Lowering of Snowline

The snowline is the line (as in a point of elevation on a mountain) at which the amount of snow falling equals the amount of snow melting in the summer.

If the climate cools enough, so that snowlines are lowered, glaciers develop in mountains, and ice sheets will develop in mid-latitudes. The sea level will fall, and the area of sea ice is increased. Some areas not affected by glaciers will become affected by permafrost. Ice in some shape or form begins to affect a larger area of Earth’s surface than usual.

Source: The Ice Age: Past and Present by Brian S. John. This idea was suggested by Prof. Frederick Shotton (a well-known Quaternary geologist)

Building up of Mountain Ranges

Building up of Mountain Ranges until they are above the regional snowline affects glaciation. As long as there is adequate snowfall for the growth of glaciers, ice affects high parts of mountains and flows to altitudes below snowlines in glacial valleys. This does not stop until the mountain is worn down below the snowline again. Source: Brian John’s book.

Location of Continents

If land moves to an area of high-latitude, ice sheets can develop. (Another idea of Shotton). (From Erickson book and John book). Arrangement of land is also important because it can prevent circulation of warm waters from the equator. (Source: John’s book.) Wandering of land increases volcanic activity. The land near the poles has a higher albedo due to the snow and ice; this leads to more snow and ice.

Volcanoes

When a volcano erupts, it emits ash and dust that form into clouds that exist for long periods of time. These ash and dust clouds reflect incoming solar radiation. An erupting volcano also emits water vapor and gases that can obscure sunlight. The dust from volcanic eruptions in the tropics is carried pole ward by wind. The effect of a volcanic eruption on climate is really dependent on the type of dust particles emitted. Another factor is the weight of ice, which can squeeze magma out of the Earth, increasing volcanic activity. (This is a positive feedback loop). Source: Jon Erickson’s Ice Ages: Past and Future. Evidence: checking the ice cores shows the composition of the atmosphere at a particular point in time. Volcanoes emit acidic gases into the atmosphere. These acidic gases conduct electricity more easily than ordinary ice. In some cases, you can compare the acidity changes in the ice with the time measurements on tree fossils that indicate cooler or dryer seasons.

Carbon Cycle (Source: pp.221-224 of textbook. Diagrams of these three hypotheses are Figures11-14, 11-15, and 11-17.)

Oceans are a huge reservoir of CO2 . The ocean is like a pump that removes CO2 from Earth’s surface and atmosphere and stores it in the sea (as long as the sea is not mostly covered with ice). The faster this works, the more CO2 is removed. This rate is partly determined by the amount of nutrients in the sea, which respond to changes in sea ice volume. The efficiency of the biological pump during the ice ages might have been greater due to higher concentrations of nutrients in the ocean. The CO2 level would be much lower because of it.

The shelf nutrient hypothesis has to do with phosphate in continental shelves being exposed due to low sea level caused by the presence of glaciers. The phosphate is carried to the sea by rivers. This increase in nutrients increases the intensity of the biological pump which makes the atmospheric concentration of CO2 go down, further decreasing the temperature and allowing the glacial ice volume to increase. This is a positive feedback loop. This feedback loop ties in well to the Milankovitch theory due to the fact that the residence time for phosphate in the ocean is 100,000 years, a significant number because of the predictable cycling of glaciation. Some people do not believe that phosphate is a significant nutrient for this, so they look towards iron for an answer instead.

Another hypothesis dealing with sea level is the Coral Reef Hypothesis. Continental shelves are a good habitat for coral, which secrete CO2 into the atmosphere. When sea level goes down due to glaciation, coral reefs are exposed, and then dissolved, so the amount of CO2 released is reduced. This in turn brings down temperature, increases glacial ice volume, and further decreases sea level. This is another positive feedback loop.

The iron fertilization hypothesis involves the delivery of iron molecules to the ocean by east-west blowing wind. The equator-to-pole temperature gradient drives the east-west winds. Wind-blown dust particles usually have a coating of iron, and the particles are delivered to the ocean. Iron is a nutrient that increases the intensity of the biological pump, which decreases atmospheric CO2, and then temperature. This is a positive feedback loop. Evidence: amount of iron in wind-blown dust increases several fold during periods of glaciation.

Bibliography

John, Brian. S. The Ice Age: Past and Present, William Collins Sons & Co Ltd. Glasgow. Collins, London. 1977, pp. 162, 163.

Erickson, John. Ice Ages: The Past and Future, Tab Books Publishing Company, USA. 1990. pp74-108.

(the textbook)

Kump, Kasting, Crane. The Earth System, Prentice Hall- Inc., New Jersey. 1999. Pp221-224.