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 quote a woodcutter telling Jean de Charpentier of the former extent of the Swiss Grimsel glacier^[1]. No single person invented the idea ^[2]. Between 1825 and 1833, Charpentier assembled evidence in support of the concept. In 1836 Charpentier convinced Louis Agassiz of the theory, and Agassiz published it in his book Étude sur les glaciers (Study of Glaciers) of 1840. e. g.: North American review. / Volume 145, Issue 368, July 1887

Evidence for ice ages

There are 3 main types of evidence for ice ages: geological, chemical and paleontological.

Major ice ages

There have been at least four major ice ages in the Earth's past.

Vicious and virtuous circles in glacial periods

Each glacial period is subject to cycles of processes which make it more severe and to cycles which mitigate and (in all cases so far) eventually end it.

Cycles which make glacial periods more severe

Ice and snow increase the Earth's albedo, i.e. they make it reflect more of the sun's energy and absorb less. Hence the air temperature decreases, ice and snow fields grow, the Earth's albedo increases again, etc.

Cycles which mitigate glacial periods

In most glacial periods the ice and snow prevent weathering of rocks and therefore slow down the part of the geological carbon cycle which removes carbon from the atmosphere. So the atmosphere's carbon content increases (especially CO₂), the greenhouse effect becomes stronger, the air temperature rises, the growth of ice and snow fields slows down and (in all cases so far) eventually stops.

Interglacials

Causes of ice ages

The cause of ice ages remains controversial for both the large-scale ice age periods and the smaller ebb and flow of glacial/interglacial periods within an ice age. The general consensus is that it is a combination of several important factors: atmospheric composition (the relative amounts of water vapor, carbon dioxide, methane, sulfur dioxide, and various other gases and particulates in the atmosphere), changes in the Earth's orbit around the Sun known as Milankovitch cycles (and possibly the Sun's orbit around the galaxy), the motion of tectonic plates resulting in changes in the relative location and amount of continental and oceanic crust on the Earth's surface, variations in solar output, the orbital dynamics of the Earth-Moon system, the impact of relatively large meteorites, and eruptions of supervolcanoes.

Changes in Earth's atmosphere

The most relevant change is in the quantity of greenhouse gases in the atmosphere. There seems to be plenty of evidence that greenhouse gas levels fell at the start of ice ages and rose during the retreat of the ice sheets, but it is hard to decide which is cause and which is effect (see the notes above on the role of weathering). Greenhouse gas levels may also have been affected by other factors which have been proposed as causes of ice ages, such as the movement of continents and vulcanism.

Position of the continents

The geological record appears to show that ice ages start when the continents are in positions which block or reduce the flow of warm water from the equator to the poles and thus allow ice sheets to form. The ice sheets increase the Earth's albedo, this reduces the air temperature, the ice sheets grow, etc., until the reduction in weathering causes an increase in the greenhouse effect.

Variations in Earth's orbit (Milankovitch cycles)

The Milankovitch cycles are a set of cyclic varions in various characteristics of the Earth's orbit round the sun. Each cycle has a different length, so at some times their effects reinforce each other and at other times they (partially) cancel each other.

Variations in the sun's energy output

There at least 2 types of variation in the sun's energy output:

Vulcanism

The largest known volcanic events, the flood basalt events which produced the Siberian Traps and Deccan traps and are both associated with mass extinctions, are not associated with ice ages. At first sight this implies that vulcanism cannot have produced ice ages.

Recent glacial and interglacial phases

See Timeline of glaciation.

Glaciation in North America

Notes