Glacial Periods

Glacial Periods refer to the longer periods of cold that separate shorter periods of warmth of the Earth, which are called interglacial periods. These occur within the major ice ages. Many glacial periods, sometimes referred to as mini ice ages, have occurred during the past few million years, initially at 40,000 year frequency, but more recently at more than 100,000 year intervals.

Scientists report that the present major ice age began 40 million years ago with the growth of an ice sheet in Antarctica. It intensified starting around 3 million years ago with the spread of ice sheets into the Northern Hemisphere.

Because the Greenland and Antarctic ice sheets still exist we are in a major ice age that has lasted for very many years. However, within that major ice age the earth experiences glacial periods with extensive ice sheets, separated by the warmer interglacial periods. The last glacial period ended some ten or twelve thousand years ago. Traditionally, the warmer interglacial periods have lasted up to fifteen thousand years, some predictions suggest periods up to fifty thousand years can be expected.

We can learn of previous glacial periods by studying the geographical distribution of fossils and the study of ice cores drilled into the ice caps and similar cores taken of ocean sediments. Scientists examine these and report on their findings. We can also spot evidence of former glacial periods like U-shaped valleys scoured by glaciersa large body of ice with definite lateral limits, which moves in a downslope direction due to its great mass, as in Alaska. and moraines left behind as the glaciers receded. The Wisconsin lakes were most probably gouged out by glacial period ice sheets.

How do glacial periods begin and end?

There is strong evidence that the Milankovitch cycles affect the occurrence of glacial and inter-glacial periods within an ice age. The combined effects of the changing distance to the Sun, the precession (wobble) of the Earth's axis, and the changing tilt of the Earth's axis redistribute the sun's energy eceived by the Earth. Of particular importance are changes in the tilt of the Earth's axis, which affect the intensity of the seasons. These effects could either begin or end a glacial period.

The present ice ages are the most studied and best understood, particularly the last 400,000 years, 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 glacial/interglacial frequencies to the Milankovich orbital forcing periods is so close that orbital forcing is generally accepted as a cause for a glacial period to begin.

Although it is difficult to be sure of cause and effect, there is no doubt that the composition of the earth's atmosphere is crucially important. From the ice cores, the earth's temperature and the concentration of greenhouse gases in the atmosphere are closely allied to each another, and appear to move virtually in lockstep.

Variations in solar out put could also be significant events, together with extreme volcanic activities causing much ash debris into the atmosphere to shield sunlight from the earth's surface. A similar effect could stem from the earth's orbit taking it into the path of cosmic dust regions.

Whatever cause my begin or end a glacial period, it seems that the events that follow accelerate the cooling or heating of the earth.

Sequences of events

First, sea ice melts and glaciersa large body of ice with definite lateral limits, which moves in a downslope direction due to its great mass, as in Alaska. retreat. Highly reflective white ice (reflectivity or, albedoThe fraction of incident light that is reflected by an object, especially the Earth or another planet reflecting the Sun's light.) is replaced by land and sea absobing more solar energyEnergy from the sun that is converted into thermal, chemical, or electrical energy. than before. The land and the oceans heat up causing melting of more ice to take place.

Second, warming increases levels of evaporationEvaporation can be defined as the process by which liquid water is converted into a gaseous state. Evaporation can only occur when water is available. It also requires that the humidity of the atmosphere be less than the evaporating surface (at 100 % relative humidity there is no more evaporation). The evaporation process requires large amounts of energy. For example, the evaporation of one gram of water at a temperature of 100° Celsius requires 540 calories of heat energy (600 calories at 0° Celsius). putting more water vapour into the air, which itself is a potent greenhouse gas. More water vapour leads to increased warming and consequent increased evaporation, and, incidentally heavier rainstorms and hurricanes.

Third, warming causes soils to dry, permafrostPermanently frozen layer of soil that underlies the arctic tundra. or tundra to melt, and the oceans to release carbon dioxideCommon gas found in the atmosphere. Has the ability to selectively absorb radiation in the longwave band. This absorption causes the greenhouse effect. The concentration of this gas has been steadily increasing in the atmosphere over the last three centuries due to the burning of fossil fuels, deforestation, and land-use change. Some scientists believe higher concentrations of carbon dioxide and other greenhouse gases will result in an enhancement of the greenhouse effect and global warming. The chemical formula for carbon dioxide is CO₂. and methane, both of which are potent greenhouse gases.

The sequence is self-accelerating.

In most glacial periods the ice and snow prevent weathering of rocks and therefore slows down the part of the geological carbon cycleThe combined processes, including photosynthesis, decomposition, and respiration, by which carbon as a component of various compounds cycles between its major reservoirs: the atmosphere, oceans, and living organisms. which removes CO₂ from the atmosphere. With higher CO₂ concentrationsThe amount of a component in a given area or volume. the greenhouse effect becomes stronger and provides a warming influence. As a result the growth of ice and snow fields slows down and hopefully eventually stops. The increase in the greenhouse effect is progressively slowed and eventually stopped by the increase in weathering as the retreat of ice and snow expose more rock to weathering.

The decrease in weathering which starts this process depends on how much land area the ice and snow fields cover.

Some of these factors are causally related to each other. For example, changes in Earth's atmospheric composition (especially the concentrationsThe amount of a component in a given area or volume. of greenhouse gases) may alter the climate, while climate change itself can change the atmospheric composition (for example by changing the rate at which weathering removes CO₂).

The models used by scientists to understand these complex events, and to discern whether the balance is shifting in favour of warming or cooling, requires a great deal of computer power and a process of continuous refinement.