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Name: Nicole
Status: Other
Grade: Other
Location: WA
Country: United States
Date: January 2009


Question:
Is Earth's tilt decreasing? If so how much would this changes the effects of global warming



Replies:
Nicole--

Earth rotates on an axis that is presently inclined at 23° 26’ 22” (almost 23.5 degrees) to the plane of the ecliptic (the plane in which all eight planets orbit the sun). The inclination (also referred to as obliquity) changes from 21° 39” to 24° 36” degrees over an average 40,600 year cycle. The inclination of the axis of rotation is decreasing about 0.5” per year. The changing obliquity affects how much sunlight a particular latitude receives at a particular season.

Consider a northern hemisphere summer when the inclination is high (25 degrees); in this situation the sun’s rays are beating more directly down on the polar surface than would be the case when the inclination is low (21 degrees). When the sun’s rays are more direct (high inclination) one would expect the polar ice to melt during summer months. The inclination is presently decreasing and we would expect the polar ice to expand during the summer months. Unfortunately, this is not the case.

Obliquity is one of the three Milankovitch orbital parameters of Earth (the other two are precession and eccentricity). These regular, predictable changes in the orientation of the Earth's axis of rotation and shape of its orbit, affect the distribution of sunlight over Earth's surface. It was first described by Milutin Milankovitch, a Serbian Civil engineer, who was also a poet, and mathematician. The Milankovitch cycles have historically been used to account for some of the variations in global atmospheric temperature. Something else must be forcing the temperature increase because the current trend in (decreasing) obliquity suggests that polar temperatures should be on a downward trend.

Leslie Kanat, Ph.D.
Professor of Geology
Department of Environmental Sciences



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