The Quaternary glaciation, also known as the Pleistocene glaciation, is an alternating series of glacial and interglacial periods during the Quaternary period that began 2.58 Ma (million years ago) and is ongoing. Although geologists describe the entire time period up to the present as an “ice age“, in popular culture the term “ice age” is usually associated with just the most recent glacial period during the Pleistocene or the Pleistocene epoch in general. Since planet Earth still has ice sheets, geologists consider the Quaternary glaciation to be ongoing, with the Earth now experiencing an interglacial period.
During the Quaternary glaciation, ice sheets appeared. During glacial periods they expanded, and during interglacial periods they contracted. Since the end of the last glacial period, the only surviving ice sheets are the Antarctic and Greenland ice sheets. Other ice sheets, such as the Laurentide Ice Sheet, formed during glacial periods, had completely melted and disappeared during interglacials. The major effects of the Quaternary glaciation have been the erosion of land and the deposition of material, both over large parts of the continents; the modification of river systems; the creation of millions of lakes, including the development of pluvial lakes far from the ice margins; changes in sea level; the isostatic adjustment of the Earth’s crust; flooding; and abnormal winds. The ice sheets themselves, by raising the albedo (the extent to which the radiant energy of the Sun is reflected from Earth) created significant feedback to further cool the climate. These effects have shaped entire environments on land and in the oceans, and in their associated biological communities.
Before the Quaternary glaciation, land-based ice appeared, and then disappeared, during at least four other ice ages.
Evidence for the quaternary glaciation was first understood in the 18th and 19th centuries as part of the scientific revolution.
Over the last century, extensive field observations have provided evidence that continental glaciers covered large parts of Europe, North America, and Siberia. Maps of glacial features were compiled after many years of fieldwork by hundreds of geologists who mapped the location and orientation of drumlins, eskers, moraines, striations, and glacial stream channels in order to reveal the extent of the ice sheets, the direction of their flow, and the locations of systems of meltwater channels. They also allowed scientists to decipher a history of multiple advances and retreats of the ice. Even before the theory of worldwide glaciation was generally accepted, many observers recognized that more than a single advance and retreat of the ice had occurred.
To geologists, an ice age is marked by the presence of large amounts of land-based ice. Prior to the Quaternary glaciation, land-based ice formed during at least four earlier geologic periods: the Karoo (360–260 Ma), Andean-Saharan (450–420 Ma), Cryogenian (720–635 Ma) and Huronian (2,400–2,100 Ma).
Within the Quaternary Period, or ice age, there were also periodic fluctuations of the total volume of land ice, the sea level, and global temperatures. During the colder episodes (referred to as glacial periods, or simply glacials) large ice sheets at least 4 km (2.5 mi) thick at their maximum existed in Europe, North America, and Siberia. The shorter and warmer intervals between glacials, when continental glaciers retreated, are referred to as interglacials. These are evidenced by buried soil profiles, peat beds, and lake and stream deposits separating the unsorted, unstratified deposits of glacial debris.
Initially the fluctuation period was about 41,000 years, but following the Mid-Pleistocene Transition it has slowed to about 100,000 years, as evidenced most clearly by ice cores for the past 800,000 years and marine sediment cores for the earlier period. Over the past 740,000 years there have been eight glacial cycles.
The entire Quaternary Period, starting 2.58 Ma, is referred to as an ice age because at least one permanent large ice sheet—the Antarctic ice sheet—has existed continuously. There is uncertainty over how much of Greenland was covered by ice during each interglacial.
Currently, Earth is in an interglacial period, which marked the beginning of the Holocene epoch. The current interglacial began between 15,000 and 10,000 years ago; this caused the ice sheets from the last glacial period to begin to disappear. Remnants of these last glaciers, now occupying about 10% of the world’s land surface, still exist in Greenland, Antarctica and some mountainous regions.
During the glacial periods, the present (i.e. interglacial) hydrologic system was completely interrupted throughout large areas of the world and was considerably modified in others. Due to the volume of ice on land, sea level was about 120 metres (394 ft) lower than present.