, the Swedish botanist Göran Wahlenberg (1780--1851) published his theory of a glaciation of the Scandinavian peninsula . He regarded glaciation as a regional phenomenon . </P>

What causes an ice age to take place