The third cycle, with a period of 100,00 years, alters the shape of the earth's orbit, and its eccentricity amplifies or minimises the effect of the procession. This was first put forward in the 1870's by a Scottish amateur scientist, James Croll, and verified in the 1920's by a Serbian mathematician, Milutin Milankovitch. He showed that these variations were the basis for the significantly cooler summers every 21,000 and 41,000 years. These could help cause an ice age by beginning the build up of ice at the poles. The additional time it took for the icesheets to grow to their maximum sustainable size has been suggested as reason for the ice ages occurring every 100,000 years.
Some scientists think the world is about due for another ice age.
The result would be global warming, a scenario which is rapidly gaining credibility and acceptance. The effects of global warming will first be visible in the ice-covered parts of the planet, for any incremental rise in temperatures would reduce the amount of water held in ice. Since Antarctica is a global repository of frozen water, we need to have a much greater understanding of Antarctic geography, glaciology and ecology in order to effectively monitor and predict such changes. The possibility of this happening in the near term is not seriously considered by scientists, for the Antarctic icecap is currently believed to be in equilibrium. This will be due to an increase in the amount of water vapor in the air as a result of higher air temperatures and will lead to greater snow fall on the continent.