Prerequisites

1. Variations in quantity or quality (wavelength) of solar radiation.

2. Changing conditions between Sun and Earth, particularly variations in earth/sun orbital geometry.

3. Varying absorption, transmission, or reflection of solar energy in earth's atmosphere or at surface.

4. Changes in heat distribution and circulation on Earth with no change in total reception of solar energy.

The Sun, photographed in red hydrogen-alpha light, which depicts the chromosphere. NASA image.

Several basic conditions are necessary for development of cold climate and growth of large ice sheets. Each major episode of ice ages in the Earth's history was characterized by these conditions. These basic conditions are controlled by tectonic and biologic factors.

• Existence of large, stable continents located in mid- to high-latitude positions = tectonic.

• Mountains uplifted, particularly in low-latitude positions = tectonic.

• Low sea level globally with open oceans at middle to high latitudes to supply moisture = tectonic.

• Low level of atmospheric greenhouse gases (CO₂ and CH₄) = tectonic and biologic.

Space-shuttle photograph of Andes Mountains and Atacama Desert, northern Chile. High-oblique view toward south--Pacific Ocean to right (west). The Andes are the world's second highest mountain system, and they span the tropical and temperate regions from north to south. These mountains have a major impact on atmospheric circulation and climate, as evidenced by the Atacama Desert, the world's driest--barren region in foreground. NASA Johnson Space Center, Imagery Services, STS065-102-034, 7/94.

Stromatolite in banded iron formation. The laminations were built by algal mats during sedimentation of the iron formation. Such fossils are found in Archean sedimentary rocks more than 3½ billion years old. Penny for scale; specimen courtesy of Dr. W.P. Lanier.

Regulation of the Earth's climate is due in large part to certain heat-trapping gases in the atmosphere--the greenhouse effect. These gases include CO₂, H₂O, methane, ozone, and most recently chlorofluorocarbons; CO₂ is most important overall. At present, CO₂ makes up only about 0.3‰ of atmospheric gases, but early in earth history is was much more abundant. This explains how an equitable climate was achieved when solar energy was much less than today. Atmospheric CO₂ has slowly declined as solar output has increased, so that a moderate greenhouse effect has maintained a suitable climate for life throughout earth history.

Atmospheric CO₂ is controlled in part by biologic activity, specifically the organic weathering of soil minerals and the burial of excess organic carbon--see Fig. 12-1. The idea that life not only contributed to development of the atmosphere, but actually created and regulates the atmosphere is called Gaia, after the Greek mother earth goddess (Lovelock 1979). Although highly controversial, there is seemingly no other way to explain the existence of the volatile, unusual composition of the Earth's modern atmosphere without strong biological influence. Atmospheric oxygen, methane, and carbon dioxide are all consumed and produced by biological processes--see Fig. 12-2.

Huge coal strip mine in eastern Germany. Miocene age "brown coal" or lignite is mined here for power production. Coal, petroleum, natural gas, and other fossil fuels represent organic carbon that was removed from the earth's surface by sediment burial in the past. Photo date 8/95; © by J.S. Aber.

The amount of CO₂ present in the atmosphere is regulated by the amount of CO₂ dissolved in sea water. The ocean is the major reservoir for CO₂ at the Earth's surface, and CO₂ exchanges freely between ocean and atmosphere. The CO₂ content of sea water is determined by buffering cations, such as K+, Ca²+ and Mg²+, derived from weathering of rocks on land, and by the rate of burial of organic carbon as carbonate (CO₃^2-) and hydrocarbon-rich sediment.

Mountain building and uplift of land during plate collisions result in more erosion and delivery of cations to the oceans. This in turn increases the ocean's buffering capacity and organic productivity, which remove CO₂ from the atmosphere, and lead to cooler climate. Times of continental breakup and high sea level have the opposite climatic effect. Major uplifts of Himalaya/Tibet Plateau and western North/South America took place in late Cenozoic time, when climate became drier and markedly cooler worldwide. Old mountains and shields were also uplifted by >1 km around the margins of the North Atlantic during the Cenozoic, as a result of the Iceland hot spot, continental rifting, and sea-floor spreading. High plateaus in eastern Canada, Greenland, the British Isles and Scandinavia provided platforms for growth of perennial snow fields and eventually ice sheets (Eyles 1996).

Space-shuttle photograph of the Ganges River delta and coastal zone, Bangladesh. Near vertical, color-visible view that depicts the heavy load of sediment (pink) transported by the Ganges from the Himalayan region to the sea. The Himalaya/Tibet region produces 40% of all sediment delivered to oceans worldwide. NASA Johnson Space Center, Imagery Services, NASA STS066-92-013, 11/94.

Land plants also have a strong influence on weathering of soil minerals. Deciduous/angiosperm ecosystems weather cations from soil three or four times faster than do evergreen/conifer ecosystems. The rise of angiosperm vegetation and mountain building during the Cenozoic combined to cause reduction of atmospheric CO₂ and climatic cooling that culminated in ice-sheet glaciation (Raymo et al. 1988; Volk 1989).

The linkage between volcanic dust and short-term climatic oscillations is fairly well understood. Periods of cool weather lasting a few years have occurred following large historical volcanic eruptions. For example, Mt. Tambora erupted in Indonesia in 1815 and injected up to 100 km³ of ash and debris into the atmosphere. The year following, 1816, was the coldest year of historical record in many places (Stommel and Stommel 1979). The massive eruption of Mt. Pinatubo in the Philippines in 1991, also caused noticeable climatic cooling during the following 2-3 years. Average cooling in the tropics was as much as 2°C, according to NOAA scientists. Other massive eruptions took place in Kamchatka and New Britain, both in September, 1994, with further cooling effects.

	Space-shuttle photograph of Mt. Tambora, Indonesia. Near-vertical, color-visible view of the volcanic cone and surrounding caldera. These features are mere remnants of the tremendous 1815 eruption. The following year, 1816, was the coldest year of historical record in many parts of the world. NASA Johnson Space Center, Imagery Services, STS049-92-40, 5/92.
	Ground photograph of Mt. Pinatubo, Philippines, as seen from Clark Air Base from several miles distance. A huge vertical ash cloud is visible during the first major eruption on June 12, 1991. This eruption was followed, on June 15-16, by a catastrophic explosion that sent tephra 30-40 km into the atmosphere. © NOAA National Geophysical Data Center.
	Space-shuttle photograph showing the limb of the Earth seen at sunrise over Hawaii. The gray-white layer is high-altitude volcanic dust from the eruption of Mt. Pinatubo, which took place more than a year before this photo was taken. This ash layer is credited with measurable climatic cooling during 1992-93. NASA Johnson Space Center, Imagery Services, STS046-105-034, 8/92.
	Space-shuttle photograph of Mt. Kliuchevskoi (snow covered) during its eruption in late September, 1994. Low-oblique, color-visible view of the ash plume drifting from the Kamchatka peninsula toward the southeast over the Pacific. NASA Johnson Space Center, Imagery Services, STS068-150-045, 9/94.

Major eruptions of this type are mostly associated with subduction zones where oceanic lithosphere is consumed. Subduction continues more-or-less constantly around the world; any long-term influence on climate would require a change in the global rate of sea-floor spreading and subduction. Volcanic eruptions also release CO₂, which is recycled from oceanic sediments and derived from the mantle. Volcanoes, thus, have an influence on the greenhouse effect. The warming effect of volcanic CO₂ is probably more important over the long term than the cooling effect of volcanic dust.

1. Obliquity - angle of Earth's axis tilt with respect to plane of its orbit. Varies from 22.1° to 24.5° (23½° at present) with a period of about 40,000 years.

2. Eccentricity - departure of Earth's orbit from a true circle. Varies from 0.005 to 0.06 (presently 0.017) with a period of about 100,000 years.

3. Precession - direction of Earth's axis moves in a circle with respect to distant stars, axis points to Polaris (North Star) at present. Precession causes shift in season of perihelion during 26,000 year cycle.

The combined action of these orbital parameters causes an irregular variation in solar energy received at high latitudes during summer. Although total solar radiation does not change, Milankovitch argued that northern summer insolation would be critical for ice sheets in Europe and North America. The so-called Milankovitch cycle is dominated by precession of the perihelion; however, major glacial cycles of the last million years lasted about 100,000 years each. Statistical analysis of oxygen-isotopes from deep-sea cores in the western Pacific has revealed a distinct peak in global ice volume with a period of around 100,000 years (Muller and MacDonald 1997), which coincides roughly with eccentricity of the orbit (Imbrie et al. 1987).

It is now generally accepted that orbital parameters have driven glacial-interglacial cycles during the Pleistocene (Nesje and Dahl 2000). However, there are several problems with accepting this mechanism as the only control of glacial cycles. The possible temperature variations due to this mechanism are much too small to account for glacial cycles; no consideration is given to atmospheric, oceanic, or land conditions. The southern hemisphere should theoretically experience interglaciation, while the northern hemisphere has glaciation and vice versa. Finally, the Milankovitch cycle has operated throughout earth history, during glacial and nonglacial times alike. At best, the Milankovitch cycle could act as a trigger for northern hemisphere glaciation once cool climate had been established by other factors (Covey 1984).

Apollo photograph of the whole Earth, showing Africa, Antarctica and surroundings. Note the different albedos of clouds, snow, water, forest, and desert. As seen from space, the Earth is mainly blue and white, with some brown desert regions. Vegetated land areas are mostly obscured by cloud cover. NASA Johnson Space Center, Imagery Services, AS17-148-22727, 12/72.

A positive feedback exists between snow/ice cover and climate: snow and ice cover increases the Earth's albedo and cools climate leading to still more snow/ice cover. This albedo effect is especially important in low-latitude regions, because most solar radiation is received between 30°N and 30°S latitudes. In the early Quaternary, development of extensive glaciers and perennial snow cover in the Himalaya/Tibet region was apparently the trigger for lowering global temperature by 1.5° to 2.0°C (Kuhle 1988). This led, in turn, to glaciation of other, lower mountains and eventually to sea-level glaciation by ice sheets of northern continents.

Some mechanism other than climatic change must be responsible for abruptly ending glacial cycles. Continental ice sheets appear to self-destruct spontaneously once a critical ice thickness is achieved. Thick ice depresses the crust and forms an insulating blanket that traps geothermal heat. As crustal depression proceeds, an increasingly large portion of the ice sheet base sinks below existing sea level. Increased basal melting forms subglacial lakes, which allow surging and massive production of icebergs. The sea ultimately invades the depressed region and causes rapid breakup of the ice sheet's interior. In effect, the ice mass is transported from cold, high-latitude land to melt in warm, low-latitude seas.

The Laurentide Ice Sheet self destructed in this manner at the end of each major glaciation--see Fig. 12-4. Rapid rise in sea level destabilized other marine ice sheets (Eurasia) leading to their collapse. Radical changes in albedo led to global warming that caused glaciers to shrink elsewhere. The glacial cycle could then begin again only when land had rebounded high enough to once more support glaciers. During the most recent deglaciation, the Greenland Ice Sheet was protected by surrounding mountains, and the Antarctic Ice Sheets were stable due to their polar positions.

Evidence from Greenland ice cores and deep sea sediments suggests that this Atlantic circulation pattern may turn on and off abruptly, as a result of minor trigger events related to dynamics of the Laurentide Ice Sheet. Such sudden changes took place at least twice at the end of the last glaciation during the Bølling and Younger Dryas episodes. Vegetation records from eastern North America and Europe confirm rapid and widespread changes in vegetation related to these climatic shifts (Williams et al. 2002). Although brief in duration, these late glacial cooling episodes apparently were global in extent (Gosse et al. 1995).

One likely effect of the Younger Dryas glaciation would be to shift the Intertropical Convergence Zone (ITCZ) farther south, so that the equatorial region came under Arctic climatic influence--see Fig. 12-6. This has been verified from pollen studies in eastern Brazil, near the equator (Ledru et al. 2002).

No such climatic events have taken place during the Holocene, but the potential is thought to exist (Broecker 1994). It has been suggested recently that continuing buildup of greenhouse gasses might lead to shutdown of the thermohaline circulation and plunge the Earth into a glacial phase similar to the Younger Dryas (Broecker 1999). However, this possiblity remains remote, and no climatic models have been able to reproduce large and abrupt changes in the Earth's atmosphere.

Emerging evidence seems to indicate the rapid climatic changes in Greenland and Antarctica may have been out of phase (Alley & Bender 1998). In other words, episodes of rapid warming in Greenland were times of cooling in Antarctica, and vice versa during the period 20,000 to 10,000 years ago. Further research is underway to better document the timing and mechanisms that may be involved. Nonetheless, an important principle is that rapid climatic change in the northern and southern hemispheres may be asynchronous for time spans of decades or centuries.