Change in Axis Inclination
Country: United States
Date: January 2009
Is Earth's tilt decreasing? If so how much would this
changes the effects of global warming
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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Update: June 2012