Ice ages can be triggered when Earth's continents are in positions that block or reduce the flow of warm water from the equator to the poles. If warm water is not allowed to travel poleward, ice sheet formation begins. Therefore, the location of Earth's continents has had a significant influence on glaciation. Ice sheets require large landmasses at polar and high-latitude locations in order to become established and grow. These conditions have been met during Earth's ice ages. During the Ice Age (the last one), the continents of Antarctica, North America, and Eurasia supported extensive ice sheets and glaciation. In the ice age prior to that (in the Pennsylvanian and Permian), the supercontinent Pangaea was located at the South Pole and was heavily glaciated. Having continents located in close proximity to Earth's polar regions and the presence of extensive amounts of sea ice are two major factors in promoting ice ages.
Once these conditions are established, a positive feedback loop can begin. Because ice sheets have a high albedo (reflectivity), they greatly reduce the absorption of solar radiation because most of it is immediately reflected back into space. When less radiation is absorbed, the atmosphere cools down, which further promotes the growth of ice sheets. This then increases the albedo, and so on. The system becomes self-perpetuating, and the ice age continues until another mechanism occurs to break the cycle, such as volcanic activity, which increases the amount of CO2 in the atmosphere and intensifies the greenhouse effect.
Just as adding carbon dioxide to the atmosphere is contributing to the warming of the atmosphere today, scientists have suggested that a lowering of atmospheric CO2 levels can lead to a cooling of the atmosphere, pushing Earth into an ice age. Several processes can lead to a decrease in the CO2 levels in the atmosphere, such as physical processes with ocean currents, erosion, and volcanoes.
One example is the Himalayas. Because of the extreme height of these mountains, they have increased the amount of rainfall Earth receives. This added precipitation serves to wash large amounts of CO2 out of the atmosphere, decreasing the greenhouse effect and contributing to the promotion of ice ages. The Himalayas' formation began around 70 million years ago when the Indo-Australian plate collided with the Eurasian plate. The Himalayas are still rising today and its documented rise fits with the climate theory of the region.
Based on data collected by the U.S. Geological Survey (USGS), another suggested influence on the occurrence of ice ages is from continental uplift. According to the USGS, major uplift at continental plate boundaries can cause significant changes in oceanic and atmospheric circulation patterns. The USGS has suggested that climatic changes caused by uplift play an important role in the development of ice ages. As Earth's plates get subducted, mountains are uplifted. A well-known example is that of the Himalayas and the Tibetan plateau. Continents there have been uplifted about 2,000 feet (600 m) during the past 15 million years. This uplift is important because it can change global ocean and atmospheric circulation patterns, which in turn control the climate. For example, if warm air in circulation from the equator to the polar regions pushes up against a tall mountain range but cannot rise and pass beyond it, the warmth that the mass of air contains, which potentially could be delivered to the polar location, becomes blocked. Because the warmer air mass is physically obstructed from being delivered to the polar region, the temperatures there remain cold, potentially encouraging ice age conditions. The same is true for ocean currents. If an uplifted landmass blocks the poleward travel of a warm ocean current, the polar areas will not receive any benefit from the warm temperatures, which could trigger an ice age-like climate at the pole. It is possible that these climate changes caused by the uplift of landmasses play a significant role in the beginning of ice ages.
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