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Climate Science: Investigating Climatic and Environmental Processes

Orbital Dynamics

(From Ice Age Slideset developed by Thomas Andrews).

Like a spinning top, the earth's orbit wobbles so that over the course of a precessional cycle, the North Pole traces a circle in space. This wobble causes the precession of the equinoxes. Adhémar... demonstrated that the cycle of precession takes about 22,000 years to complete. As shown in the figures to the left, the position of the equinoxes and solstices shifts slowly around the earth's elliptical orbit. Precession changes the date at which the earth reaches its perihelion serving to amplify or dampen seasonal climatic variability.


For example, the earth currently reaches its perihelion on January 3, close to the Northern Hemisphere's winter solstice. This timing of the perihelion and Northern Hemisphere's winter solstice reduces seasonal differences in insolation in the Northern Hemisphere because the hemisphere is closer to the sun in winter and hence relatively warmer. On the other hand, the earth is further away from the sun and
relatively cooler during the Northern Hemisphere's summer, reaching its aphelion on July 5. However, 11,000 years ago, the reverse was true: the earth reached its perihelion during the northern summer, increasing the seasonal variability of earth's climate
As the graphic to the right illustrates, the shape of the earth's orbit varies from nearly circular (eccentricity approaching 0.00) to more elliptical (eccentricity=0.06). These variations occur at a frequency of 100,000 years and 400,000 years. Variations in orbital eccentricity have a small impact on the total amount of radiation received at the top of earth's atmosphere (on the order of 0.1%), but that the eccentricity cycle modulated the amplitude of the precession cycle. During periods of high eccentricity (a more elliptical orbit), the effect of precession on the seasonal cycle is strong. When eccentricity is low (more circular), the position along the orbit at which the equinoxes occur is irrelevant because all points on the orbit become, in effect, perihelia. (From Ice Age Slideset developed by Thomas Andrews).

Earth's axial tilt or obliquity varies from 24.5 degrees to 22.1 degrees over the course of a 41,000-year cycle. The current angle is 23.4 degrees. Changes in axial tilt affect the distribution of solar radiation received at the earth's surface. When the angle of tilt is low, polar regions receive less insolation. When the tilt is greater, the polar regions receive more insolation during the course of a year. Like precession and eccentricity, changes in tilt thus influence the relative strength of the seasons, but the effects of the tilt cycle are particularly pronounced in the high latitudes where the great ice ages began.

There is ongoing debate within the paleoclimate community as to whether these forces and their impact on insolation are sufficient to explain Ice Age dynamics.


Image of axial tilt
Image of axial tilt

(From Ice Age Slideset developed by Thomas Andrews).

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