An ice age is a period of long-term downturn in the temperature of Earth's climate, resulting in an expansion of the polar ice caps and mountain glaciers ("glaciation"). 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. More colloquially, and of the last 4 Myr, ice age is used to refer to colder periods with extensive ice sheets over the North American and European continents: in this sense, the last ice age ended about 10,000 years ago. This article will use the term 'glacial periods' for colder periods during ice ages and 'interglacial' for the warmer periods.
|Table of contents|
2 Major periods of glaciation
4 Causes of ice ages
5 Recent glacial and interglacial phases
6 See also
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, p25, quote a woodcutter telling de Charpentier of the former extent of the Swiss Grimsel glacier). No single person invented the idea . Between 1825 and 1833 Jean de Charpentier assembled evidence in support of this idea. In 1836 Charpentier convinced Louis Agassiz of the theory, and Agassiz published it in his book Étude sur les glaciers of 1840.
Major periods of glaciation
There have been four major periods of glaciation in the Earth's past. The first, and possibly most severe, occurred from 800 to 600 million years ago (the late Proterozoic Age). It has been suggested that the end of this cold period was responsible for the subsequent Cambrian Explosion, though this theory is recent and controversial. A minor series of glaciations occurred from 460 to 430 million years ago. There was extensive glaciations from 350 million years before present to 250 million. The present Pleistocene ice age has seen more or less extensive glaciation on 40,000 and 100,000 year cycles. The last glacial period ended about 10,000 years ago.
In between periods of glaciation, there are multi-million year periods of more temperate climate, but also within these abovementioned periods (or at least within the last one), temperate and severe periods occur. The colder periods are called 'glacial periods', the warmer periods 'interglacials'.
We are in an interglacial period now, the last retreat ending about 10,000 years ago. There appears to be a folk wisdom that "the typical interglacial period lasts ~12,000 years" but this is hard to substantiate from the evidence of ice core records. Nonetheless, this led to some fear of a new glacial period starting soon, a global cooling concern. Many now believe that anthropogenic forcing from increased "greenhouse gases" would outweigh any Milankovitch (orbital) forcing; and more recent consideration of the orbital forcing suggests that even in the absence of human perturbation the present interglacial would last at least 50 kyr.
Causes of ice ages
The cause of ice ages remain controversial, but the general consensus is that it is a combination of up to three different factors: atmospheric composition (particularly the fraction of CO2 and methane), changes in the Earth's orbit around the Sun known as Milankovitch cycles (and possibly the Sun's orbit around the galaxy), and the arrangement of the continents.
The present ice ages are the most studied and best understood, particularly the last 400 kyr, since this is the period covered by ice cores that record atmospheric composition and proxies for temperature and ice volume. Within this period, the match of ice age frequencies to orbital forcing periods is so good that orbital forcing is the generally accepted explanation, even though the strength of the forcing appears to be too small. Feedback from CO2 may explain this mismatch.
The first of these three factors is probably responsible for much of the change, especially for the first ice age. The "Snowball Earth" hypothesis maintains that the severe freezing in the late Proterozoic was both caused and ended by changes in CO2 levels in the atmosphere. However, the other two factors do matter.
For reasons that are unclear, an abundance of land in the arctic and antarctic circles appears to be a necessity for an ice age. The Earth's orbit does not have a great effect on the long term causation of ice ages, but does seem to dictate the pattern of multiple freezings and thawings that take place within the current ice age. The complex pattern of changes in Earth orbit and the change of albedo may influence the occurrence of glacial and interglacial phases - this was first explained by the theory of Milutin Milankovic which is supported by the recently discovered details about the last ice age.
While Milankovic forcing predicts that cyclic changes in the Earth's orbital parameters can be expressed in the glaciation record, additional explanations are necessary to explain which cycles are observed to be most important in the timing of glacial periods. In particular, during the last 800 thousand years the dominant glacial oscillation has been 100 thousand, which corresponds to changes in Earth's eccentricity and inclination, and yet is by far the weakest of the three frequencies predicted by Milankovic. During the period 3.0 - 0.8 million years ago the dominant pattern of glaciation corresponded to the 41 thousand year period of changes in Earth's obliquity. The reasons for preferring one frequency to another are poorly understood and an active area of current research, but the answer probably relates to some form of resonance in the Earth's climate system.
Richard A. Muller and Gordon J. MacDonald    have proposed that orbital inclination coupled to variations in dust, rather than eccentricity variations, are responsible for the ice age periods. This has been challenged by Rial .
Recent glacial and interglacial phases
The last glacial and interglacial phases of the Pleistocene are named, from most recent to most distant, as follows (names before the '/' are North America, names after it Northern European, dates in thousand years BCE - note that in Eastern Europe and the Alps yet other names are used):
- Wisconsinan/Weichsel (or Vistula) (glacial period, 70-15)
- Sangamon/Eem (interglacial, 130-70)
- Illinoian/Saale (glacial, 180-130)
- Yarmouth/Holstein (interglacial, 230-180)
- Kansan/Elster (glacial, 300-230)
- Aftonian/Cromer (interglacial, 330-300)
- Nebraskan/Gunz (glacial, 470-330)
- -/Waalian (interglacial, 540-470)
- -/Donau II (glacial, 550-540)
- -/Tiglian (interglacial, 585-550)
- -/Donau I (glacial, 600-585)
The end of the last glaciation also corresponds quite closely to the development of permanent human settlements and agriculture, and it is possible that there is a connection between the two events.
Imbrie and Imbrie, "Ice ages - solving the mystery", Harvard University Press, 1979.