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Why were there four long, generally cool periods during which continent-sized glaciers advanced and retreated?


Although scientists cannot answer this question with certainty, they know that a number of factors interact to produce conditions favoring the formation of ice sheets. Some of these factors include
  1. changing continental positions
  2. uplift of continental blocks
  3. reduction of CO2 in the atmosphere
  4. changes in the Earth's orbit
Long ice age intervals did not just suddenly occur. Instead, they seem to have been the culmination of even longer periods of worldwide climatic cooling. This cooling took place for tens of millions of years before the beginning of glaciation.

Once ice sheets start to grow, they probably contribute to their own further development. This positive feedback occurs because ice sheets reflect more sunlight back into space than does ground not covered by ice. The reflected sunlight would otherwise warm the Earth's surface. Consequently, the presence of ice sheets may lead to more cooling and continued development of ice sheets.

Changing continental positions

Plate tectonics is an important process influencing when ice ages occur, and the position of the continents is probably one of the most important factors controlling long periods of multiple glaciations. The presence of large land masses at high latitudes appears to be a prerequisite for the development of extensive ice sheets, because the large accumulations of ice associated with ice sheets cannot form over the ocean.

During the current ice age, which began slightly less than 3 million years ago, several large land masses have been at high latitudes. These include Antarctica, much of North America and much of Eurasia. This continental configuration led to extensive glaciation of both North America and Eurasia.

During the ice age that occurred in the Pennsylvanian and Permian, the southern portion of the supercontinent Pangea was at the south pole. The result was extensive glaciation of what is now Africa, South America, India, Antarctica, Australia, and the Arabian peninsula.

The position of the continents during the Late Proterozoic glaciation (around 700 million years ago) is not well-known. Evidence of glaciers exists from North America, Australia, and Africa.

Uplift of continental blocks

Plate tectonics probably contributes to the development of long periods with many glaciations in another, more subtle way. Plate movements sometimes cause uplift of large continental blocks. Major uplift can cause profound changes in the global oceanic and atmospheric circulation patterns. Changing circulation patterns cause climate change. Some scientists hypothesize that climatic changes caused by uplift are critical to the development of ice ages.

Over the past 15 million years, the continents have risen about 600 meters (2000 feet) on average. The uplift of the Himalayas and the Tibetian Plateau probably contributed to the initiation of the current cool period.

Similar tectonic uplift appears to have been involved in the three other long, ice age intervals.

Reduction of CO2 in the atmsophere

A general reduction in the amount of carbon dioxide (CO2) in the atmosphere may contribute to the development of ice ages. Carbon dioxide is an important greenhouse gas. Decreases in the amount of CO2 in the atmosphere may lead to global cooling.

Many processes can cause a long-term decrease in the amount of CO2 in the atmosphere. These processes include many complex interactions among organisms, ocean currents, erosion, and volcanism. Important relationships exist between ice ages and the composition of the atmosphere; however, many scientists are unsure whether the changes in atmosphere cause cool periods or whether cool periods cause atmospheric changes. Also, many scientists are not sure the magnitude of past CO2 changes was large enough to initiate ice ages.

Changes in the Earth's orbit

The Earth's orbit varies through time. Important parameters that vary include the eccentricity of the orbit around the sun, the tilt of the Earth's axis, and the direction the north pole points. Variation in these three factors changes the amount and distribution of incoming solar radiation. Variations in the distribution of solar radiation affect and initiate glaciations. The next section of this exhibit describes and illustratesthese three factors.

However, the variation of the orbital parameters seems to be on too short a time scale to explain the timing of the long, cool intervals with many glaciations. Variations in orbital factors are probably more important in controlling the advance and retreat of large glaciers during the four long, cool periods than they are for controlling the larger-scale patterns.


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