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.
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