Over its long history, much larger parts of the Earth's surface than at present have been periodically covered with ice, periods known as the ice ages. Although the cause of ice ages is not known for certain, the Milankovich Model is proposed as a large part of the explanation.
Over the past several million years, ice ages have occurred regularly, each lasting roughly 100,000 years and separated from each other by interglacial periods each lasting between 10,000 and 15,000 years. We are now living in an interglacial period which began 10,000 years ago.
During the last ice age, great ice sheets covered Canada and Scandinavia. Throughout Europe, the ice extended to the latitude of the English midlands and almost totally covered Ireland. Apart from high-mountain glaciers, there are no ice bodies so far south today.
Our climate is largely determined by how the Earth receives heat from the sun. Consider the basis for the four seasons. The Earth is roughly spherical, turning on its axis every 24 hours and orbiting the sun once every year. The axis of rotation is an imaginary line joining the North to the South Pole and is tilted by 23.5 degrees relative to the plane of orbit around the sun. The tilt remains constant over the year, always pointing in the same direction.
So, as we orbit the sun, the northern hemisphere is sometimes tilted towards the sun, and sometimes (six months later) away from the sun, with the southern hemisphere experiencing the opposite. We enjoy summer when tilted towards the sun and winter when tilted away. This explains the cycle of the seasons.
Just as the Earth's annual orbit of the sun determines our annual seasons, long-term periodic fluctuations in this same orbit determine two longer-term seasons; in one of which much of the Earth is covered in ice and in the other when less ice is present.
The Earth's orbit around the sun changes slowly with respect to three parameters: (a) the tilt of the Earth's axis, (b) the direction in which the North Pole points, and (c) the eccentricity of the orbit.
The tilt of the Earth's axis with respect to its orbit around the sun varies between 21.6 and 24.5 degrees in a periodic manner, taking about 41,000 years to complete a cycle. Changes in axis tilt cause large changes in the seasonal distribution of sunlight at high latitudes but not at low latitudes.
The Earth's axis wobbles slowly, changing the direction in which the North Pole points at the heavens, but not the tilt of the Earth's axis. This slowly-changing direction of the pointing of the North Pole traces out an imaginary circle in the heavens with a cycle approximately every 20,000 years.
The effects of changes of axis tilt are closely related to the changing direction in which the North Pole points. Variation in these two factors causes radiation changes of up to 15 per cent at high latitude, which greatly influences the growth and melting of ice sheets.
The Earth is gravitationally attracted to all the other bodies in the solar system. This attraction varies gradually with time in a cyclical manner and the net effect is to change the orbit of the earth around the sun slightly from a more circular orbit to a more elliptical orbit and back again with a rhythm of approximately 100,000 years.
The interaction of these three long cycles of the orbit of the Earth results in long-term cyclical changes in global heat balance, although the total amount of annual heat received by the Earth from the sun is always the same. Ice ages occur when the long-term cycle produces cold summers in the Northern hemisphere, and, although this means that winters are relatively warm, they remain cold enough for snow to fall at high latitudes.
The important point is that summers should be cool enough at high latitudes for snow to remain on the ground throughout the year. This allows ice sheets to grow rapidly. Ice ages end when the three long-term cycles work together to make the summers unusually warm, melting the ice. This explanation is called the Milankovich Model after the Yugoslav astronomer Milutin Milankovich, who worked out the details.
Summer snow on the ground reflects incoming solar heat which would otherwise warm the ground below. The ground is, therefore, cooler at the start of the next winter and it is easier for the next snowfall to lie there without melting. A full ice age can develop quickly but warming out of an ice age is much slower because ice and snow reflect incoming heat.
Human civilisation developed during this interglacial period. The Milankovich rhythms would plunge the Earth back into ice age within the next few thousand years but human activities may indefinitely postpone this.
Atmospheric carbon-dioxide, the principal "greenhouse gas", has increased by 25 per cent since the 19th century because of human activities: the burning of fossil fuel and destruction of forests. This has warmed world temperature about one degree Celsius since the 1880s and we continue to pour carbon-dioxide gas into the atmosphere.
Conservative estimates predict a further warming of one degree over the next 20 years, which will take temperatures beyond anything experienced on Earth for over 100,000 years. The rate of warming will be 50 times faster than the rate at the end of the last ice age. We don't know how this dramatic change will affect the physical environment.
William Reville is a senior lecturer in biochemistry and director of microscopy at UCC