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What controls the advance and retreat of these large glaciers during the four long, cool periods?


Scientists understand more about why glaciers advance during cool periods than they do about why large scale cool periods occur, because they have gathered large quantities of data about the current cool period.

Variation in the Earth's orbit through time causes changes in the amount and distribution of sunlight (and other solar radiation) reaching the Earth's surface. These changes are thought to affect the development of ice sheets.

Although the idea that variation in the Earth's orbit causes glacial-interglacial cycles originated in the mid 1800s, Milutin Milankovitch first popularized it in about 1920. Although Milankovitch's hypothesis was not widely accepted initially; data collected during the 1970's have generated broad support for it.

Three orbital parameters are especially important in causing ice sheet waxing and waning:

  1. Changes in the eccentricity of the Earth's orbit
  2. Changes in the tilt of the Earth's axis
  3. The precession of the equinoxes

In combination these factors influence the amount and distribution of solar radiation reaching the Earth. Changes vary with both latitude and season. Because of the different periodicities of variation for the three factors, the composite variations in solar radiation are very complex.

Although the connections are not obvious and direct, changes in the amount of solar radiation are thought to drive the growth and melting of major ice sheets. Over the last 750,000 years ice sheets have expanded into the midwestern United States at least 8 major times. The timing of the earlier of these advances is not well known.

The last glaciation of the midwestern United States had its maximum extent approximately 20,000 years ago. The animals and plants discussed in this exhibit are the ones that were living in the midwestern U.S. during and just following that glaciation.

Eccentricity

The Earth's orbit around the sun is not a circle, but rather it is an ellipse. The shape of the elliptical orbit, which is measured by its eccentricity, varies from between one and five percent through time.

The eccentricity affects the difference in the amounts of radiation the Earth's surface receives at aphelion and at perihelion. The effect of the radiation variation is to change the seasonal contrast in the northern and southern hemispheres. For example, when the orbit is highly elliptical, one hemisphere will have hot summers and cold winters; the other hemisphere will have warm summers and cool winters. When the orbit is nearly circular, both hemispheres will have similar seasonal contrasts in temperature.

Although the amount of change in radiation is very small (less than 0.2%), it is apparently extremely important in the expansion and melting of ice sheets.

The eccentricity of the Earth's orbit varies in a periodic manner. The primary periodicity is approximately 100,000 years.

Tilt

The Earth's axis is tilted with respect to its orbit around the sun. Today the tilt is approximately 23.5 degrees. The tilt varies from between 21.6 and 24.5 degrees in a periodic manner. A graph of the tilt over the last 750,000 years shows that the dominant period of this variation is approximately 41,000 years.

Changes in the tilt of the Earth's axis cause large changes in the seasonal distribution of radiation at high latitudes and in the length of the winter dark period at the poles. Changes in tilt have very little effect on low latitudes.

The effects of tilt on the amount of solar radiation reaching the Earth are closely linked to the effects of precession. Variation in these two factors cause radiation changes of up to 15% at high latitudes. Radiation variation of this magnitude greatly influences the growth and melting of ice sheets.

Precession of the equinoxes

Twice a year, the equinoxes, the sun is positioned directly over the equator. Currently the equinoxes occur on approximately March 21 and September 21. However, because the Earth's axis of rotation "wobbles" (like a spinning top), the timing of the equinoxes changes . The change in the timing of the equinoxes is known as precession.

Although the timing of the equinoxes is not in itself important in determining climate, the timing of the Earth's aphelion and perihelion also changes. Like the timing of the equinoxes, the timing of the aphelion and perihelion is also affected by the wobble of the axis of rotation.

The changing aphelion and perihelion is important for climate because it affects the seasonal balance of radiation. For example, when perihelion falls in January the northern hemisphere winter and southern hemisphere summer are slightly warmer than the corresponding seasons in the opposite hemispheres.

The aphelion and perihelion change position on the orbit through a cycle of 360 degrees. The cycle has two periods of approximately 19,000 and 23,000 years. Together these combine to produce a generalized periodicity of about 22,000 years.

The effects of precession on the amount of solar radiation reaching the Earth are closely linked to the effects of tilt. Variation in these two factors cause radiation changes of up to 15% at high latitudes. Radiation variation of this magnitude greatly influences the growth and melting of ice sheets.


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