An ice age is a period of long-term reduction in the temperature of Earth's climate, resulting in an expansion of the continental ice sheets, polar ice sheets and mountain glaciers. Glaciologically, ice age is often used to mean a period of ice sheets in the northern and southern hemispheres; by this definition we are still in an ice age (because the Greenland and Antarctic ice sheets still exist). More colloquially, when speaking of the last few million years, ice age is used to refer to colder periods with extensive ice sheets over the North American and Eurasian continents: in this sense, the most recent ice age ended about 10,000 years ago. This article will use the term ice age in the former, glaciological, sense; glacials for colder periods during ice ages; and interglacials for the warmer periods.
Origin of ice age theory:
The idea that in the past glaciers had been far more extensive was folk knowledge in some alpine regions of Europe: Imbrie and Imbrie (1979) quote a woodcutter telling Jean de Charpentier of the former extent of the Swiss Grimsel glacier. Macdougall (2004) claims the person was a Swiss engineer named Ignatz Venetz, but no single person invented the idea. Between 1825 and 1833, Charpentier assembled evidence in support of the concept. According to Macdougall (2004), Charpentier and Venetz disapproved of the ideas of Agassiz who extended their work claiming that most continents were once covered by ice. At this early stage of knowledge, what was being studied were the glacial periods within the past few hundred thousand years, during the current ice age. The existence of ancient ice ages was as yet unsuspected. Evidence for ice ages: There are three main types of evidence for ice ages: geological, chemical, and paleontological. Geological evidence for ice ages comes in various forms, including rock scouring and scratching, glacial moraines, drumlins, valley cutting, and the deposition of till or tillites and glacial erratics. Successive glaciations tend to distort and erase the geological evidence, making it difficult to interpret. It took some time for the current theory to be worked out. The chemical evidence mainly consists of variations in the ratios of isotopes in fossils present in sediments and sedimentary rocks, ocean sediment cores, and for the most recent glacial periods, ice cores. Because water containing heavier isotopes has a higher heat of evaporation, its proportion decreases with colder conditions. This allows a temperature record to be constructed. However, this evidence can be confounded by other factors recorded by isotope ratios; for example, a mass extinction increases the proportion of lighter isotopes in sediments and ice because biological processes preferentially use lighter isotopes so a reduction in land or ocean biomass makes larger quantities of lighter isotopes available for deposition. The paleontological evidence consists of changes in the geographical distribution of fossils. During a glacial period cold-adapted organisms spread into lower latitudes, and organisms that prefer warmer conditions become extinct or are squeezed into lower latitudes. This evidence is also difficult to interpret because it requires (1) sequences of sediments which cover a long time-span and wide range of latitudes and are easily correlated; (2) ancient organisms which survive for several million years without change and whose temperature preferences are easily diagnosed; and (3) the finding of the relevant fossils, which requires a lot of luck. Despite the difficulties, analyses of ice cores and ocean sediment cores clearly show the record of glacials and interglacials over the past few million years. These also confirm the linkage between ice ages and continental crust phenomena such as glacial moraines, drumlins, and glacial erratics. Hence the continental crust phenomena are accepted as good evidence of earlier ice ages when they are found in layers created much earlier than the time range for which ice cores and ocean sediment cores are available. Major ice ages: There have been at least four major ice ages in the Earth's past. Outside these periods, the Earth seems to have been ice-free even in high latitudes. The earliest hypothesized ice age, called the Huronian, was around 2.7 to 2.3 billion years ago during the early Proterozoic Eon. The earliest well-documented ice age, and probably the most severe of the last 1 billion years, occurred from 850 to 630 million years ago (the Cryogenian period) and may have produced a Snowball Earth in which permanent ice covered the entire globe. This ended very rapidly as water vapor returned to Earth's atmosphere. It has been suggested that the end of this ice age was responsible for the subsequent Ediacaran and Cambrian Explosion, though this theory is recent and controversial. A minor ice age, the Andean-Saharan, occurred from 460 to 430 million years ago, during the Late Ordovician and the Silurian period. There were extensive polar ice caps at intervals from 350 to 260 million years ago, during the Carboniferous and early Permian Periods, associated with the Karoo Ice Age. The present ice age began 40 million years ago with the growth of an ice sheet in Antarctica. It intensified during the late Pliocene, around 3 million years ago, with the spread of ice sheets in the Northern Hemisphere, and has continued in the Pleistocene. Since then, the world has seen cycles of glaciation with ice sheets advancing and retreating on 40,000- and 100,000-year time scales. The most recent glacial period ended about ten thousand years ago. Ice ages can be further divided by location and time; for example, the names Riss (180,000–130,000 years bp) and Würm (70,000–10,000 years bp) refer specifically to glaciation in the Alpine region. Note that the maximum extent of the ice is not maintained for the full interval. Unfortunately, the scouring action of each glaciation tends to remove most of the evidence of prior ice sheets almost completely, except in regions where the later sheet does not achieve full coverage. It is possible that glacial periods other than those above, especially in the Precambrian, have been overlooked because of scarcity of exposed rocks from high latitudes from older periods.
