Australia: The Land Where Time Began

A biography of the Australian continent 

Ice Age Induction

According to the author¹ there are a number of criteria that must be met before sufficient cooling occurs for an ice age to start, that include the right conditions in the atmosphere, oceans and geology. During the Oligocene and Miocene cooling led to the possibility of ice ages but the climate changed before an ice age began. The author¹ suggests the fact that ice ages are not common indicates that exact conditions required for them to begin don't often arise. He suggests that the arrangement of continents must be such that the flow of warm water to the poles is prevented, or at least seriously limited, resulting in cooler conditions at both poles. The ocean waters at high latitudes mix freely with warmer water from the tropics when the polar regions are not landlocked, the temperature gradient from the equator to the pole reduced to 30° C. Though sea ice forms under these conditions at both poles is is not persistent and does not accumulate, the average temperatures at the poles not being below freezing permanently. There is a dramatic reduction of mixing of the colder polar waters with warmer waters from the tropical regions when there is a landmass at the poles or polar waters are landlocked this situation leads to a temperature gradient between the equator and the poles that is much greater. Such a situation prevails at the present in which the gradient between the equator and Antarctica of 65° C and between the equator and the Arctic region of 50° C. There are no polar waters to mix with warmer waters in the south as there is a landmass, the Antarctic continent, and the continent is cooled even further by the presence of the continuously circulating Antarctic Circum Polar Current that constantly drains heat from the continent and discharges it to the 3 main oceans dividing the southern continents, the Indian, Pacific and Atlantic Oceans. There is also a solid base on which ice can accumulate in the form of the Antarctic continent. In the Arctic Ocean the cooler polar waters and the sea ice that forms from them are almost completely landlocked, being surrounded by a barrier formed by North America and Siberia with only a comparatively narrow gap in the North Atlantic through which water can move. The geographic distribution of the continents is close to ideal for ice age conditions to prevail, though this can easily be changed by anthropogenic warming.

These conditions are not considered to have been sufficient on their own to trigger an ice age, especially as the disposition of the high latitude landmasses has been much the same for about the past 100 My, but in spite of previous abortive ice ages, the Ice Age didn't really start until about 2.5 Ma. According to the author¹ this suggests other factors are required before a cooling can become an ice age, one suggested possibility being the closure of the Isthmus of Panama that occurred between about 4 and 2.5 Ma. The author¹ suggests this may have been a critical factor that set the Earth on track to experience full ice age conditions. Once the Isthmus closed the last connection between the North Atlantic Ocean and the Pacific Ocean was blocked, the only remaining connection between these 2 oceans then being via the Southern Ocean to the south of the southern tip of South America. The result was a major reconfiguration of ocean currents. The corollary of this is suggested to have been the sudden, dramatic climate change of the North Atlantic that allowed the formation of ice sheets across North America and Europe, even if the reasons can be considered counterintuitive. The warm tropical Atlantic waters could no longer leak into the Pacific, instead being forced to the north where they strengthened the Gulf Stream and associated currents moving warm water towards the Arctic. It has been suggested that this change in currents might have been predicted to reduce the chances of ice sheet formation, but in the event it appears to have had the opposite effect. The invigorated Gulf Stream carried more moisture to high latitudes, as well as more heat. This is suggested to have stimulated snowfall once all the other factors required for the establishment of ice age conditions were in place, the extra snowfall promoting the growth and expansion of ice sheets.

The level of atmospheric carbon dioxide is an additional global factor in the establishment of icehouse conditions, the requirement being relatively low concentrations of CO2 in the atmosphere in order to limit the amount of greenhouse warming. The carbon dioxide concentrations in the atmosphere has followed a trend that was broadly downward from more than 1,000 ppm at the warmest part of the Eocene to less than 180 ppm during the glacial phases of about the last million years, though punctuated by occasional episodes warming in interglacial phases. According to the author¹ it is not fully understood why the levels of carbon dioxide fell the way they did. A suggestion has been made by Maureen Raymo, Flip Froelich and Bill Ruddiman (the latter pointing out a connection between the beginning of agriculture and the start of anthropogenic warming) that a bout of mountain-building activity in the latter part of the Cenozoic involving the Andes, western North America, the Swiss Alps, and the Southern Alps in New Zealand. About 50-40 Ma India collided with the southern part of Asia and triggered the uplift of the Himalayas and the neighbouring Tibetan Plateau, the largest geographic feature on the Earth's surface. An enormous physical barrier was created by this event that caused the intensification of the Asian monsoon as the new upland area was encountered by the humid air masses from the south. Raymo et al. suggested that the chemical weathering was greatly increased, taking enormous volumes of carbon dioxide out of the atmosphere in the process, the carbon dioxide combining with the rain water to form a weak solution of carbonic acid, a very effective dissolving agent of rocks and minerals. The soluble products of chemical weathering were then carried down the large rivers from the uplands to the ocean. Once in the oceans the carbon dioxide was utilised in the making of the skeletons of tiny marine organisms and eventually sequestered in the sediments. The researchers' proposition is that the growth of the Tibetan Plateau and other upland areas during later stages of of the Cenozoic greatly increased the opportunity for chemical weathering by the rainfall that was also greatly increased by the uplift of these upland areas. The result being the progressive extraction of carbon dioxide from the atmosphere, cooling the Earth in the process, eventually to the degree that it was easier for ice sheets to form and survive.

Sources & Further reading

  1. McGuire, Prof. Bill, 2012, Waking the Giant: How a changing climate triggers earthquakes, tsunamis, and volcanoes, Oxford University Press.


Author: M. H. Monroe
Last updated 25/08/2012

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