5.These two timescales are instantly recognisable to anyone familiar with Earths ice ages: for the past million years, ice ages have occurred roughly every 100,000 years.Before that, they occurred roughly every 41,000 years.
6.Most scientists believe that the ice ages are the result of subtle changes in Earths orbit, known as the Milankovitch cycles.One such cycle describes the way Earths orbit gradually changes shape from a circle to a slight ellipse and back again roughly every 100,000 years.The theory says this alters the amount of solar radiation that Earth receives, triggering the ice ages.However, a persistent problem with this theory has been its inability to explain why the ice ages changed frequency a million years ago.
7.In Milankovitch, there is certainly no good idea why the frequency should change from one to another, says Neil Edwards, a climatologist at the Open University in Milton Keynes, UK.Nor is the transition problem the only one the Milankovitch theory faces.Ehrlich and other critics claim that the temperature variations caused by Milankovitch cycles are simply not big enough to drive ice ages.
8.However, Edwards believes the small changes in solar heating produced by Milankovitch cycles are then amplified by feedback mechanisms on Earth.For example, if sea ice begins to form because of a slight cooling, carbon dioxide that would otherwise have found its way into the atmosphere as part of the carbon cycle is locked into the ice.That weakens the greenhouse effect and Earth grows even colder.
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