Atmosphere And Weather

Ice Ages Recurrent possible causes

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The Ice Age, contrary to what we believe, has not ended; we are currently in the Holocene epoch, an interglacial, when the climate has warmed and most of the ice sheets have retreated, with the remaining glaciers being confined to Greenland and Antarctica. In the last glacial epoch of the Pleistocene, ice sheets covered about 30% of the earth’s surface, mostly in the Northern hemisphere. At present, there is no satisfactory theory to explain the Ice Age. The Ice Ages may have happened due to a combination of factors, chief among them, rotation of the earth, and orbit of the planet around the sun, ocean currents and plate tectonics.

The most plausible theory is by a Serbian geophysicist and astronomer Milutin Milankovitch. He linked astronomical variables such as the tilt in the rotation of the earth, its orbit around the sun and the resultant distribution of solar radiation, with the periodic cooling of the earth. His mathematical calculations came to be known as Milankovitch cycles. The Milankovitch cycles offers an explanation for at least four major glacial events, in the Pleistocene, much of his theory has been confirmed by deep sea core samples. The main drawback of the theory is it works well once glaciation cycles have begun, it does not predict when the cooling will begin.

The earth rotates at a tilt to its orbital plan, called the obliquity. The obliquity varies in a narrow range narrow range of 21.1º to 24.5º over a period of approximately 41,000 years; presently the tilt is 23.4º from its orbital plane. When the obliquity increases both hemispheres are warmer in the summer and in the winters less. The opposite is true when the obliquity is decreases, summers are cooler and winters are warmer. Cooler summers are believed to lead to the onset of an ice age because less ice from previous winters will melt.

Eccentricity is the change of the orbit around the sun, from circular to elliptical in a 100,000 year cycle. When the earth’s orbit is elliptical and at perihelion, its closest approach to the sun, it would be in a much warmer period than at aphelion, its furthest approach. At perihelion the earth receives 23% more radiation than aphelion       

Another theory explaining these changes in climate involves the opening and closing of gateways for the flow of ocean currents. This theory suggests that the redistribution of heat on the planet by changing ocean circulation can isolate Polar Regions, cause the growth of ice sheets and sea ice, and increase temperature differences between the equator and the poles.

The climate could have also been affected by the ocean currents. It is generally discredited as being an initiator of glaciation but it could have been a contributing factor. The region of the North Pole has been an open ocean throughout most of geological history Equatorial currents warmed the Polar Regions, and once the Artic Ocean became isolated from these warm currents, cooling of the world’s climate could have accelerated and ushered in the Ice Ages

The key to glaciation may lie in the southern hemisphere spring time, beginning 19,000 years ago.

Sediment core samples indicate deep-sea temperatures warmed about 1,300 years before the tropical surface ocean, and well before the rise in atmospheric CO2,   the result was the sea ice melted quicker, exposing more of the ocean to the sun and a feedback loop was created. The rise in ocean temperatures was preceded by a warming Antarctica, while temperatures in the Northern Hemisphere remained low. The retreat of the Antarctic sea ice began much earlier than the northern hemisphere’s ice retreat.

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